React Context & Hooks Tutorial

Learn React Context and Hooks with a comprehensive tutorial series. Explore state management, component logic, and more with practical examples.

Introduction to React Context and Hooks

Welcome to your first tutorial on React Context and Hooks! These two relatively new features in the React ecosystem have significantly impacted how we develop React applications. They offer powerful tools for state management and component logic, streamlining development and enhancing code organization.

Understanding React Context and Hooks

So, what exactly are React Context and Hooks?

React Context API: Simplifying State Sharing

The React Context API provides a clean and efficient way to share state between different components without the need for prop drilling.

React Context API:

React Context API is a feature in React that provides a way to share data between components without explicitly passing props through every level of the component tree. It simplifies state management for data that needs to be accessible to many components.

Prop drilling, also known as “props passing”, is the process of passing data down multiple levels of components via props. While props are fundamental to React, excessive prop drilling can make code harder to read and maintain, especially in larger applications. Context offers a solution by making data available to components throughout the component tree, regardless of their nesting level.

React Hooks: Empowering Functional Components

React Hooks revolutionize functional components by enabling them to perform tasks that were previously exclusive to class components.

React Hooks:

React Hooks are functions that let you “hook into” React state and lifecycle features from within functional components. They enable the use of state and other React features in functional components, which were previously only available in class components.

Before Hooks, functional components were primarily used for presentational purposes, lacking the ability to manage local state or tap into React lifecycle methods. Hooks like useState and useEffect bridge this gap, allowing functional components to handle state and side effects, making them just as powerful and versatile as class components.

Context and Hooks: A Powerful Combination

When used together, Context and Hooks create a potent synergy. They offer an elegant approach to managing shared data within your application, conceptually similar to state management libraries like Redux, but without the need to install external dependencies.

You might have heard discussions about Context and Hooks potentially replacing Redux. While it’s still early to definitively say they are a complete replacement, their combined power is undeniable. They offer a compelling alternative for managing application state, especially in scenarios where the complexity of Redux might be overkill.

Prerequisites for this Course

Before diving into this course, it’s essential to have a solid foundation in certain web development concepts.

• JavaScript Proficiency: This course is heavily JavaScript-focused. A strong understanding of JavaScript fundamentals is crucial for grasping the concepts and code examples effectively.
• Moderate React Understanding: You should be familiar with the basics of React development, including:

  • React CLI (Create React App): Knowing how to use the React command-line interface to create and manage React projects.

    React CLI (Create React App):

    React CLI, often referring to Create React App, is a command-line interface tool for quickly setting up a new React project with a pre-configured development environment. It simplifies project initialization by handling build configurations, tooling, and project structure.

  • Component Creation: Understanding how to create both class and functional components.

    • Class Components: React components defined using ES6 classes.

      Class Components (in React):

      Class components are a way to define React components using ES6 classes. They have access to lifecycle methods and state management using this.state and this.setState, and were the traditional way to create stateful components before Hooks.

    • Functional Components: React components defined as JavaScript functions.

      Functional Components (in React):

      Functional components are simpler ways to define React components as JavaScript functions. They primarily receive props as input and return JSX to describe the UI, and with Hooks, they can now manage state and lifecycle effects.

  • Props: Understanding how to use props to pass data from parent to child components.

    Props (in React):

    Props, short for properties, are read-only inputs passed from a parent component to a child component in React. They are used to customize and configure child components, making them reusable and dynamic.

If you lack any of these prerequisites, don’t worry! The instructor recommends two resources:

• Modern JavaScript Course (Udemy): A comprehensive course to build a strong JavaScript foundation. (Link provided in the video description).
• Free React Course for Beginners: An introductory course to learn the fundamentals of React development. (Link provided in the video description).

Course Overview: Building a Reading List Application

In this course, we will embark on a practical journey to master React Context and Hooks. The curriculum is structured in a progressive manner:

1. Separate Learning: We will first explore the fundamentals of the Context API and React Hooks individually. This approach ensures a clear understanding of each concept before combining them.

2. Integration and Application: We will then merge our knowledge of Context and Hooks to build a simple reading list application.

   Dummy Application:

   A dummy application, also known as a sample or example application, is a simple, often minimal, application created for demonstration or learning purposes. It’s designed to illustrate specific concepts or functionalities without the complexity of a full-fledged application.

   This application will also incorporate browser local storage functionality.

   Local Storage:

   Local storage is a web storage feature that allows web applications to store key-value pairs in a web browser with no expiration time. It’s often used to persist user data or application settings across browser sessions.

   While the application’s design may not be visually groundbreaking, its primary purpose is to provide hands-on practice with Context API and Hooks in a realistic context.

Application Features:

• Book Listing: Display a list of books.
• Book Deletion: Allow users to delete books from the list.
• Local Storage Persistence: Ensure that the book list persists even after the browser is refreshed by utilizing local storage.
• Book Addition: Enable users to add new books to the list, which are also saved to local storage.

This project will provide excellent practical experience in utilizing Context API and Hooks for managing application state and data persistence.

Course Resources and Setup

To facilitate your learning experience, several resources and setup steps are provided:

Course Files Repository

All code examples and project files for each lesson are available in a dedicated Git repository.

Git Repo (Git Repository):

A Git repository is a storage location for tracking changes to a set of files, often used for version control in software development. It contains the entire history of changes and allows collaboration and management of codebases.

The repository link is provided in the video description.

• Branch-Based Lessons: Each lesson in this course has a corresponding branch in the Git repository.

  Branch (in Git):

  In Git, a branch is a parallel version of a repository, allowing developers to work on new features or fixes without affecting the main codebase. Branches can be created, merged, and deleted, enabling collaborative and organized development workflows.

  For example, to access the code for lesson 9, you would switch to the lesson-9 branch.

• Code Access: You can access the code in two ways:

  • Download ZIP: Download a ZIP archive of the repository.

  • Clone Repository: Clone the repository to your local machine using Git.

    Clone (in Git):

    Cloning a Git repository means creating a local copy of a remote repository on your computer. This allows you to work on the code locally, make changes, and contribute back to the original repository.

    You will need Git installed on your system to use this method.

Recommended Code Editor

Visual Studio Code (VS Code) is the recommended code editor for this course.

VS Code (Visual Studio Code):

Visual Studio Code (VS Code) is a popular source code editor developed by Microsoft, known for its extensive features, debugging capabilities, and support for various programming languages. It is widely used for web development and React development due to its extensions and integrated terminal.

Using VS Code ensures consistency with the instructor’s environment and workflow. You can download VS Code for free from the official website (code.visualstudio.com).

VS Code Extension: Simple React Snippets

To enhance your React development experience in VS Code, consider installing the “Simple React Snippets” extension.

Extensions (in VS Code):

Extensions in VS Code are add-ons that enhance the editor’s functionality and support for specific programming languages, tools, or workflows. They can provide features like code snippets, linting, debugging, and more.

Simple React Snippets (VS Code Extension):

Simple React Snippets is a VS Code extension that provides code snippets to quickly generate common React code structures, such as functional components, class components, and imports. It speeds up development by reducing boilerplate code writing.

This extension provides shortcuts for generating common React code structures, such as components and imports, speeding up your coding process.

Project Setup: Creating a React Application

Let’s begin by setting up a new React project using the React CLI.

1. Open Terminal: Open your system’s terminal or command prompt.

   Terminal (Command Line Interface):

   A terminal, also known as a command-line interface, is a text-based interface for interacting with a computer’s operating system. It allows users to execute commands, manage files, and run programs using text input instead of a graphical user interface.

2. Create React App: Use MPX (npm package runner) to run the create-react-app command to generate a new React project.

   MPX (npm package runner):

   MPX, or npm package runner, is a command-line tool that comes with npm and allows you to execute packages without installing them globally. It’s often used to run tools like create-react-app to initialize projects.

   Create-react-app:

   Create React App is a command-line tool and React CLI that sets up a React development environment with no build configuration. It’s the recommended way to start new single-page React applications.

   Run the following command in your terminal:

   npx create-react-app context-app

   This command creates a new React project named “context-app”.

3. Navigate to Project Directory: Once the project is created, navigate into the project directory using the cd command:

   cd context-app

4. Start Development Server: Start the local dev server to view the application in your browser.

   Local Dev Server:

   A local development server is a lightweight server that runs on a developer’s local machine to host and serve web applications during development. It typically provides features like hot-reloading and makes it easy to test and debug the application in a browser.

   Run the following command:

   npm start

   NPM start:

   npm start is a command used in projects set up with npm that typically starts the development server for the application. It’s often configured in the package.json file to run scripts that build and serve the application locally for development.

   This command will compile the React application and open it in your default web browser. You should see the default React starter page.

Project Structure and Initial Setup

Let’s clean up the initial project structure and create our basic components.

1. Source Folder (src): Navigate to the src folder in your project directory. This folder contains the main source code of your React application.

   Source folder (in React projects):

   The src folder in a React project created with Create React App is the directory where most of the application’s source code resides. It usually contains components, JavaScript files, CSS files, and other assets.

2. Cleanup Files: Remove unnecessary files to simplify the project structure for this tutorial:

   • Delete App.css
   • Delete App.test.js
   • Delete logo.svg

3. Modify App.js: Edit the App.js file, which is the root component of your application.

   App.js (in React projects):

   App.js is typically the root component of a React application created with Create React App, located in the src folder. It serves as the entry point and often contains the main structure and logic of the application.

   • Remove import statements for the deleted files (e.g., import './App.css'; and import logo from './logo.svg';).

   • Clear out the content within the main div with className="App". Retain the surrounding div element.

     ClassName (in JSX):

     className in JSX is an attribute used to specify CSS classes for HTML elements rendered by React components. It’s analogous to the class attribute in HTML but uses className because class is a reserved keyword in JavaScript.

   Your App.js should now look something like this:

   import React from 'react';
   
   function App() {
     return (
       <div className="App">
         {/* Content will go here */}
       </div>
     );
   }
   
   export default App;

4. Create Components Folder: Create a new folder named components within the src folder to organize your React components.

5. Create Navbar Component: Inside the components folder, create a new file named Navbar.js.

   • Import React and Component: Import React and Component from the ‘react’ library.

     Imports (in JavaScript/React):

     Imports in JavaScript, particularly in React, are statements used to bring code from other modules or files into the current file. They allow you to reuse code, access libraries, and structure your application into modular pieces.

   • Class Component Structure: Create a class component named Navbar. Use the “CC” snippet from the “Simple React Snippets” extension for quick boilerplate code generation.

     Boilerplate (code):

     Boilerplate code refers to sections of code that are repeated in many situations with minimal variation. It’s often used as a starting point to reduce repetitive coding and set up standard structures.

   • Render Method: Within the render method, return a simple template for the navbar using JSX.

     Template (in programming/JSX):

     In programming, a template refers to a pre-designed structure or format that serves as a starting point for creating something new. In JSX, templates are the HTML-like structures returned by React components to define the UI.

     import React, { Component } from 'react';
     
     class Navbar extends Component {
       render() {
         return (
           <nav>
             <h1>Context App</h1>
             <ul>
               <li>Home</li>
               <li>About</li>
               <li>Contact</li>
             </ul>
           </nav>
         );
       }
     }
     
     export default Navbar;

   • Export Component: Export the Navbar component to make it available for use in other parts of your application.

     Export (in JavaScript modules):

     export in JavaScript modules is a keyword used to make functions, objects, or values defined in a module available for use in other modules. It enables modularity and code reuse by controlling what parts of a module are accessible externally.

6. Create BookList Component: Create another file in the components folder named BookList.js.

   • Follow the same steps as for Navbar.js to create a class component named BookList.

   • In the render method, return a template for the book list.

     import React, { Component } from 'react';
     
     class BookList extends Component {
       render() {
         return (
           <div className="book-list">
             <ul>
               <li>The Way of Kings</li>
               <li>The Name of the Wind</li>
               <li>The Final Empire</li>
             </ul>
           </div>
         );
       }
     }
     
     export default BookList;

7. Nest Components in App.js: In App.js, nest the Navbar and BookList components within the main div.

   Nesting (components in React):

   Nesting components in React refers to placing one component inside another component in the JSX structure. This creates a parent-child relationship and allows for composition and hierarchical organization of the UI.

   import React from 'react';
   import Navbar from './components/Navbar'; // Import Navbar
   import BookList from './components/BookList'; // Import BookList
   
   function App() {
     return (
       <div className="App">
         <Navbar /> {/* Nest Navbar component */}
         <BookList /> {/* Nest BookList component */}
       </div>
     );
   }
   
   export default App;

8. Preview in Browser: Save all files and preview the application in your browser. You should see the Navbar and BookList components displayed on the page.

   Preview (in development):

   Preview in development refers to viewing the application or changes in a browser or development environment as they are being built. It allows developers to immediately see the results of their code changes and test the application interactively.

Basic Styling

Let’s add some basic CSS styles to improve the appearance of our application.

1. Edit index.css: Open the index.css file in the src folder. This file is for global styles.

2. Replace Default Styles: Replace the existing CSS rules with the styles provided in the lesson 1 branch of the course’s Git repository. These styles include:

   • Basic layout for the App class to center content and set a maximum width.

   • Styling for the nav and ul elements within the Navbar component for basic navigation links.

   • Styling for the book-list and ul elements within the BookList component to create a visually distinct list.

   • Styling for li elements in both components for padding, margins, and border-radius.

     Border-radius (in CSS):

     border-radius is a CSS property used to round the corners of HTML elements. It allows you to create elements with curved edges instead of sharp corners, improving visual appeal.

   • Padding (in CSS):

     Padding in CSS refers to the space between the content of an HTML element and its border. It’s used to create visual spacing inside the element, making the content less cramped and more readable.

   • Margin (in CSS):

     Margin in CSS refers to the space outside the border of an HTML element, separating it from neighboring elements. It’s used to control the spacing between elements and create layout structures.

   • List-style-type (in CSS):

     list-style-type is a CSS property used to control the marker (like bullets or numbers) for list items in HTML lists (<ul> and <ol>). Setting it to none removes the default markers.

   The provided CSS styles are as follows:

   body {
     margin: 0;
     font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
       'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
       sans-serif;
     -webkit-font-smoothing: antialiased;
     -moz-osx-font-smoothing: grayscale;
   }
   
   code {
     font-family: source-code-pro, Menlo, Monaco, Consolas, 'Courier New',
       monospace;
   }
   
   .App {
     max-width: 400px;
     margin: 30px auto;
     text-align: center;
   }
   
   nav {
     padding: 5px;
     background: #f2f2f2;
   }
   
   nav ul {
     padding: 0;
   }
   
   nav li {
     display: inline-block;
     margin: 0 10px;
   }
   
   .book-list {
     padding: 20px;
     margin-bottom: 20px;
   }
   
   .book-list ul {
     padding: 0;
     list-style-type: none;
   }
   
   .book-list li {
     padding: 10px;
     margin: 10px auto;
     border-radius: 10px;
     background: #eee;
   }

3. Preview Styles: Save index.css and refresh your browser. The application should now have basic styling applied, making it more visually organized.

With the project set up and basic styling in place, we are now ready to delve into the React Context API in the next tutorial!

Understanding the React Context API: A Solution for Prop Drilling

This chapter introduces the React Context API, a powerful tool for managing and sharing state within your React applications. We will explore the problems it solves, how it works, and when it is most appropriate to use.

The Challenge of Prop Drilling

In React applications, data is typically passed down the component tree using props.

Props (Properties): Short for ‘properties’, props are data passed from a parent component to a child component in React, enabling data flow down the component tree. They are read-only from the child component’s perspective.

This unidirectional data flow is a core principle of React and helps maintain predictable data management. However, as applications grow in complexity, passing props through multiple levels of components can become cumbersome and inefficient. This issue is commonly referred to as “prop drilling.”

Imagine a scenario where you have a deeply nested component that needs access to some state defined at the root of your application. To achieve this using only props, you would have to pass that state down through every intermediary component in the component tree, even if those components themselves don’t need to use that state.

Component Tree: A hierarchical structure representing the parent-child relationships between components in a React application. It visually represents how components are nested within each other to build the user interface.

Consider the following component structure as an example:

• App (Root Component)
  • PageView
    • Navbar
      • StatusBar
      • BookList
        • BookDetails
        • AddBook

Let’s say the App component holds state related to the UI theme (e.g., light or dark mode). If components like StatusBar, BookList, BookDetails, and AddBook need to access this theme state, traditionally, you would pass it down as props through PageView and Navbar, even though these intermediary components might not directly use the theme.

This approach, while functional, has several drawbacks:

• Code becomes verbose and harder to maintain: Passing props through multiple levels increases code complexity and makes it harder to track data flow.
• Reduced reusability: Components become tightly coupled to the specific props they are expected to receive, even if they don’t directly use them, limiting their reusability in different contexts.
• Potential for errors: As the component tree grows, managing and passing props correctly across multiple levels increases the risk of introducing bugs.

Introducing the Context API: A Centralized State Management Solution

The React Context API provides an elegant solution to the prop drilling problem. It offers a way to share state that can be considered “global” for a subtree of React components, without having to manually pass props at every level.

Context API: A React feature that provides a way to share values like state between components without explicitly passing props through every level of the component tree. It’s particularly useful for data that needs to be accessible by many components at different nesting levels.

The Context API essentially creates a centralized “context” where you can store data (or state) and then allow components within a specified subtree to access that data directly, without relying on prop passing.

Think of it as establishing a shared data layer that components can tap into when needed. This approach offers several advantages:

• Eliminates prop drilling: Components can access shared data directly from the context, bypassing the need to pass props through intermediary components.
• Cleaner and more maintainable code: Reduces code verbosity and improves code readability by simplifying data flow.
• Improved component reusability: Components become less dependent on specific prop structures, enhancing their reusability.

Context API and State Management: A Comparison to Redux

If you are familiar with state management libraries like Redux, the concept of the Context API might seem familiar.

Redux: A popular JavaScript library for managing application state, often used with React for complex applications. It provides a centralized store for application state and enforces a strict unidirectional data flow, making state changes predictable and debuggable.

Both Redux and the Context API provide a centralized place to manage state and make it accessible to different parts of your application. In fact, when used in conjunction with hooks, the Context API can behave very similarly to Redux for many use cases.

Hooks: Functions in React that let you “hook into” React state and lifecycle features from within functional components. They were introduced to make state and other React features accessible in functional components, simplifying component logic.

However, it’s important to understand the nuances:

• Scope: The Context API is generally designed for sharing data that is “global” within a specific component subtree, such as theme settings, user authentication, or language preferences. Redux, on the other hand, is a more comprehensive state management solution suitable for managing complex application state across the entire application.
• Complexity: The Context API is simpler to set up and use for basic state sharing scenarios. Redux has a steeper learning curve and involves more boilerplate code but offers more advanced features for managing complex state, such as time-travel debugging and middleware.

The Context API is often a suitable alternative to Redux for smaller to medium-sized applications or when you need to share specific types of data across a component subtree.

How to Use the Context API: Providers and Consumers

To utilize the Context API, you need to perform the following steps:

1. Create a Context: You start by creating a context object using React.createContext(). This context object will hold the shared data.

2. Provide the Context: You use a Context Provider component to make the context available to a subtree of components.

   Context Provider: A React component provided by the Context API, used to wrap a section of your component tree. It makes the context’s value available to all descendant components within that subtree.

   The Provider component accepts a value prop, which is the data you want to share via the context. Any component wrapped within the Provider can access this value.

3. Consume the Context: Components that need to access the shared context data can “consume” it. This is typically done using hooks like useContext in functional components or the Context.Consumer render prop in class components.

By wrapping a portion of your component tree with a Context Provider, you effectively create a scope within which components can access the shared data defined in the context without prop drilling.

When to Use Context API: Global Data for a Subtree

React’s official documentation provides guidance on when to consider using the Context API. It is designed for sharing data that can be considered “global” for a tree of React components. Examples include:

• Current authenticated user: Sharing user authentication status across components that need to know if a user is logged in.
• UI theme: Providing a consistent theme (e.g., light or dark mode) across various UI elements.
• Preferred language: Sharing the user’s language preference throughout the application.

Before using the Context API, it’s helpful to ask yourself: “Is the data I want to share global within a specific part of my application, and will it be used by several components in that subtree?” If the answer is yes, then the Context API is likely a good choice.

However, it’s important to note that the Context API is not a universal solution for all state management scenarios. React encourages component composition as a primary pattern for building UI.

Component Composition: A React pattern where complex UI is built by combining simpler, reusable components, rather than inheritance. It emphasizes building UIs by composing components together, promoting modularity and flexibility.

For more localized state management within a single component or for passing data between closely related parent-child components, props might still be the most appropriate and straightforward approach.

React Developer Tools: Inspecting Context in Your Application

To aid in development and debugging, especially when working with context and state, the React Developer Tools browser extension is highly recommended.

React Developer Tools: A browser extension for Chrome and Firefox that allows you to inspect React component hierarchies, props, state, and context values directly in the browser’s developer tools. It’s an invaluable tool for understanding and debugging React applications.

This extension allows you to inspect the component tree, view the props and state of each component, and crucially, examine the context values provided by Context Providers. This makes it much easier to understand how context data is flowing through your application and to debug any issues related to context usage.

You can easily install the React Developer Tools from the Chrome Web Store or Firefox Add-ons.

Conclusion and Next Steps

The React Context API is a valuable tool for managing and sharing state in React applications, particularly for data that is considered “global” within a component subtree. It effectively addresses the problem of prop drilling, leading to cleaner, more maintainable, and more reusable code.

In the following chapters, we will delve into practical examples and learn how to create and use contexts and providers to manage shared state in a real React application. We will explore how to integrate context with hooks to build efficient and well-structured React components.

Creating a Theme Context in React: A Step-by-Step Guide

This chapter will guide you through the process of creating and implementing a theme context in a React application. Contexts in React are a powerful way to share data between components without having to pass props manually at every level. This chapter will focus on creating a ThemeContext to manage the visual theme (light or dark) of an application and make theme-related data accessible to various components.

Introduction to Contexts

In React applications, it’s often necessary for different components to access the same data. Traditionally, this is achieved by passing data down the component tree using props. However, this can become cumbersome and inefficient, especially when deeply nested components need access to data from components higher up in the tree. React Context provides a solution to this problem by allowing you to share data globally across your component tree without prop drilling.

Context: In React, Context provides a way to share values like themes, user authentication, or preferred locale between components without explicitly passing a prop through every level of the component tree. It effectively creates a “global” state for a specific part of your application.

In this chapter, we will create a context to manage the theme of our application, specifically whether it’s a light or dark theme, and the associated colors for text, UI elements, and background.

Setting Up the Project and Components

For this demonstration, we assume you have a React application set up with at least two components: Navbar and BookList. These components will be used to illustrate how to consume and utilize the theme context.

Component: In React, a component is a reusable, self-contained building block of a user interface. Components can be functional or class-based and are responsible for rendering a part of the UI based on their props and state.

Our goal is to share theme-related data between Navbar and BookList, and potentially other components we might add in the future.

Creating the Context Folder and File

To organize our project, we will create a dedicated folder for our context files. While not mandatory, this practice helps maintain a clean project structure.

1. Create a contexts folder: Inside your src directory, create a new folder named contexts.
2. Create ThemeContext.js: Inside the contexts folder, create a new JavaScript file named ThemeContext.js. This file will house the code for our theme context.

Defining the Theme Context

Inside ThemeContext.js, we will define our context.

1. Import React and createContext: Begin by importing React and the createContext function from the React library.

   import React, { createContext } from 'react';

   Import: In JavaScript, import is used to bring in modules, functions, objects, or primitive values from external files or libraries into the current module’s scope, making them available for use.

   Function: In programming, a function is a block of organized, reusable code that performs a specific task. Functions are essential for modularizing code and making it more readable and maintainable.

2. Create and Export the Context: Use the createContext() function to create the context and export it.

   export const ThemeContext = createContext();

   Export: In JavaScript modules, export is used to make functions, objects, or primitive values defined in the current module available for use in other modules.

   Constant: In JavaScript, const is used to declare a variable with a constant value. Once a constant is declared and assigned a value, it cannot be reassigned.

   createContext(): This is a React function that creates a Context object. When React renders a component that subscribes to this Context object, it will read the current context value from the closest matching Provider above it in the tree.

At this stage, ThemeContext is created but doesn’t contain any data or provider logic.

Creating the Context Provider Component

To supply data to our context, we need to create a Provider component. This component will manage the theme data and make it available to consuming components.

1. Create a Class Component: We will create a class component named ThemeContextProvider.

   class ThemeContextProvider extends React.Component {
       // Component logic will be added here
   }
   export default ThemeContextProvider;

   Class Component: In React, a class component is a component defined using ES6 classes. It has access to features like state, lifecycle methods, and can manage more complex logic compared to functional components (before hooks).

   React.Component: This is the base class for creating class-based React components. By extending React.Component, a class inherits the necessary methods and properties to function as a React component.

   export default: In JavaScript modules, export default is used to export a single value (function, class, object, etc.) as the default export from a module. This is often used when a module is intended to export only one primary thing.

2. Initialize State: Inside ThemeContextProvider, initialize the state to hold theme-related data. This state will include:

   • isLightTheme: A boolean value to determine whether the light theme is active.
   • light: An object containing color values for the light theme.
   • dark: An object containing color values for the dark theme.

   state = {
       isLightTheme: true,
       light: { syntax: '#555', ui: '#ddd', bg: '#eee' },
       dark: { syntax: '#ddd', ui: '#333', bg: '#555'}
   }

   State: In React, state is a JavaScript object that holds data that may change over time and affects the component’s rendering. When state changes, React re-renders the component and its children.

   Boolean: A boolean is a data type that has one of two possible values: true or false. It is often used to represent logical values and conditions.

   Object: In JavaScript, an object is a collection of key-value pairs. Objects are used to represent complex data structures and entities with properties and methods.

   Property: In the context of JavaScript objects, a property is a named value associated with an object. Properties define the characteristics or attributes of an object.

3. Implement the render Method and Provider: The render method of ThemeContextProvider will return the ThemeContext.Provider. The Provider component is crucial for making the context data available to consuming components.

   render() {
       return (
           <ThemeContext.Provider value={{...this.state}}>
               {this.props.children}
           </ThemeContext.Provider>
       );
   }

   render(): In React class components, the render() method is a lifecycle method that is responsible for describing what the UI should look like, based on the component’s props and state. It must return JSX, null, or false.

   JSX (JavaScript XML): JSX is a syntax extension to JavaScript that looks similar to HTML. It’s used in React to describe the structure of the user interface and is then transformed into regular JavaScript function calls.

   props (Properties): In React, props are inputs to a component. They are read-only values passed from a parent component to a child component, allowing parent components to configure and control child components.

   children prop: In React, every component has a special prop called children. It represents the content passed between the opening and closing tags of a component when it’s used in JSX.

   Spread syntax (...): The spread syntax in JavaScript allows an iterable such as an array or object to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) or key-value pairs (for object literals) are expected. Here, ...this.state spreads all properties of the state object into the value prop of the Provider.

   • ThemeContext.Provider: Every Context object comes with a Provider React component that allows consuming components to subscribe to context changes. It accepts a value prop, which is the data you want to provide to the context.

   • value prop: This prop of the Provider component accepts the data that you want to share through the context. In this case, we are passing an object containing all the state properties using the spread syntax ({...this.state}).

   • {this.props.children}: This renders any components that are wrapped by the ThemeContextProvider component. These “children” components will be able to access the context value provided by ThemeContext.Provider.

4. Wrap Components with the Provider in App.js: To make the theme context available to Navbar and BookList, we need to wrap them with ThemeContextProvider in our main App.js component.

   // App.js
   import ThemeContextProvider from './contexts/ThemeContext';
   import Navbar from './Navbar'; // Assuming Navbar component exists
   import BookList from './BookList'; // Assuming BookList component exists
   
   function App() {
     return (
       <div className="App">
         <ThemeContextProvider>
           <Navbar />
           <BookList />
         </ThemeContextProvider>
       </div>
     );
   }
   
   export default App;

   Auto import: Modern code editors often provide “auto import” functionality. When you start typing a component or module name, the editor can automatically suggest and insert the import statement at the top of the file.

5. Verify in React DevTools: Open your browser’s developer tools, navigate to the React tab (if you have the React Developer Tools extension installed). You should see the ThemeContextProvider component wrapping your Navbar and BookList components. Expanding the ThemeContextProvider in the React panel will show the context provider and the data it’s providing ( dark, light, and isLightTheme).

   React Panel (React DevTools): React DevTools is a browser extension that allows you to inspect React component hierarchies, props, state, and performance. It’s an invaluable tool for debugging and understanding React applications.

At this point, the ThemeContext is set up, and the ThemeContextProvider is providing theme data to any components it wraps. In the next steps, we will learn how to consume this context data within the Navbar and BookList components.

Consuming Context in React Class Components: A Deep Dive into contextType

This chapter explores how to consume context data within React class components using contextType. We will build upon the concept of context providers and demonstrate how to access shared data in components that are nested within the provider. This method, leveraging contextType, is specifically designed for class components and offers a straightforward way to interact with context data.

Introduction to Context Consumption

In React, the Context API provides a way to share values like themes, user authentication, or preferred language globally across a component tree without manually passing props down at every level. In the preceding setup (as referenced in the video), we have established a ThemeContext and a ThemeContextProvider. The provider component wraps other components, making the context data available to them.

Theme Context: In React’s Context API, the Theme Context is an object created using React.createContext(). It serves as the container for context data and is used by both the Provider and Consumer components. It does not hold the actual data itself but rather acts as an identifier for the context.

Theme Context Provider: A component associated with a Theme Context that makes the context data available to its descendant components. It uses the Provider component from the Context API and accepts a value prop, which is the data to be shared.

Our objective now is to access and utilize the data provided by the ThemeContextProvider within the Navbar and BookList components.

Accessing Context with contextType in Class Components

For class components, React offers a special static property named contextType to consume context. This approach is exclusive to class components and cannot be used in functional components directly.

Setting contextType

To consume a specific context in a class component, you need to declare the static contextType property and assign it the desired context object. In our example, we want to consume the ThemeContext within the Navbar component.

import React, { Component } from 'react';
import { ThemeContext } from '../contexts/ThemeContext'; // Assuming ThemeContext is defined here

class Navbar extends Component {
  static contextType = ThemeContext; // Setting contextType to ThemeContext

  // ... rest of the component
}

export default Navbar;

Class Component: In React, a class component is a component defined using ES6 classes. It must extend React.Component and typically implements a render() method that returns the React elements to be rendered. Class components can manage their own state and have access to lifecycle methods.

Functional Component: In React, a functional component is a component defined as a JavaScript function. It’s a simpler way to define components, primarily for rendering UI based on props. Functional components can now also manage state and lifecycle behavior using Hooks.

By setting static contextType = ThemeContext, React automatically connects the Navbar component to the nearest ThemeContextProvider in the component tree.

Component Tree: The hierarchical structure of React components in an application. Components are nested within each other, forming a tree-like structure where parent components pass data and behavior down to their children.

React will then traverse upwards in the component tree starting from the Navbar component. The first time it encounters a ThemeContextProvider that provides the ThemeContext, it establishes a connection.

Provider: In the Context API, the Provider component is a component that makes context data available to its descendants. It accepts a value prop which holds the data to be shared. Any component within the Provider’s subtree can access this data.

Accessing Context Data via this.context

Once contextType is set, the context data provided by the ThemeContextProvider becomes accessible within the class component instance through this.context. This this.context property will hold the value prop that was passed to the ThemeContextProvider.

Value Property: The value prop is passed to the Provider component in React’s Context API. It determines the data that will be made available to all consuming components within the Provider’s subtree. This value can be any JavaScript value, including objects, arrays, or primitive types.

To verify this, we can use console.log(this.context) within the render method of the Navbar component:

class Navbar extends Component {
  static contextType = ThemeContext;

  render() {
    console.log(this.context); // Logging the context data
    return (
      // ... JSX for Navbar
    );
  }
}

Render Method: In a React class component, the render() method is a lifecycle method that is responsible for describing the UI that the component should display. It must return a React element, or null if nothing should be rendered.

Console.log: A JavaScript function used for outputting information to the browser’s console, primarily for debugging and logging purposes.

Upon inspecting the browser’s console, you will observe that this.context contains the data passed through the value prop of the ThemeContextProvider. This confirms that we have successfully accessed the shared context data within the Navbar component.

Destructuring Context Data

To efficiently use the context data, we can employ destructuring to extract specific properties from this.context. In our example, the context data includes properties like isLightTheme, light, and dark.

Destructuring: A JavaScript feature that allows you to unpack values from arrays or properties from objects into distinct variables. It provides a concise way to extract multiple values at once.

Inside the render method, we can destructure these properties:

render() {
  const { isLightTheme, light, dark } = this.context; // Destructuring context properties
  // ... rest of the render method
}

This code snippet extracts isLightTheme, light, and dark from this.context and assigns them to respective constants, making them easily accessible within the component’s scope.

Dynamically Applying Themes

Now that we have access to the theme data, we can dynamically apply styles based on the current theme. We use a ternary operator to determine which theme (light or dark) to apply based on the isLightTheme boolean value from the context.

Ternary Operator: A concise conditional operator in JavaScript, often used as a shorthand for if...else statements. It takes three operands: a condition followed by a question mark (?), an expression to execute if the condition is truthy, and an expression to execute if the condition is falsy, separated by a colon (:).

render() {
  const { isLightTheme, light, dark } = this.context;
  const theme = isLightTheme ? light : dark; // Determining theme based on isLightTheme

  return (
    <nav style={{ background: theme.ui, color: theme.syntax }}>
      {/* ... Navbar content */}
    </nav>
  );
}

Here, if isLightTheme is true, the theme constant will be assigned the light theme object; otherwise, it will be assigned the dark theme object. We then use this theme object to style the Navbar component, setting the background color to theme.ui and the text color to theme.syntax.

Applying Context to the BookList Component

We can apply the same process to the BookList component to consume the ThemeContext and dynamically style it based on the current theme. The steps are identical to those performed for the Navbar component:

1. Set contextType: Add static contextType = ThemeContext; to the BookList class component.
2. Destructure Context Data: In the render method, destructure isLightTheme, light, and dark from this.context.
3. Determine Theme: Use a ternary operator to determine the appropriate theme (light or dark) based on isLightTheme.
4. Apply Styles: Use the determined theme to dynamically style the BookList container and individual list items (li tags).

class BookList extends Component {
  static contextType = ThemeContext;

  render() {
    const { isLightTheme, light, dark } = this.context;
    const theme = isLightTheme ? light : dark;

    return (
      <div className="book-list" style={{ color: theme.syntax, background: theme.bg }}>
        <ul>
          <li style={{ background: theme.ui }}>Book title 1</li>
          <li style={{ background: theme.ui }}>Book title 2</li>
          <li style={{ background: theme.ui }}>Book title 3</li>
        </ul>
      </div>
    );
  }
}

Dynamic Theme Switching and React DevTools

By modifying the isLightTheme value within the ThemeContextProvider’s state, we can dynamically switch between light and dark themes. This change is reflected in both the Navbar and BookList components because they are consuming the same context and re-rendering whenever the context value updates.

React DevTools: A browser extension for Chrome and Firefox that provides debugging and inspection tools specifically for React applications. It allows developers to examine the component tree, props, state, context, and performance of their React applications.

Using React DevTools, you can inspect the ThemeContextProvider component and directly manipulate the isLightTheme property within its state.

State: In React, state is a JavaScript object that holds data that may change over time. State is local to a component and controls the component’s behavior and rendering. When state changes, React re-renders the component and its children.

By toggling the isLightTheme property in React DevTools, you can observe the theme switching dynamically in the application, demonstrating the effectiveness of context consumption using contextType.

Conclusion

Using contextType provides a clean and efficient way for class components to consume context data in React. It simplifies accessing shared values and allows for dynamic updates across components that are part of the context’s provider tree. This method is particularly useful for managing application-wide themes, user settings, or other global data that needs to be accessible in multiple components without prop drilling. In the subsequent chapter, we will explore alternative methods for consuming context, including approaches suitable for functional components.

Consuming Context in React Components: Exploring the Context Consumer

Introduction to React Context

React’s Context API provides a way to share values like themes, user authentication, or preferred language between components without explicitly passing them down through every level of the component tree via props. This is particularly useful for data that needs to be accessible by many components within an application.

Context API: A feature in React that allows you to share state and functions (collectively known as “context”) across your application without prop drilling. It provides a way to pass data through the component tree without having to pass props down manually at every level.

In previous approaches, managing global data often involved prop drilling, where props are passed down through multiple layers of components that may not actually need the data themselves, just to get it to a component deep down in the tree that does. Context provides a more elegant solution to this problem.

This chapter will explore different methods for consuming context within React components, focusing primarily on the Context Consumer.

Methods for Consuming Context in React

There are two primary ways to consume context in React components:

1. static contextType Property (for Class Components): This method, while functional, is limited to class components and only allows consuming a single context.
2. Context.Consumer Component: This approach is more flexible, working in both functional and class components, and allows for consuming multiple contexts within a single component.

Let’s delve into the Context.Consumer approach, building upon the concept of context creation and provision.

Understanding the Context Consumer

When you create a context using React.createContext(), you receive both a Provider and a Consumer component. The Provider is used to supply the context value to its descendants in the component tree. The Consumer, on the other hand, is used by components that need to access and utilize the context value.

Consumer (Context Consumer): A component provided by the Context API that allows functional or class components to subscribe to context changes. It enables components to access the current context value and re-render whenever that value changes.

Provider (Context Provider): A component provided by the Context API that makes a context value available to all descendant components, no matter how deeply nested. It is used to wrap parts of the component tree that need access to the context.

Let’s consider an example where we have a ThemeContext that manages the current theme of an application.

import React from 'react';

const ThemeContext = React.createContext('light'); // Default theme is light

function ThemedComponent() {
  // ... component logic here ...
}

export default ThemedComponent;

In this setup, ThemeContext is our created context. Now, let’s explore how to consume this ThemeContext within the ThemedComponent using the Context.Consumer.

Implementing Context Consumer in a Functional Component

To consume the ThemeContext in our ThemedComponent, we will use ThemeContext.Consumer. Instead of directly accessing context through a static property, we will wrap a portion of our JSX with the Consumer component.

JSX (JavaScript XML): A syntax extension to JavaScript that allows you to write HTML-like code within your JavaScript files. It is used in React to describe the user interface.

Here’s how to modify the ThemedComponent to use ThemeContext.Consumer:

import React from 'react';

const ThemeContext = React.createContext('light');

function ThemedComponent() {
  return (
    <ThemeContext.Consumer>
      {(theme) => (
        <div style={{ backgroundColor: theme === 'dark' ? 'black' : 'white', color: theme === 'dark' ? 'white' : 'black' }}>
          <p>The current theme is: {theme}</p>
        </div>
      )}
    </ThemeContext.Consumer>
  );
}

export default ThemedComponent;

Breakdown of the Code:

• <ThemeContext.Consumer>: We are using the Consumer component directly from the ThemeContext we created.
• Function as a Child: The Consumer component expects a function as its child. This function is often referred to as a “render prop” or “function as child”.
• Context Value as Argument: This function receives the current context value as its argument. In our example, this argument is named theme.
• Returning JSX: The function must return the JSX that you want to render. This JSX will have access to the theme value from the context.

Explanation of the Process:

1. The ThemeContext.Consumer component subscribes to changes in the ThemeContext.
2. When the context value changes (provided by a ThemeContext.Provider higher up in the component tree), React will re-render any Consumer components that are subscribed to that context.
3. During the re-render, the function passed as a child to the Consumer is executed.
4. The current context value is passed as an argument to this function.
5. The function returns JSX, which is then rendered by React.

In our example, the function receives the theme value and uses it to dynamically style a div and display the current theme in a paragraph. If the theme is ‘dark’, the background will be black and text white; otherwise (if it’s ‘light’ or the default), the background will be white and text black.

Accessing Context Data

Inside the function within the Consumer, you have direct access to the context value. You can use this value to dynamically render content, apply styles, or control component behavior, as demonstrated in the example above.

If the context value is an object, you can use destructuring to easily access specific properties from the context object.

Destructuring: A JavaScript feature that allows you to unpack values from arrays, or properties from objects, into distinct variables. It provides a concise way to extract multiple values at once.

For instance, if our ThemeContext provided an object like { theme: 'dark', fontSize: 'large' }, we could destructure it like this:

<ThemeContext.Consumer>
  {({ theme, fontSize }) => ( // Destructuring theme and fontSize from context object
    <div style={{ backgroundColor: theme === 'dark' ? 'black' : 'white', color: 'white', fontSize: fontSize }}>
      <p>The current theme is: {theme} and font size is {fontSize}</p>
    </div>
  )}
</ThemeContext.Consumer>

This demonstrates how to access and utilize multiple values provided by the context within the Consumer component.

Benefits of Using Context Consumer

The Context.Consumer approach offers several advantages:

• Works in Functional Components: Unlike the static contextType approach, Context.Consumer can be used effectively within functional components, which are increasingly favored in modern React development.

• Consuming Multiple Contexts: A significant benefit of Context.Consumer is its ability to consume multiple contexts within a single component. This is not possible with the static contextType method. To consume multiple contexts, you can nest multiple Consumer components.

  <ThemeContext.Consumer>
    {(theme) => (
      <UserContext.Consumer>
        {(user) => (
          <div>
            {/* Access theme and user context values here */}
            <p>Theme: {theme}</p>
            <p>User: {user.name}</p>
          </div>
        )}
      </UserContext.Consumer>
    )}
  </ThemeContext.Consumer>

  In this example, we are consuming both ThemeContext and UserContext within the same component by nesting the Consumer components. The inner Consumer for UserContext is placed within the JSX returned by the outer Consumer for ThemeContext.

Choosing Between contextType and Context.Consumer

While static contextType is a valid approach for class components consuming a single context, Context.Consumer is generally preferred due to its flexibility and compatibility with both functional and class components, as well as its ability to handle multiple contexts.

For modern React development, especially with the increasing use of functional components and Hooks (another way to consume context, not covered in detail here), Context.Consumer provides a robust and versatile method for accessing context values within your components.

Conclusion

The Context Consumer is a powerful tool within React’s Context API for accessing shared state and data across your application. By using Context.Consumer, you can create components that are dynamically influenced by context values, leading to more maintainable and adaptable applications. Understanding and utilizing the Context.Consumer is essential for effectively leveraging React’s Context API for state management and data sharing.

Chapter: Modifying Context Data in React Components

This chapter expands upon the concept of Context in React, building on the understanding of how components can access shared data. We will now explore the mechanism for modifying this shared data from within components that are consuming the context. This is crucial for creating dynamic and interactive applications where changes in one part of the application can trigger updates across other relevant components.

Introduction: The Need for Dynamic Context

In the previous chapter (or as demonstrated earlier in the video), we established how React’s Context API allows us to share data between components without the need for prop drilling. Components can “consume” context and access shared state. However, in real-world applications, data is rarely static. We often need to update this shared data in response to user interactions or other application events.

This chapter focuses on how to enable components to not only read data from a context but also to modify it. We will achieve this by implementing a function within the context provider that updates the shared state, and then making this function accessible to context consumers.

Creating a Theme Toggle Component

To illustrate the process of modifying context data, we will create a new React component called ThemeToggle. The purpose of this component is to provide a user interface element – a button – that allows users to switch between different application themes (e.g., light mode and dark mode).

import React, { Component } from 'react';

class ThemeToggle extends Component {
  render() {
    return (
      <button>
        Toggle the theme
      </button>
    );
  }
}

export default ThemeToggle;

This initial ThemeToggle component is a basic class component that currently renders a button with the text “Toggle the theme.” It does not yet have any functionality to actually toggle the theme. We will add this functionality by connecting it to our theme context.

Component: In React, a component is a reusable building block that represents a part of the user interface. Components can be functional or class-based and are responsible for rendering and managing their own UI and logic.

JSX (JavaScript XML): JSX is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript files. It is used in React to describe the structure of user interfaces in a declarative way. JSX is transformed into regular JavaScript function calls at runtime.

Implementing the Theme Toggling Logic in the Context Provider

To enable theme toggling, we need to modify our ThemeContext component. We will introduce a function within the ThemeContext that is responsible for updating the theme state.

Let’s assume we have a ThemeContext that currently provides a boolean value isLightTheme to determine the current theme. We want to add a function, toggleTheme, to this context. This function will reverse the value of isLightTheme, effectively switching between themes.

import React, { Component, createContext } from 'react';

export const ThemeContext = createContext();

class ThemeContextProvider extends Component {
  state = {
    isLightTheme: true,
    light: { syntax: '#555', ui: '#ddd', bg: '#eee' },
    dark: { syntax: '#ddd', ui: '#333', bg: '#555'}
  }
  toggleTheme = () => {
    this.setState({ isLightTheme: !this.state.isLightTheme });
  }
  render() {
    return (
      <ThemeContext.Provider value={{...this.state, toggleTheme: this.toggleTheme }}>
        {this.props.children}
      </ThemeContext.Provider>
    );
  }
}

export default ThemeContextProvider;

In this updated ThemeContextProvider:

1. We define a new method within the class called toggleTheme.
2. Inside toggleTheme, we use this.setState to update the component’s state.
3. We are specifically modifying isLightTheme by setting it to the opposite of its current value (!this.state.isLightTheme). This achieves the toggling effect.
4. Crucially, we now include toggleTheme as part of the value prop passed to the ThemeContext.Provider. This makes the toggleTheme function accessible to any component that consumes this context.

Context: In React, Context provides a way to share values like data, functions, or objects between components without explicitly passing them down through each level of the component tree (prop drilling). It is particularly useful for sharing global data that many components might need to access.

Shared State/Data: This refers to the data that is managed by a Context Provider and made available to all consuming components. Changes to this shared state will be reflected in all components that are subscribed to the context.

State: In React components, state is an object that holds data that can change over time. When the state of a component changes, React re-renders the component to reflect those changes in the user interface. State is managed internally within a component using this.setState in class components or useState hook in functional components.

this.setState: In React class components, this.setState() is the primary method used to update the component’s state. It triggers a re-rendering process, allowing React to efficiently update the user interface based on the new state.

Arrow Function: An arrow function is a concise syntax for writing function expressions in JavaScript. In this context, using an arrow function for toggleTheme ensures that the this keyword within the function correctly refers to the component instance, particularly when the function is passed down through props or context.

Providing the Toggle Function via Context Value

As highlighted above, the key step in making the toggleTheme function available to consumers is including it in the value prop of the ThemeContext.Provider.

<ThemeContext.Provider value={{...this.state, toggleTheme: this.toggleTheme }}>
    {this.props.children}
</ThemeContext.Provider>

Here, we are constructing the value object to include:

• ...this.state: This uses the spread syntax to include all properties from the component’s state object (i.e., isLightTheme, light, dark).
• toggleTheme: this.toggleTheme: This explicitly adds the toggleTheme function as a property named toggleTheme to the value object.

By including toggleTheme in the value, any component that consumes ThemeContext will now have access to this function in addition to the theme data itself.

Provider: In React Context API, the Provider component is responsible for making the context value available to its descendant components. It wraps a part of the component tree and dictates what value is shared within that subtree.

Props/Properties: “Props” is short for properties. In React, props are used to pass data from a parent component to a child component. They are read-only from the perspective of the child component. In the case of Context Provider, the value prop is a special prop that determines what data is shared via the context.

Consuming the Context and Utilizing the Toggle Function in ThemeToggle

Now, let’s return to our ThemeToggle component and enable it to consume the ThemeContext and utilize the toggleTheme function.

import React, { Component } from 'react';
import { ThemeContext } from '../contexts/ThemeContext';

class ThemeToggle extends Component {
  static contextType = ThemeContext;
  render() {
    const { toggleTheme } = this.context;
    return (
      <button onClick={toggleTheme}>
        Toggle the theme
      </button>
    );
  }
}

export default ThemeToggle;

In this updated ThemeToggle component:

1. We import ThemeContext from its file.
2. We use static contextType = ThemeContext; to specify that this class component wants to consume the ThemeContext. This is the standard way for class components to consume context.
3. Inside the render method, we access the context value using this.context. We then use object destructuring (const { toggleTheme } = this.context;) to extract the toggleTheme function from the context value.
4. Finally, we attach the toggleTheme function to the onClick event of the button. When the button is clicked, toggleTheme will be executed.

Consumer/Consume: In React Context API, “consuming” context refers to the process by which a component accesses the value provided by the nearest Context Provider in the component tree. Components can consume context using static contextType (for class components) or useContext hook (for functional components).

static contextType: In React class components, static contextType is a static property that you can add to a class to specify the context type that the component wants to consume. When you define static contextType = MyContext, React will make the context value available to instances of your component as this.context.

onClick event: onClick is a standard event handler in JavaScript and HTML DOM (Document Object Model). In React, you can attach functions to the onClick event of JSX elements (like buttons) to execute that function when the element is clicked.

Integration and Testing

To see the ThemeToggle component in action, we need to integrate it into our App component and ensure that the ThemeContextProvider is wrapping the relevant parts of our application.

Assuming we have an App component and a BookList component that also consumes the ThemeContext to style the book list based on the theme, the integration would look something like this:

// App.js
import React from 'react';
import BookList from './components/BookList';
import Navbar from './components/Navbar';
import ThemeToggle from './components/ThemeToggle';
import ThemeContextProvider from './contexts/ThemeContext';

function App() {
  return (
    <div className="App">
      <ThemeContextProvider>
        <Navbar />
        <BookList />
        <ThemeToggle />
      </ThemeContextProvider>
    </div>
  );
}

export default App;

By wrapping Navbar, BookList, and ThemeToggle within ThemeContextProvider, all these components now have access to the ThemeContext and can react to changes in the theme state.

When you run this application, clicking the “Toggle the theme” button in the ThemeToggle component will execute the toggleTheme function from the context provider. This will update the isLightTheme state, and because BookList and potentially other components are consuming this context, they will re-render to reflect the new theme (e.g., changing background colors, text colors, etc.).

If you encounter an error like “isLightTheme is not defined”, it is likely due to accessing isLightTheme directly instead of through the component’s state. Ensure you are using this.state.isLightTheme within the toggleTheme function in ThemeContextProvider to correctly reference the state property.

Conclusion: Dynamic Context and Function Passing

This chapter demonstrated how to modify shared context data by:

1. Defining a function within the context provider component that updates the state.
2. Passing this function as part of the value prop of the Context.Provider.
3. Consuming the context in a component and accessing the function from the context value.
4. Using the function (e.g., in an event handler) to trigger state updates in the context provider, which in turn updates all context consumers.

This pattern is fundamental for creating dynamic React applications that utilize Context API to manage and share application-wide state and behavior across components. It allows for a clean and efficient way to handle interactions that need to propagate changes throughout different parts of your application, without resorting to complex prop drilling or global state management solutions.

Chapter 2: Managing Application State with Multiple Contexts in React

Introduction to Context API and the Need for Multiple Contexts

In React applications, managing state efficiently is crucial for building complex user interfaces. The Context API provides a powerful mechanism to share data across components without the need for prop drilling, where props are passed down through multiple levels of the component tree. We previously explored how to create a single context to manage a specific type of data, such as application theming. However, as applications grow in complexity, they often require managing different categories of global data.

Consider a scenario where you need to manage both the application’s theme and user authentication status. While it might be tempting to combine all global data into a single context, this approach can lead to tightly coupled and less maintainable code. A more organized and scalable solution is to leverage multiple contexts, each dedicated to managing a specific domain of application-wide data.

Context: In React, Context provides a way to share values like data, functions, or objects between components without explicitly passing them through every level of the component tree. It’s a mechanism for global state management within a React application.

This chapter will guide you through the process of creating and integrating multiple contexts within a React application, using the example of managing both theme and authentication data.

Creating a New Context: Authentication Context

Building upon our understanding of context creation, let’s create a new context specifically for managing authentication data. We will name this context AuthContext.

Setting up AuthContext.js

To create a new context, we follow a similar pattern to creating our theme context. We begin by creating a new file, typically named after the context, for example, AuthContext.js. Within this file, we need to import necessary modules from React:

import React, { createContext, Component } from 'react';

Here, we import React itself, the createContext function, and the Component class.

Component: In React, a component is a reusable, self-contained building block that encapsulates UI and logic. Components can be functional or class-based and are used to create dynamic and interactive user interfaces.

The createContext function is essential for creating a new context object. We then export a constant variable, AuthContext, and assign it the result of calling createContext():

export const AuthContext = createContext();

This line of code creates our authentication context. Now we need to create a Provider component for this context, which will be responsible for supplying the authentication data to the components that need it.

Provider: In the React Context API, a Provider component makes context available to all its descendant components. It accepts a value prop that contains the data you want to share and wraps the components that should have access to this context.

We will create a class-based component named AuthContextProvider for this purpose. Inside this component, we will manage the authentication state.

class AuthContextProvider extends Component {
  state = {
    isAuthenticated: false
  };

  toggleAuth = () => {
    this.setState({ isAuthenticated: !this.state.isAuthenticated });
  }

  render() {
    return (
      <AuthContext.Provider value={{ ...this.state, toggleAuth: this.toggleAuth }}>
        {this.props.children}
      </AuthContext.Provider>
    );
  }
}

export default AuthContextProvider;

Let’s break down the AuthContextProvider component:

• State Initialization: We initialize the component’s state with a single boolean property, isAuthenticated, set to false by default. This represents the authentication status of the user.

  State: In React, state is a JavaScript object that represents the internal data of a component and can change over time. When the state of a component changes, React re-renders the component to reflect the updated data in the UI.

• toggleAuth Function: This function is designed to toggle the isAuthenticated state value. It uses this.setState to update the state, setting isAuthenticated to the opposite of its current value.

  Arrow Function: An arrow function in JavaScript is a concise way to write function expressions. It uses a shorter syntax compared to traditional function expressions and lexically binds this, which can be beneficial in certain contexts.

• render() Method: The render() method is responsible for returning the JSX that defines the component’s output.

  JSX: JSX (JavaScript XML) is a syntax extension for JavaScript that allows you to write HTML-like structures within your JavaScript code. It is commonly used in React to describe the structure of user interfaces and is transformed into regular JavaScript function calls at runtime.

  Inside render(), we return AuthContext.Provider. This is the Provider component associated with our AuthContext.

  • value Prop: The Provider component requires a value prop. This prop accepts the data we want to share through the context. Here, we are passing an object as the value.

    value={{ ...this.state, toggleAuth: this.toggleAuth }}

    We use the spread syntax (...this.state) to include all properties from the component’s state (isAuthenticated in this case) into the value object. We also include the toggleAuth function, making both the state and the function available to consuming components.

    Spread Syntax: The spread syntax (...) in JavaScript allows an iterable such as an array or object to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) or key-value pairs (in object literals) are expected. In React, it’s often used to pass all properties of an object as props to a component.

  • this.props.children: The AuthContext.Provider wraps {this.props.children}. This is crucial because it makes the context available to all components that are nested as children within the AuthContextProvider component in the component tree.

    Props (props.children): Props (short for properties) are read-only inputs passed down from parent components to child components in React. props.children is a special prop that represents the components or elements that are nested directly within a component’s opening and closing tags.

Finally, we export AuthContextProvider as the default export of this module, making it available for use in other parts of our application.

Integrating Multiple Context Providers

Now that we have both ThemeContextProvider and AuthContextProvider, we need to integrate them into our application so that components can access data from both contexts. We typically do this in our main App.js file or the root component of our application.

Nesting Context Providers in App.js

To make both theme and authentication contexts available to our components, we need to wrap the relevant parts of our component tree with both Providers. In App.js (or your main application component), you would import both ThemeContextProvider and AuthContextProvider.

We can choose to nest the providers. For example, we can wrap AuthContextProvider inside ThemeContextProvider, or vice versa. The order of nesting may matter depending on your specific application logic, but in many cases, it is flexible. Let’s consider nesting AuthContextProvider inside ThemeContextProvider:

import ThemeContextProvider from './contexts/ThemeContext';
import AuthContextProvider from './contexts/AuthContext';
import Navbar from './components/Navbar';
import BookList from './components/BookList';
import ThemeToggler from './components/ThemeToggler';

function App() {
  return (
    <div className="App">
      <ThemeContextProvider>
        <AuthContextProvider>
          <Navbar />
          <BookList />
          <ThemeToggler />
        </AuthContextProvider>
      </ThemeContextProvider>
    </div>
  );
}

export default App;

In this setup, both Navbar, BookList, and ThemeToggler components, and any of their child components, will have access to both the theme context provided by ThemeContextProvider and the authentication context provided by AuthContextProvider.

Alternatively, you could choose to wrap them separately at the same level:

function App() {
  return (
    <div className="App">
      <ThemeContextProvider>
        <Navbar />
        <BookList />
        <ThemeToggler />
      </ThemeContextProvider>
      <AuthContextProvider>
         {/* Components that only need AuthContext could be placed here */}
      </AuthContextProvider>
    </div>
  );
}

However, in our example, we want all these components to potentially access both contexts, so nesting as shown in the first example is a suitable approach.

Verifying Context Providers in the Browser

After integrating the context providers, it’s essential to verify that they are correctly set up and providing the intended data. A valuable tool for this is the React DevTools.

React DevTools: React DevTools is a browser extension (available for Chrome, Firefox, and Edge) that allows developers to inspect React component hierarchies, props, state, and performance. It is an essential tool for debugging and understanding React applications.

By opening React DevTools in your browser and inspecting the component tree, you can locate the ThemeContextProvider and AuthContextProvider components. Expanding these components in the DevTools will allow you to examine their value props. You should see the data and functions you are providing through each context.

For instance, in the AuthContextProvider, you should be able to see the isAuthenticated state and the toggleAuth function within the value prop. This visual confirmation in React DevTools helps ensure that your contexts are correctly providing the intended data to the wrapped components.

Conclusion and Next Steps

This chapter has demonstrated how to create and integrate multiple contexts in a React application. By creating separate contexts for different domains of application data, like theme and authentication, we achieve better organization and maintainability in our code. We successfully set up an AuthContext and wrapped our components with both ThemeContextProvider and AuthContextProvider to make both contexts accessible.

In the next chapter, we will explore how to consume multiple contexts within a single component, allowing components like our Navbar to utilize data and functions from both the theme and authentication contexts simultaneously.

Consuming Multiple Contexts in React Components

Introduction to React Context

React Context provides a way to share data across your component tree without manually passing props at every level. This is particularly useful for data that can be considered “global” for a tree of React components, such as theme settings, user authentication, or locale preferences.

In React, context is created using React.createContext. A Context object comes with a Provider component that allows consuming components to subscribe to context changes.

This chapter will explore how to consume multiple contexts within a single React component. We will examine scenarios where a component needs access to values from more than one context and demonstrate effective techniques to achieve this.

Reviewing Methods for Consuming Context

Before tackling multiple contexts, let’s briefly review the standard methods for consuming a single context in React. The transcript mentions two primary approaches: static contextType and Context.Consumer.

1. static contextType

For class components, static contextType provides a direct and concise way to access context.

Static contextType: A class property in React components that can be set to a Context object to subscribe to context changes. It allows accessing context values using this.context.

To use static contextType, you assign your Context object to the static contextType property of your class component. The context value then becomes accessible within the component instance via this.context.

Example (Conceptual - not from transcript, but for illustration):

import React, { useContext } from 'react';

const ThemeContext = React.createContext('light');

class MyComponent extends React.Component {
  static contextType = ThemeContext;

  render() {
    const theme = this.context; // Accessing context value
    return (
      <div>Current theme: {theme}</div>
    );
  }
}

2. Context.Consumer

The Context.Consumer component offers a more flexible approach and works for both functional and class components.

Consumer (Context.Consumer): A React component that subscribes to context changes. It requires a function as a child, which receives the current context value and returns a React node.

Context.Consumer requires a function as its child. This function is called with the current context value when rendering. You can then use this value to render your desired output.

Example (Conceptual - not from transcript, but for illustration):

import React, { useContext } from 'react';

const ThemeContext = React.createContext('light');

function MyComponent() {
  return (
    <ThemeContext.Consumer>
      {theme => ( // Context value passed to the function
        <div>Current theme: {theme}</div>
      )}
    </ThemeContext.Consumer>
  );
}

The Challenge: Using Multiple Contexts

The transcript highlights the challenge of consuming multiple contexts. Consider a scenario where a component needs to access both theme settings and user authentication status.

If we attempt to use static contextType for multiple contexts in a class component, we encounter an issue. We can only define one static contextType per component. Attempting to define multiple static contextType properties will lead to conflicts and is not a valid approach in React.

This limitation necessitates alternative strategies when dealing with multiple contexts.

Solution: Nesting Context.Consumer Components

The transcript focuses on a solution using nested Context.Consumer components. This approach allows a component to subscribe to multiple contexts by nesting consumers and accessing each context’s value within their respective consumer functions.

Let’s break down the example from the transcript, which demonstrates consuming both an “auth context” and a “theme context.”

Scenario: A component needs to access:

• Theme Context: Provides theme-related data (e.g., current theme, colors).
• Auth Context: Provides authentication-related data (e.g., user authentication status, toggle authentication function).

Implementation Steps (following the transcript’s example):

1. Import Contexts: Begin by importing the necessary Context objects. In the transcript’s example, these are auth context and theme context. Let’s assume these are imported as AuthContext and ThemeContext respectively.

   import { AuthContext } from './AuthContext'; // Assuming AuthContext is defined elsewhere
   import { ThemeContext } from './ThemeContext'; // Assuming ThemeContext is defined elsewhere
   import React from 'react';

2. Outer Consumer (AuthContext.Consumer): Wrap the component’s JSX with the outer AuthContext.Consumer. This consumer will provide access to the authentication context’s values.

   <AuthContext.Consumer>
     {(authContext) => ( // Function receiving auth context value
       // ... inner content ...
     )}
   </AuthContext.Consumer>

   Context object: An object provided by a React Context Provider component. It contains the data that is shared through the context. In this case, authContext is the context object provided by the AuthContext.Provider.

3. Inner Consumer (ThemeContext.Consumer): Nest the ThemeContext.Consumer inside the AuthContext.Consumer. This allows access to the theme context within the scope of the authentication context.

   <AuthContext.Consumer>
     {(authContext) => (
       <ThemeContext.Consumer>
         {(themeContext) => ( // Function receiving theme context value
           // ... component JSX accessing both authContext and themeContext ...
         )}
       </ThemeContext.Consumer>
     )}
   </AuthContext.Consumer>

4. Accessing Context Values: Within the innermost function (the one provided to ThemeContext.Consumer), both authContext and themeContext are available as function arguments. You can then access specific values from these context objects.

   In the transcript, the component destructures values from both contexts:

   <AuthContext.Consumer>
     {(authContext) => {
       const { isAuthenticated, toggleAuth } = authContext; // Destructuring auth context values
       return (
         <ThemeContext.Consumer>
           {(themeContext) => {
             const { theme } = themeContext; // Destructuring theme context value
             return (
               <div>
                 <h1>Navbar (Theme: {theme})</h1>
                 <div>
                   {/* Conditional rendering based on isAuthenticated */}
                   {isAuthenticated ? 'Logged in' : 'Logged out'}
                 </div>
                 <div onClick={toggleAuth}> {/* Using toggleAuth function */}
                   Toggle Auth
                 </div>
               </div>
             );
           }}
         </ThemeContext.Consumer>
       );
     }}
   </AuthContext.Consumer>

   Destructuring (Destructuring assignment): A JavaScript feature that allows you to unpack values from arrays or properties from objects into distinct variables. Here, it’s used to extract isAuthenticated and toggleAuth from the authContext object, and theme from the themeContext object.

5. Utilizing Context Values in JSX: The example demonstrates using the context values in the component’s JSX:

   • Displaying Theme: <h1>Navbar (Theme: {theme})</h1> displays the current theme obtained from themeContext.

   • Conditional Rendering based on Authentication: {isAuthenticated ? 'Logged in' : 'Logged out'} uses a ternary operator to conditionally render “Logged in” or “Logged out” based on the isAuthenticated value from authContext.

     Ternary operator: A shorthand way of writing conditional expressions in programming. It takes three operands: a condition followed by a question mark (?), an expression to execute if the condition is truthy, and an expression to execute if the condition is falsy, separated by a colon (:). Here, it concisely checks isAuthenticated and renders different text accordingly.

   • Triggering Context Update: <div onClick={toggleAuth}>Toggle Auth</div> attaches the toggleAuth function (from authContext) to the onClick event of a div. Clicking this div will call toggleAuth, presumably updating the authentication status in the AuthContext.Provider and causing the component to re-render with the updated context value.

Code Structure and JSX

The example code utilizes JSX to define the component’s structure.

JSX: A syntax extension to JavaScript that looks similar to HTML or XML. It is used in React to describe the structure of user interfaces and is transformed into regular JavaScript function calls. JSX allows developers to write UI structures in a declarative way, making code more readable and maintainable.

The JSX returned within the innermost consumer function defines the visual output of the component, incorporating the context values to dynamically render content.

Output and Behavior

When this component is rendered within a React application where AuthContext.Provider and ThemeContext.Provider are providing context values, it will:

• Display the current theme from the ThemeContext.
• Display “Logged in” or “Logged out” based on the isAuthenticated value from AuthContext.
• Toggle the displayed authentication status when the “Toggle Auth” div is clicked, due to the toggleAuth function from AuthContext.

The transcript mentions inspecting the React DevTools to observe context value changes, which is a valuable debugging technique when working with React Context.

Looking Ahead: React Hooks and Context API

The transcript concludes by mentioning React Hooks as a more streamlined approach to context consumption, especially when dealing with multiple contexts.

React Hooks: Functions that let you “hook into” React state and lifecycle features from within functional components. They were introduced in React 16.8 and provide a way to reuse stateful logic without class components. Hooks like useContext offer a more direct and often cleaner way to access context values in functional components compared to Context.Consumer.

Context API: A React feature that provides a way to share values like themes, user authentication, or preferred language between components without explicitly passing props through every level of the component tree. The Context API is the underlying mechanism for both Context.Consumer and hooks like useContext.

Using the useContext hook, introduced in React 16.8, simplifies context consumption in functional components, often making the code more readable and less nested compared to using Context.Consumer for multiple contexts. This will be explored in more detail in subsequent learning materials.

Conclusion

Consuming multiple contexts in React components is a common requirement in complex applications. While static contextType is limited to a single context, nesting Context.Consumer components provides a viable solution to access values from multiple contexts. This method involves nesting consumers and accessing context values through function arguments within each consumer. While functional, React Hooks, specifically useContext, offer a more modern and often simplified approach to achieve the same outcome, which will be a topic for further exploration.

Introduction to React Hooks: Expanding Functionality in Functional Components

This chapter introduces React Hooks, a significant addition to the React library that has revolutionized how developers write React components. We will explore what React Hooks are, why they were introduced, and how they empower functional components with capabilities previously exclusive to class components.

Understanding the Context Before Hooks

Before diving into Hooks, it’s important to acknowledge a concept we’ve previously discussed: the Context API.

The Context API in React provides a way to share values like themes, user authentication, or preferred language globally across a component tree without explicitly passing props through every level. It simplifies state management for components that need access to the same data.

Now, we shift our focus from the Context API to another crucial feature in React: React Hooks.

The Advent of React Hooks

React Hooks are a relatively new feature in the React library and have generated considerable excitement within the React community. They offer a powerful and elegant way to enhance functional components.

What are React Hooks?

At their core, React Hooks are special JavaScript functions. They unlock the ability to use state and other React features within functional components. Prior to Hooks, these functionalities were primarily limited to class components.

React Hooks are functions that let you “hook into” React state and lifecycle features from within functional components. They do not work inside class components — they let you use React without classes.

For instance, managing component state, which is data that can change over time and affect the component’s rendering, was traditionally done within class components. However, with Hooks, functional components can now also manage their own state.

State in React refers to data that is local to a component and can change over time. When state changes, React re-renders the component to reflect the updated data in the user interface.

This capability means that developers are no longer forced to use class components solely for state management. Functional components, often considered more concise and easier to understand, can now handle state and other complex logic.

Benefits of Using Hooks

The introduction of Hooks offers several advantages:

• Functional Components with State: Hooks empower functional components to manage state, making them more versatile.
• Improved Readability and Reusability: Functional components are often perceived as easier to read and can be more readily reused across applications.
• Simplified Component Logic: Hooks can help organize component logic into smaller, reusable functions, leading to cleaner and more maintainable code.

Class Components: Still Relevant

It’s important to emphasize that class components are not being deprecated or becoming obsolete. They remain a significant part of existing React codebases and continue to be fully supported. Hooks provide an alternative approach, not a replacement, and developers can choose the style that best suits their needs and project requirements.

Types of React Hooks

React offers a variety of built-in Hooks, each designed for specific purposes. All Hooks are essentially functions that tap into React’s core functionalities. We will explore several key Hooks in detail throughout this learning journey.

Here are a few important Hooks we will be examining:

• useState Hook: This hook allows functional components to manage their own state.

• useEffect Hook: This hook enables functional components to perform side effects, such as data fetching or DOM manipulations, after every render or re-render.

  Render in React refers to the process of React creating the user interface based on the component’s code and data. It involves creating a virtual DOM representation of the UI.

  Re-render occurs when a component’s state or props change, causing React to update the user interface to reflect these changes.

• useContext Hook: This hook allows functional components to easily consume context values, integrating seamlessly with the Context API.

We will delve into each of these Hooks and more as we progress. To begin our exploration of Hooks practically, we will set up a new React application.

Setting Up a New React Application for Hook Exploration

To effectively learn and experiment with React Hooks, we will create a brand new React application. This fresh project will provide a clean environment, free from any existing code related to the Context API or other features, ensuring a focused learning experience.

Creating a New React Project

We will use create-react-app, a popular and officially supported command-line tool for quickly setting up React projects.

create-react-app is a command-line interface (CLI) tool that simplifies the process of creating new React projects. It sets up a basic project structure with all the necessary configurations and dependencies, allowing developers to start building React applications quickly.

To create a new application, open your terminal or command prompt, navigate to the desired directory where you want to store your projects, and execute the following command:

npx create-react-app hooks-app

A directory is a container in a computer’s file system that organizes files and other directories. It is also commonly referred to as a folder.

This command will create a new React project named “hooks-app” within a folder of the same name. Once the process is complete, navigate into the newly created project directory:

cd hooks-app

Cleaning Up the Project

After creating the project, we will perform some initial cleanup to streamline our learning environment. We will remove some default files that are not essential for our current focus on Hooks.

1. Remove Unnecessary CSS and Logo: In the src directory, delete App.css and logo.svg files.

2. Modify App.js: Open src/App.js in your code editor. Remove the import statement for logo and App.css at the top of the file. Also, delete the content within the div element with the className="App" and replace it with a simple empty element or comment.

   import React from 'react';
   // import logo from './logo.svg'; // REMOVE THIS LINE
   // import './App.css'; // REMOVE THIS LINE
   
   function App() {
     return (
       <div>
         {/* Your components will go here */}
       </div>
     );
   }
   
   export default App;

Creating a Components Folder and a First Component

To organize our project, we will create a components folder within the src directory to house our React components.

1. Create components Folder: In the src directory, create a new folder named components.

2. Create SongList Component: Inside the components folder, create a new file named SongList.js. This file will contain our first functional component.

   A component in React is a reusable, self-contained building block that represents a part of the user interface. Components can be functional (using functions) or class-based (using classes) and are responsible for rendering and managing their own UI elements and logic.

Writing the SongList Component

Open SongList.js and add the following code. This creates a simple functional component called SongList that displays a list of songs.

import React from 'react';

const SongList = () => {
  return (
    <div className="song-list">
      <ul>
        <li>This Wild Darkness</li>
        <li>Memory Gospel</li>
      </ul>
    </div>
  );
};

export default SongList;

This code snippet demonstrates the basic structure of a functional component.

• Import React: We begin by importing the React module.

  In JavaScript, a module is a self-contained unit of code that can be reused in different parts of a program or in other programs. Modules help organize code and prevent naming conflicts.

• Functional Component Definition: const SongList = () => { ... }; defines a functional component named SongList.

• JSX Template: The code within the return() statement is JSX (JavaScript XML), a syntax extension that allows writing HTML-like structures within JavaScript code.

  JSX (JavaScript XML) is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript files. It is used in React to describe the user interface of components in a declarative way. JSX code is transformed into regular JavaScript function calls at build time.

• Component Structure: The component returns a div with a class name “song-list” containing an unordered list (ul) of songs.

• Export Component: export default SongList; makes the SongList component available for use in other modules or components.

  Export in JavaScript modules makes functions, objects, or primitive values available for use in other modules. It is a way to share code between different parts of an application.

Integrating SongList into App.js

To display the SongList component in our application, we need to import and use it within App.js.

Import in JavaScript modules is the process of bringing exported functions, objects, or primitive values from one module into another module so they can be used in the current module’s code.

Open App.js and modify it as follows:

import React from 'react';
import SongList from './components/SongList'; // IMPORT SongList

function App() {
  return (
    <div>
      <SongList /> {/* USE SongList COMPONENT */}
    </div>
  );
}

export default App;

We have now imported the SongList component from its file and used it within the App component by adding the <SongList /> tag.

Starting the Development Server

To view our application in the browser, we need to start the development server.

A development server is a local server that runs during the development process of a web application. It typically provides features like hot reloading, which automatically updates the browser when code changes are saved, making development faster and more efficient.

In your terminal, within the hooks-app directory, run the command:

npm start

npm (Node Package Manager) is a package manager for JavaScript. The start command, as defined in the project’s package.json file, typically initiates the development server.

NPM (Node Package Manager) is a package manager for JavaScript and the world’s largest software registry. It is used to install, manage, and share packages of JavaScript code, including libraries and tools.

After running npm start, your default web browser should open automatically and display the React application, showing the “SongList” component with the list of songs. You might see the application running on http://localhost:3000 or a similar address.

With our basic React application set up, we are now ready to begin exploring our first React Hook in the next chapter.

Using State in Functional Components with React Hooks

Introduction to State Management in React

In React development, managing state is crucial for building dynamic and interactive user interfaces. State represents the data that an application needs to remember and respond to changes. Traditionally, state management in React was primarily handled within class components. However, with the introduction of React Hooks, functional components gained the ability to manage state as well. This section will explore how to use state in functional components using the useState Hook.

Functional Components vs. Class Components

Historically, React components were broadly categorized into two types: functional components and class components.

• Functional Components: These are simpler JavaScript functions that accept props (properties) as arguments and return React elements describing what should be rendered on the screen. Initially, functional components were primarily used for presentational purposes and lacked the ability to manage their own state or lifecycle methods.
• Class Components: These are ES6 classes that extend React.Component. They have access to features like state and lifecycle methods, making them suitable for more complex components that require internal data management and component lifecycle control.

Functional Component: A JavaScript function in React that accepts props as an argument and returns JSX (or null). Functional components are often used for simpler UI elements and are now capable of managing state and lifecycle features through React Hooks.

Class Component: A React component defined as an ES6 class that extends React.Component. Class components have access to state, lifecycle methods, and other features not originally available in functional components.

Before React Hooks, managing state required using class components. However, React Hooks, introduced in React 16.8, revolutionized functional components by enabling them to utilize state and other React features previously exclusive to class components.

Introducing the useState Hook

The useState Hook is a fundamental React Hook that allows functional components to have state. It is imported from the React library and used within a functional component to declare and manage state variables.

React Hooks: Functions that let you “hook into” React state and lifecycle features from within functional components. Hooks do not work inside classes.

useState Hook: A React Hook that enables functional components to declare and manage state variables. It returns an array with two elements: the current state value and a function to update it.

Importing and Using useState

To use the useState Hook, you must first import it from the react library:

import React, { useState } from 'react';

Once imported, you can invoke the useState Hook inside your functional component. The useState Hook accepts one argument: the initial value of the state. This initial value can be of any JavaScript data type, such as a number, string, boolean, object, or array.

const [songs, setSongs] = useState([]);

In this example, useState([]) is called with an empty array [] as the initial value. The useState Hook returns an array containing two elements, which are then array destructured into songs and setSongs.

Array Destructuring: A JavaScript feature that allows you to unpack values from arrays, or properties from objects, into distinct variables.

• songs: This is the current state value. In this case, songs will initially be an empty array as defined by the initial value passed to useState. You can access and read the state value through this variable.
• setSongs: This is a function that is used to update the songs state variable. It allows you to modify the state and trigger a re-render of the component whenever the state changes. React automatically handles re-rendering the component when the state is updated using the setSongs function.

Initializing State with Data

Let’s initialize the songs state with an array of song objects. Each song object will have a title and a unique id.

const [songs, setSongs] = useState([
  { title: 'Song Title 1', id: '1' },
  { title: 'Song Title 2', id: '2' },
  { title: 'Song Title 3', id: '3' }
]);

This sets the initial state of songs to an array containing three song objects. Now, the functional component can access and render this list of songs dynamically.

Dynamically Rendering State in JSX

To display the list of songs in the user interface, we can use JSX (JavaScript XML) and the map function.

JSX (JavaScript XML): A syntax extension to JavaScript that looks similar to HTML. It is used in React to describe the user interface and is transformed into regular JavaScript function calls at runtime.

The map function is a JavaScript array method that iterates over each item in an array and applies a provided function to each item, returning a new array of results. In this context, we use map to transform the songs array into an array of JSX list items (<li>) to be rendered in the component’s template.

<ul>
  {songs.map((song) => (
    <li key={song.id}>
      {song.title}
    </li>
  ))}
</ul>

• songs.map((song) => ...): This iterates over each song object in the songs array. For each song, the provided function is executed.
• <li key={song.id}>: For each song, an <li> (list item) element is created. The key prop is crucial in React when rendering lists of components. It helps React identify which items have changed, are added, or are removed. It should be a unique and stable identifier for each item in the list. In this case, we use song.id as the key, assuming each song object has a unique id property.

key property: A special attribute in React that should be used when creating lists of elements. Keys help React identify which items have changed, are added, or removed in a list. Keys should be unique and stable identifiers.

• {song.title}: Inside each <li> element, we render the title property of the current song object.

This code snippet dynamically renders a list of songs based on the current state stored in the songs variable.

Updating State in Functional Components

To make the application interactive, we need to be able to update the state. The setSongs function, obtained from the useState Hook, is used to update the songs state. Let’s create a button that, when clicked, adds a new song to the list.

Adding an “Add Song” Button

First, add a <button> element within the JSX:

<button onClick={addSong}>Add a song</button>

• onClick={addSong}: This sets up an event handler for the button’s click event. When the button is clicked, the addSong function will be executed.

Event Handler: A function that is executed when a specific event occurs, such as a user clicking a button or typing in an input field. In React, event handlers are often used to update the component’s state in response to user interactions.

Creating the addSong Function

Now, define the addSong function that will update the songs state:

const addSong = () => {
  setSongs([...songs, { title: 'New song', id: '4' }]);
};

• const addSong = () => { ... }: This defines a new function called addSong.

• setSongs([...songs, { title: 'New song', id: '4' }]): Inside the addSong function, we use the setSongs function to update the state.

  • [...songs]: This uses the spread syntax to create a new array containing all the existing songs from the current songs state.

  Spread Syntax: A JavaScript feature that allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected. In this context, it’s used to create a new array containing all elements of the original songs array.

  • { title: 'New song', id: '4' }: This creates a new song object with the title “New song” and a hardcoded id of ‘4’.
  • By combining the spread syntax and the new song object within an array, we are creating a brand new array that includes all the old songs and the new song. The setSongs function then updates the songs state with this new array.

Important Note on State Updates: When updating state in React, especially when dealing with arrays or objects, it is crucial to create a new copy of the state rather than modifying the existing state directly. React relies on detecting changes in the object reference to trigger re-renders efficiently. By using the spread syntax, we ensure that setSongs receives a new array, triggering a state update and component re-render.

Addressing Unique Keys and the UUID Package

While the “Add Song” button functionality works, there’s a potential issue with the id property. Currently, the id for new songs is hardcoded as ‘4’. If we keep adding songs, all new songs will have the same id, which violates React’s requirement for unique keys in lists and can lead to errors or unexpected behavior.

To generate unique IDs for each new song, we can use a library called uuid.

UUID (Universally Unique Identifier): A 128-bit number used to uniquely identify information in computer systems. UUIDs are designed to be unique across space and time.

Installing the uuid Package

To use uuid, you need to install it using a package manager like npm (Node Package Manager):

npm install uuid

npm (Node Package Manager): A package manager for JavaScript, primarily used for managing dependencies in Node.js projects. It allows developers to easily install, update, and uninstall packages and libraries.

After installation, import the uuid function into your component:

import { v1 as uuidv1 } from 'uuid';

Here, we are importing version 1 of the UUID generator and aliasing it as uuidv1.

Using UUID to Generate Unique IDs

Now, modify the addSong function to use uuidv1 to generate a unique ID for each new song:

const addSong = () => {
  setSongs([...songs, { title: 'New song', id: uuidv1() }]);
};

• id: uuidv1(): Instead of hardcoding ‘4’, we now call uuidv1() to generate a unique ID every time a new song is added.

With this change, each new song will have a unique and dynamically generated ID, resolving the key issue and ensuring that React can efficiently manage the list of songs.

Conclusion

The useState Hook is a powerful tool that enables functional components to manage state effectively. By using useState, functional components can become dynamic and interactive, just like class components, but with a more concise and readable syntax. Understanding how to initialize, access, and update state using useState is fundamental for building modern React applications. This chapter has demonstrated the basic usage of useState for managing a list of songs, including adding new items and ensuring unique keys for efficient rendering. This fundamental concept forms the basis for more complex state management patterns in React applications.

Working with Forms and useState Hook in React

This chapter explores how to manage form inputs in React using the useState hook. We will learn how to capture user input, store it in the component’s state, and update the user interface in real-time. This is a fundamental skill for building interactive web applications with React.

Introduction to Form Handling with useState

In React applications, forms are a crucial element for user interaction. When a user types into an input field, we often need to track and store that input. React’s useState hook provides an elegant way to manage this process. Instead of directly manipulating the Document Object Model (DOM), we leverage React’s state management to handle form inputs declaratively.

We will demonstrate this by building a simple form that allows users to add new songs to a list. This form will consist of a single input field for the song title and a submit button.

Creating a New Song Form Component

To encapsulate the form logic, we will create a new React component called NewSongForm. This component will be responsible for rendering the form and managing its internal state.

1. Component Setup: Create a new file named NewSongForm.js.

2. Import Statements: Begin by importing React and the useState hook from the React library. These imports are essential for creating React components and utilizing state management.

   import React from 'react';
   import { useState } from 'react';

   Hook: In React, a Hook is a special function that lets you “hook into” React state and lifecycle features from within functional components. They were introduced to make state and other React features accessible in functional components, which were traditionally limited to presentational logic.

   Component: A component is a reusable, self-contained building block in React that encapsulates UI (User Interface) and logic. Components can be functional or class-based and are used to create complex UIs by composing smaller, independent parts.

3. Stateless Functional Component Structure: Define a stateless functional component named NewSongForm. Although we will be using state within this component, the term “stateless functional component” is often used as a starting point for functional components in React. We will use useState to make it stateful.

   const NewSongForm = () => {
       return (
           // Form JSX will go here
           <div>
               {/* Form elements */}
           </div>
       );
   }
   
   export default NewSongForm;

   Stateless Functional Component: A functional component in React that does not manage its own state. It primarily receives data through props and renders UI based on those props. With Hooks, functional components can now manage state, blurring the lines of the “stateless” definition, but the term is still often used for functional components.

   JSX (JavaScript XML): A syntax extension to JavaScript that looks similar to HTML. JSX is used in React to describe the structure of the user interface. It gets transformed into regular JavaScript function calls that create React elements, which then describe what should be rendered to the DOM.

4. Form Template (JSX): Inside the return statement, construct the basic form structure using JSX. This will include:

   • A <form> element to wrap the form inputs.
   • A <label> element to provide a descriptive label for the input field (e.g., “Song Name”).
   • An <input type="text"> element for users to type in the song title. We will set the type to “text” for a standard text input. The required attribute ensures that the form cannot be submitted without this field being filled.
   • An <input type="submit"> element to create a submit button. The value attribute sets the text displayed on the button (e.g., “Add Song”).

   const NewSongForm = () => {
       return (
           <form>
               <label>Song name:</label>
               <input type="text" required />
               <input type="submit" value="add song" />
           </form>
       );
   }
   
   export default NewSongForm;

Integrating the New Song Form into the Song List Component

Now, let’s integrate the NewSongForm component into an existing SongList component. Assume we have a SongList component where we want to add this form.

1. Import NewSongForm: In your SongList component file (e.g., SongList.js), import the NewSongForm component.

   import NewSongForm from './NewSongForm'; // Adjust path as needed
   // ... rest of SongList component

2. Render NewSongForm: Place the <NewSongForm /> component within the SongList component’s JSX, wherever you want the form to appear. For example, you might replace an existing button with the form.

   const SongList = () => {
       // ... song list logic
       return (
           <div className="song-list">
               {/* ... song list display */}
               <NewSongForm /> {/* Render the NewSongForm component */}
           </div>
       );
   }

Handling Form Events and State with useState

Currently, the form is displayed, but it doesn’t yet interact with React’s state. We need to add event listeners to the form and input field to capture user input and update the component’s state using the useState hook.

1. onSubmit Event Handler for the Form: To handle form submission, we’ll attach an onSubmit event listener to the <form> element. This event is triggered when the user submits the form (e.g., by pressing Enter or clicking the submit button).

   <form onSubmit={handleSubmit}> {/* handleSubmit function will be defined later */}
       {/* ... form elements */}
   </form>

   Event Listener: In web development, an event listener is a procedure or function in JavaScript that waits for a specific event to occur (like a click, form submission, or key press) and then executes code in response to that event. React uses synthetic events which are a wrapper around native browser events.

2. onChange Event Handler for the Input Field: To track changes in the input field as the user types, we’ll use the onChange event listener on the <input type="text"> element. This event fires every time the value of the input field changes (after each keystroke).

   <input type="text" required onChange={handleTitleChange} /> {/* handleTitleChange function will be defined later */}

3. Using useState to Manage Input Value: We need to store the song title in the component’s state. Inside the NewSongForm component, call the useState hook.

   const NewSongForm = () => {
       const [title, setTitle] = useState(''); // Initialize state with an empty string
   
       // ... rest of component
   };

   State: In React, state is a plain JavaScript object that represents the internal data of a component. State is dynamic and can change over time, triggering re-renders of the component and its children when it’s updated. useState is a Hook that allows functional components to have state.

   The useState('') call does the following:

   • It declares a state variable named title.
   • It initializes title with an initial value of an empty string (''). This means when the component first renders, the input field will be empty.
   • It returns an array containing two elements:
     • The current value of the state variable (title).
     • A function to update the state variable (setTitle).

4. handleTitleChange Function: Create a function called handleTitleChange. This function will be triggered by the onChange event. It will receive an event object. Inside this function, we’ll use setTitle to update the title state with the current value from the input field.

   const NewSongForm = () => {
       const [title, setTitle] = useState('');
   
       const handleTitleChange = (e) => {
           setTitle(e.target.value); // Update title state with input value
       };
   
       return (
           <form onSubmit={handleSubmit}>
               <label>Song name:</label>
               <input type="text" required onChange={handleTitleChange} />
               <input type="submit" value="add song" />
           </form>
       );
   };

   Event Object: When an event occurs in the browser (like a click or a change in an input field), JavaScript creates an event object. This object contains information about the event that occurred, such as the target element that triggered the event, the type of event, and any other relevant data. In React event handlers, you receive a synthetic event object which is a cross-browser wrapper around the native event.

   Within handleTitleChange:

   • e represents the event object.
   • e.target refers to the HTML element that triggered the event (in this case, the input field).
   • e.target.value retrieves the current value of the input field.
   • setTitle(e.target.value) updates the title state with the new input value.

5. Connecting State to Input Value: To ensure the input field always reflects the current state value, we need to set the value property of the <input> element to the title state variable.

   <input type="text" required onChange={handleTitleChange} value={title} />

   By setting value={title}, we create a controlled component. The input field’s value is now directly controlled by the React state. Whenever title state changes (via setTitle), the input field’s value will also update.

6. handleSubmit Function: Create the handleSubmit function. This function will be triggered when the form is submitted via the onSubmit event.

   const NewSongForm = () => {
       const [title, setTitle] = useState('');
   
       const handleTitleChange = (e) => {
           setTitle(e.target.value);
       };
   
       const handleSubmit = (e) => {
           e.preventDefault(); // Prevent default form submission behavior
           console.log('Song title submitted:', title); // For now, just log the title
           // In a real application, you would add logic to add the song to a list
       };
   
       return (
           <form onSubmit={handleSubmit}>
               <label>Song name:</label>
               <input type="text" required onChange={handleTitleChange} value={title} />
               <input type="submit" value="add song" />
           </form>
       );
   };

   Inside handleSubmit:

   • e.preventDefault(): This line is crucial in React forms. It prevents the default HTML form submission behavior, which would cause a page refresh. In single-page applications (SPAs) built with React, we typically want to handle form submissions using JavaScript without page reloads.
   • console.log('Song title submitted:', title);: For now, we are simply logging the submitted song title to the console to verify that the form submission is working and capturing the correct title from the state. In a real application, this is where you would add logic to add the song to a list, send data to a server, or perform other actions.

Passing Data Back to the Parent Component (SongList)

To make the form useful, we need to pass the newly entered song title back to the parent SongList component so it can be added to the list of songs. We can achieve this using props.

1. Pass addSong Function as a Prop: In the SongList component, when rendering NewSongForm, pass a function called addSong as a prop to NewSongForm. Assume SongList has a function named addSong that handles adding new songs to its internal song list state.

   const SongList = () => {
       const [songs, setSongs] = useState([ /* ... initial songs ... */ ]);
   
       const addSong = (title) => {
           setSongs([...songs, { title: title, id: uuidv4() }]); // Example addSong function
       };
   
       return (
           <div className="song-list">
               {/* ... song list display */}
               <NewSongForm addSong={addSong} /> {/* Pass addSong as a prop */}
           </div>
       );
   };

   Prop (Properties): Props are inputs to React components. They are data passed down from parent components to child components. Props are read-only from the perspective of the child component and are used to customize and configure components.

2. Receive and Call addSong Prop in NewSongForm: In NewSongForm, receive the addSong prop. Then, in the handleSubmit function, instead of just logging the title, call the addSong function, passing the title state as an argument.

   const NewSongForm = ({ addSong }) => { // Destructure addSong prop
       const [title, setTitle] = useState('');
   
       const handleTitleChange = (e) => {
           setTitle(e.target.value);
       };
   
       const handleSubmit = (e) => {
           e.preventDefault();
           addSong(title); // Call the addSong prop, passing the title
           setTitle(''); // Clear the input field after submission
       };
   
       return (
           <form onSubmit={handleSubmit}>
               <label>Song name:</label>
               <input type="text" required onChange={handleTitleChange} value={title} />
               <input type="submit" value="add song" />
           </form>
       );
   };

   ES6 Shorthand: A feature in ECMAScript 6 (ES6), also known as ECMAScript 2015, that provides concise syntax for object literals and function parameters. In this context, when passing props, if the prop name and the variable name are the same, you can use shorthand like addSong instead of addSong={addSong}. Similarly, in object literals, if the property name and the variable holding the value are the same, you can use shorthand like { title } instead of { title: title }.

   In NewSongForm:

   • const NewSongForm = ({ addSong }) => { ... } uses object destructuring to directly extract the addSong prop from the props object passed to the component.
   • addSong(title) calls the addSong function that was passed as a prop from the parent component, sending the current title state value to it.

Clearing the Input Field After Submission

For a better user experience, it’s common to clear the input field after the form is successfully submitted. We can easily do this by resetting the title state back to an empty string within the handleSubmit function, after calling addSong.

const handleSubmit = (e) => {
    e.preventDefault();
    addSong(title);
    setTitle(''); // Reset title state to empty string, clearing the input field
};

Because we have bound the input field’s value to the title state using value={title}, when we call setTitle(''), React re-renders the component, and the input field’s value is updated to reflect the new empty string state, effectively clearing the input field.

Conclusion

This chapter demonstrated how to use the useState hook to effectively manage form inputs in React. We covered:

• Creating a form component (NewSongForm).
• Using useState to store and update input values.
• Handling onChange events to capture user input in real-time.
• Handling onSubmit events to process form submissions.
• Passing data from child to parent components using props.
• Clearing input fields after form submission for improved user experience.

By understanding these concepts, you can build interactive forms in your React applications, enabling users to input data and interact with your application effectively. The useState hook is a powerful tool for managing local component state, making form handling in React both efficient and straightforward.

Understanding the useEffect Hook in React

This chapter delves into the useEffect Hook, a fundamental tool in React for managing side effects in functional components. Building upon the concept of the useState Hook, useEffect allows developers to perform actions that go beyond simply rendering UI, such as data fetching, subscriptions, or manually changing the DOM.

Introduction to useEffect

After exploring the useState Hook, which enables state management in functional components, the useEffect Hook is introduced as the next essential tool in React development.

useEffect as a Functional Alternative to Lifecycle Methods

In React class components, lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount are used to handle side effects at different stages of a component’s lifecycle. However, functional components, by design, do not have direct access to these lifecycle methods.

Lifecycle Method: In React class components, lifecycle methods are special methods that allow you to run code at specific points in a component’s life, such as when it’s initially rendered, updated, or about to be removed from the DOM.

To bridge this gap and enable side effects in functional components, React provides the useEffect Hook. Think of useEffect as a functional equivalent to lifecycle methods, allowing you to “hook into” the component’s lifecycle within a functional component.

Hook: In React, a Hook is a special function that lets you “hook into” React state and lifecycle features from within functional components. Hooks do not work inside class components — they let you use React without classes.

Basic Syntax and Behavior of useEffect

The useEffect Hook follows a naming convention common to all React Hooks: useSomething. It is a function that you import from React and call within your functional component.

import React, { useState, useEffect } from 'react';

The basic structure of the useEffect Hook is as follows:

useEffect(() => {
  // Your effect logic here
});

useEffect accepts two arguments:

1. A Callback Function (Effect Function): This function contains the code you want to run as a side effect. This function will be executed after every render of the component, including the initial render.
2. An Optional Dependency Array: This is an array of values that useEffect depends on. We will explore this in detail later.

Basic Usage: Executing Effects on Every Render

By default, when you use useEffect with only the callback function, the effect will run after every render and re-render of the component, as well as on the initial render. This means that whenever the component’s data changes, causing it to re-render, the code within the useEffect callback function will be executed.

Render/Re-render: In React, rendering is the process of creating or updating the user interface based on the component’s current state and props. A re-render occurs when a component’s state or props change, causing React to update the UI to reflect these changes.

Consider the following example, which demonstrates this behavior by logging a message and the current songs state to the console whenever the component renders:

import React, { useState, useEffect } from 'react';

function SongList() {
  const [songs, setSongs] = useState(['Song 1', 'Song 2']);

  useEffect(() => {
    console.log('useEffect Hook run');
    console.log('Current Songs:', songs);
  });

  const addSong = () => {
    setSongs([...songs, `Song ${songs.length + 1}`]);
  };

  return (
    <div>
      <ul>
        {songs.map((song, index) => (
          <li key={index}>{song}</li>
        ))}
      </ul>
      <button onClick={addSong}>Add Song</button>
    </div>
  );
}

export default SongList;

In this example:

• useState is used to manage a list of songs.
• useEffect is used with a callback function that logs a message and the songs array to the console.

When this component initially renders, and every time the songs state is updated (e.g., when you click “Add Song”), you will see “useEffect Hook run” and the updated list of songs logged in the browser’s console. This demonstrates that the effect function runs on the initial render and on every subsequent re-render caused by changes in the component’s data.

This behavior is useful for performing actions that need to occur whenever the component’s UI is updated, such as:

• Data Fetching: Making requests to fetch data from an API when the component loads or when certain data changes.
• Setting up Subscriptions: Subscribing to external data sources or events when the component is mounted and unsubscribing when it unmounts.
• Manual DOM Manipulations: Directly interacting with the Document Object Model (DOM) if necessary.
• Logging and Analytics: Tracking component renders or user interactions.

API Endpoint: An API endpoint is a specific URL (web address) that a server exposes to allow client applications to interact with its data and functionalities. It’s like a doorway to access information or services provided by a server.

Controlling useEffect Execution with the Dependency Array

While running an effect on every render is sometimes desired, there are scenarios where you need more control over when the effect function executes. For instance, you might only want to run an effect when a specific piece of data changes, rather than on every render caused by any data change in the component.

This is where the dependency array, the second optional argument to useEffect, comes into play. By providing an array of values as the second argument, you instruct useEffect to only run the effect function when one of the values in this array has changed since the last render.

Let’s extend the previous example to include an age state and demonstrate how the dependency array can be used to control effect execution based on specific state variables.

import React, { useState, useEffect } from 'react';

function SongList() {
  const [songs, setSongs] = useState(['Song 1', 'Song 2']);
  const [age, setAge] = useState(20);

  useEffect(() => {
    console.log('useEffect for songs run');
    console.log('Current Songs:', songs);
  }, [songs]); // Dependency array: [songs]

  useEffect(() => {
    console.log('useEffect for age run');
    console.log('Current Age:', age);
  }, [age]);     // Dependency array: [age]

  const addSong = () => {
    setSongs([...songs, `Song ${songs.length + 1}`]);
  };

  const incrementAge = () => {
    setAge(age + 1);
  };

  return (
    <div>
      <ul>
        {songs.map((song, index) => (
          <li key={index}>{song}</li>
        ))}
      </ul>
      <button onClick={addSong}>Add Song</button>
      <br />
      <button onClick={incrementAge}>Add 1 to Age</button>
      <span>Age: {age}</span>
    </div>
  );
}

export default SongList;

In this modified example:

• We’ve added a new state variable age using useState.
• We now have two useEffect Hooks:
  • The first useEffect has [songs] as its dependency array. This means it will only run its callback function when the songs array changes.
  • The second useEffect has [age] as its dependency array. This means it will only run its callback function when the age value changes.

Now, when you interact with the component:

• Clicking “Add Song” will only trigger the first useEffect (the one watching songs), and you will see “useEffect for songs run” in the console.
• Clicking “Add 1 to Age” will only trigger the second useEffect (the one watching age), and you will see “useEffect for age run” in the console.

This demonstrates the power of the dependency array in controlling when useEffect executes. By specifying dependencies, you can fine-tune the behavior of your effects and ensure they only run when necessary, leading to more efficient and predictable component behavior.

Using Multiple useEffect Hooks

As shown in the example above, you can use multiple useEffect Hooks within a single functional component. This is a common and recommended practice in React. It allows you to separate concerns and manage different side effects independently.

Each useEffect Hook is responsible for a specific effect and can have its own dependency array. This makes your code more organized, readable, and maintainable, as each effect is clearly defined and isolated.

In summary, the useEffect Hook is a versatile tool for managing side effects in React functional components. By understanding its basic behavior and the use of the dependency array, you can effectively control when and how your side effects are executed, creating more robust and efficient React applications.

---


# Consuming Context in React Functional Components using Hooks

## Introduction to Context API and React Hooks

React's Context API provides a powerful mechanism for sharing data across your component tree without the need for prop drilling. This is particularly useful for data that can be considered "global" to a section of your application, such as themes, user authentication, or locale settings.

> The **Context API** is a React feature that provides a way to pass data through the component tree without having to pass props down manually at every level. It is designed to share data that can be considered "global" for a tree of React components.

Traditionally, accessing context in class components was done using `contextType` or `Context.Consumer`. However, with the introduction of React Hooks, functional components gained the ability to directly access context in a cleaner and more streamlined manner.

> **React Hooks** are functions that let you "hook into" React state and lifecycle features from within functional components. They were introduced in React 16.8.

This chapter will explore how to consume context within functional components using the `useContext` hook. We will refactor a component that previously used `contextType` in a class component to a functional component leveraging `useContext`, demonstrating the simplicity and elegance of this approach.

## Understanding Context Consumption in Class Components (Brief Review)

Before diving into hooks, let's briefly revisit how context is consumed in class components. Typically, you would use `contextType` to assign a context object to a class, making the context value accessible via `this.context`.

For example, consider a scenario with a `ThemeContext` providing theme information:

```javascript
class BookList extends React.Component {
  static contextType = ThemeContext; // Assuming ThemeContext is created using React.createContext()

  render() {
    const theme = this.context; // Accessing context value
    // ... component logic using theme
  }
}

While effective, this method is specific to class components and cannot be directly used in functional components.

The Need for Context Consumption in Functional Components

Functional components are a more concise and often preferred way to build React components. Before Hooks, functional components were primarily used for presentational purposes as they lacked internal state and lifecycle methods. To consume context in functional components before Hooks, you would typically use the Context.Consumer render prop, which could sometimes lead to nested and less readable code.

Functional Component: A simpler way to define components in React, written as JavaScript functions. They can now manage state and lifecycle events using Hooks.

Introducing the useContext Hook

The useContext Hook provides a direct and elegant way for functional components to access context values. It simplifies context consumption and aligns with the functional paradigm of React development.

The useContext Hook is a React Hook that allows functional components to subscribe to React context. It accepts a context object (the value returned from React.createContext) and returns the current context value for that context.

To use useContext, you need to:

1. Import useContext from React: import { useContext } from 'react';
2. Call useContext within your functional component: Pass the context object (created using React.createContext) as an argument to useContext.
3. Access the context value: useContext returns the current context value, which you can then use within your component.

Practical Example: Refactoring to useContext

Let’s illustrate the use of useContext by refactoring a hypothetical BookList component. Imagine this component previously consumed theme context using class component syntax. We will transform it into a functional component and use useContext to achieve the same functionality.

Original Class Component (Conceptual):

// Conceptual Example (Similar functionality to transcript example)
import React, { Component } from 'react';
import { ThemeContext } from './contexts/ThemeContext'; // Assuming ThemeContext is defined

class BookList extends Component {
  static contextType = ThemeContext;

  render() {
    const { isLightTheme, light, dark } = this.context;
    const theme = isLightTheme ? light : dark;
    return (
      <div style={{ background: theme.bg, color: theme.text }}>
        {/* Book list content */}
      </div>
    );
  }
}

export default BookList;

Refactoring to a Functional Component with useContext:

1. Import useContext:

   import React, { useContext } from 'react';

2. Convert to a Functional Component: Create a functional component named BookList.

   const BookList = () => {
       // ... component logic will go here
       return (
           // ... JSX to be returned
       );
   }

   JSX is a syntax extension to JavaScript that looks similar to HTML. It is used in React to describe the structure of the user interface.

3. Use useContext to Consume Context: Inside the functional component, call useContext and pass in the ThemeContext. This will return the current value of the ThemeContext.

   const BookList = () => {
       const context = useContext(ThemeContext); // Assuming ThemeContext is imported
       // ... rest of the component logic
   }

   Context in React, context refers to the data provided by the Context API that is available to components within a certain scope. It is typically used for themes, user authentication, and other global settings.

4. Destructure Context Values: The context value is often an object containing multiple properties. We can use JavaScript destructuring to extract the specific properties we need, such as isLightTheme, light, and dark from the theme context.

   const BookList = () => {
       const { isLightTheme, light, dark } = useContext(ThemeContext);
       // ... use isLightTheme, light, dark
   }

   Destructuring is a JavaScript feature that allows you to unpack values from arrays, or properties from objects, into distinct variables.

5. Implement Component Logic and JSX: Now you can use the destructured context values to determine the theme and apply styles to your component’s JSX.

   import React, { useContext } from 'react';
   import { ThemeContext } from '../contexts/ThemeContext'; // Adjust path as needed
   
   const BookList = () => {
       const { isLightTheme, light, dark } = useContext(ThemeContext);
       const theme = isLightTheme ? light : dark;
   
       return (
           <div style={{ background: theme.bg, color: theme.text }}>
               <ul>
                   <li style={{ background: theme.ui, color: theme.syntax }}>The way of kings</li>
                   <li style={{ background: theme.ui, color: theme.syntax }}>The name of the wind</li>
                   <li style={{ background: theme.ui, color: theme.syntax }}>The final empire</li>
               </ul>
           </div>
       );
   }
   
   export default BookList;

Benefits of Using useContext:

• Cleaner Syntax: useContext provides a more direct and less verbose way to access context in functional components compared to Context.Consumer.
• Improved Readability: The code becomes easier to read and understand, especially when dealing with multiple context values.
• Functional Paradigm: It aligns perfectly with the functional programming approach encouraged by React Hooks, making functional components more powerful and versatile.

Conclusion

The useContext hook is a fundamental tool for consuming context within React functional components. It simplifies context access, promotes cleaner code, and enhances the overall development experience. By embracing useContext, you can effectively manage and utilize context data in your functional React components, building more maintainable and scalable applications. This approach significantly improves upon older methods and provides a modern, hook-centric way to work with React’s Context API.

Utilizing the useContext Hook in React Functional Components

This chapter explores the application of the useContext hook in React for managing and consuming context within functional components. We will transition class-based components to functional components, leveraging useContext to access and utilize different contexts efficiently.

Introduction to useContext Hook

The useContext hook is a fundamental tool in React for accessing context values within functional components. It provides a cleaner and more streamlined approach compared to older methods, especially when dealing with multiple contexts.

useContext hook: A React Hook that allows functional components to subscribe to React Context. It accepts a context object and returns the current context value for that context, making it easy to access shared data throughout your application.

One of the key advantages of the useContext hook is its ability to be used multiple times within a single component, allowing consumption of various contexts as needed. This is particularly useful in complex applications that rely on different types of global state management.

Component: A reusable building block in React that represents a part of the user interface. Components can be functional or class-based and are responsible for rendering and managing their own UI and logic.

Consuming Multiple Contexts in a Functional Component: Navbar Example

Let’s consider a practical example of a Navbar component that needs to access both auth context and theme context. Initially, this Navbar was implemented as a class-based component. We will refactor it into a functional component and use the useContext hook to manage these contexts.

Navbar: Short for navigation bar, a common user interface element found in websites and applications. It typically contains links to different sections of the site or app, aiding user navigation.

auth context: A specific implementation of React Context designed to manage authentication-related data, such as user login status and user information, making it accessible across different components.

theme context: A specific implementation of React Context designed to manage the visual theme of an application, such as light or dark mode, ensuring a consistent look and feel across components.

Transitioning from Class-Based to Functional Component

To begin, we convert the existing class-based Navbar component into a functional component. This involves replacing the class definition with a function definition and using hooks instead of lifecycle methods and this.context.

functional component: A way to define React components using JavaScript functions. They are simpler and often preferred over class-based components for presentational logic and when combined with Hooks for state and lifecycle management.

class-based component: An older way to define React components using JavaScript classes, extending React.Component. While still functional, functional components with Hooks are generally favored for new React development.

Inside our new functional Navbar component, we will use the useContext hook to access the theme context first.

import React, { useContext } from 'react';

function Navbar() {
  // Component logic will be added here
  return (
    // JSX to be returned here
    <nav>
      {/* Navbar content */}
    </nav>
  );
}

export default Navbar;

JSX: A syntax extension to JavaScript that resembles HTML. It’s used in React to describe the structure of user interfaces and gets transformed into regular JavaScript function calls during compilation.

Consuming Theme Context with useContext

To consume the theme context, we import the useContext hook and the ThemeContext itself (assuming it is defined elsewhere). Inside the functional component, we call useContext(ThemeContext) and destructure the properties we need, such as isLightTheme, light, and dark themes.

import React, { useContext } from 'react';
import { ThemeContext } from './contexts/ThemeContext'; // Assuming ThemeContext is defined here

function Navbar() {
  const { isLightTheme, light, dark } = useContext(ThemeContext);
  // ... rest of component
}

destructure: A JavaScript feature that allows you to extract values from arrays or properties from objects and assign them to distinct variables in a concise way.

properties: In the context of JavaScript objects or React components, properties refer to the attributes or data associated with them. In React, props (short for properties) are used to pass data from parent components to child components.

context object: The object created by React.createContext(). It contains a Provider and a Consumer component, along with the displayName property. It’s the object passed to the useContext hook to access the context’s current value.

This line of code utilizes the useContext hook to retrieve the current value of the ThemeContext. The returned value is an object containing the theme properties, which are then destructured into isLightTheme, light, and dark variables for easy access within the component.

Consuming Auth Context with useContext

Similarly, we can consume the auth context within the same functional component. We again use the useContext hook, this time passing the AuthContext. We destructure the properties we need from the auth context, such as isAuthenticated and toggleAuth.

import React, { useContext } from 'react';
import { ThemeContext } from './contexts/ThemeContext';
import { AuthContext } from './contexts/AuthContext'; // Assuming AuthContext is defined here

function Navbar() {
  const { isLightTheme, light, dark } = useContext(ThemeContext);
  const { isAuthenticated, toggleAuth } = useContext(AuthContext);

  // ... rest of component logic and JSX
}

This demonstrates the ability to use useContext multiple times in a single functional component to consume different contexts. This approach is cleaner and more readable than older methods involving Consumer components, especially when dealing with several contexts.

consumer tags: Refers to the <ContextName.Consumer> component in React’s Context API. This is an older method (render props pattern) for subscribing to context changes, primarily used in class-based components, before the introduction of the useContext hook. It can become verbose when consuming multiple contexts.

Implementing Theme Logic and Rendering

With both contexts consumed, the component can now implement its logic based on the context values. For example, the Navbar can dynamically adjust its styling based on the theme context (light or dark theme) and display user-specific information based on the auth context (isAuthenticated status).

The rendering logic within the JSX remains largely the same as in the class-based component, but now it utilizes the variables obtained from the useContext hooks.

function Navbar() {
  const { isLightTheme, light, dark } = useContext(ThemeContext);
  const { isAuthenticated, toggleAuth } = useContext(AuthContext);
  const theme = isLightTheme ? light : dark;

  return (
    <nav style={{ background: theme.ui, color: theme.syntax }}>
      <h1>Context App</h1>
      <div>
        {isAuthenticated ? 'Logged in' : 'Logged out'}
        <button onClick={toggleAuth}>
          {isAuthenticated ? 'Logout' : 'Login'}
        </button>
      </div>
      <ul>
        <li>Home</li>
        <li>About</li>
        <li>Contact</li>
      </ul>
    </nav>
  );
}

Converting Theme Toggle Component to Functional Component

To further illustrate the use of useContext, let’s convert another class-based component, ThemeToggle, into a functional component using the useContext hook.

hook: In React, Hooks are special functions that let you “hook into” React state and lifecycle features from within functional components. They were introduced to enable state management and side effects in functional components, making them as powerful as class components in many scenarios.

We can use a shortcut, often available in code editors, to quickly generate a functional component structure. In the transcript, “SFC tap” is mentioned, likely referring to a snippet or shortcut that creates a “Stateless Functional Component” template.

SFC tap: Likely refers to a code snippet shortcut in some IDEs or text editors, often standing for “Stateless Functional Component” or “Stateless Function Component.” Typing “SFC” and then pressing Tab (or Enter) may auto-generate the basic structure of a functional component.

template: In programming, a template often refers to a pre-designed structure or format that can be used as a starting point for creating something new. In UI development, it often refers to the JSX or HTML structure that defines the layout and elements of a component.

Implementing Theme Toggle Functionality with useContext

Inside the functional ThemeToggle component, we import useContext and the ThemeContext. We then use useContext(ThemeContext) to access the toggleTheme function from the ThemeContext.

import React, { useContext } from 'react';
import { ThemeContext } from '../contexts/ThemeContext';

function ThemeToggle() {
  const { toggleTheme } = useContext(ThemeContext);

  return (
    <button onClick={toggleTheme}>Toggle Theme</button>
  );
}

export default ThemeToggle;

click event handler: A function in JavaScript that is designed to be executed when a user performs a “click” action on a specific HTML element (like a button, link, or div). It’s a fundamental part of making web pages interactive.

The toggleTheme function, obtained from the theme context via useContext, is then directly used in the onClick event handler of the button. This demonstrates how functional components, combined with useContext, provide a concise and efficient way to implement context-aware components.

Advantages of useContext in Functional Components

Using the useContext hook offers several advantages:

• Readability: Functional components with useContext are often more readable and easier to understand compared to class-based components with Consumer components, especially when dealing with multiple contexts.
• Conciseness: The code becomes less verbose, eliminating the need for nested Consumer components.
• Efficiency: useContext is designed to be efficient and performant for context consumption in functional components.
• Flexibility: You can use useContext as many times as needed within a single functional component to consume multiple different contexts.

Conclusion

The useContext hook is a powerful and convenient tool for consuming context in React functional components. It simplifies context management, enhances code readability, and promotes a more functional programming style in React development. By transitioning from class-based components and Consumer components to functional components and useContext, developers can create cleaner, more maintainable, and efficient React applications.

Understanding Context API with Functional Components in React

This chapter explores how to implement the Context API in React using functional components and hooks. Traditionally, context providers were often built using class components to manage state. However, with the introduction of hooks like useState, functional components now offer a more concise and modern approach to managing and providing context. This chapter will guide you through the process of creating a context and provider using functional components, demonstrating how to consume this context within other components.

Transitioning to Functional Components for Context Management

In previous approaches to React context, class components were frequently employed. This was primarily because class components had built-in mechanisms for managing state, which is often necessary when creating a context provider. The provider component typically needs to hold the data that will be shared via the context, and class components offered the this.state and this.setState methods for this purpose.

However, the advent of React Hooks, specifically the useState hook, has revolutionized functional components. Functional components can now manage their own state, making them equally capable of creating context providers. This shift towards functional components often leads to cleaner and more readable code. While class components remain a valid option, understanding how to implement context with functional components is a valuable skill in modern React development.

Setting up Context with Functional Components

Let’s walk through the steps of setting up a context and provider using functional components. We’ll use an example involving a list of books, where the book data will be provided through a context.

Creating the Context

The first step is to create the context itself. This is done using the createContext function from React.

import React, { createContext, useState } from 'react';

const BookContext = createContext();

createContext: This React function is used to create a context object. Context objects are used by React’s Context API to share data throughout the component tree without having to pass props manually at every level.

Here, createContext() is invoked to create a context object, which we’ve named BookContext. This object will be used to provide and consume values within our application.

Building the Context Provider Component

Next, we need to create a provider component. This component will be responsible for holding the data we want to share through the context and making it available to consuming components. We will create this provider as a functional component named BookContextProvider.

const BookContextProvider = (props) => {
  const [books, setBooks] = useState([
    { title: 'The Name of the Wind', id: 1 },
    { title: 'The Wise Man\'s Fear', id: 2 },
    { title: 'The Slow Regard of Silent Things', id: 3 },
    { title: ' двери of Stone', id: 4 },
  ]);

  return (
    <BookContext.Provider value={{ books }}>
      {props.children}
    </BookContext.Provider>
  );
};

Let’s break down this component:

• Functional Component Structure: BookContextProvider is a functional component that accepts props as an argument.

  Functional Components: In React, functional components are simpler ways to define components by writing JavaScript functions. They can now manage state and lifecycle events using Hooks, making them as powerful as class components for many use cases.

  Props: Short for “properties,” props are inputs to React components. They are read-only values passed down from parent components to child components to customize their behavior and appearance. In functional components, props are passed as the first argument to the function.

• useState Hook for State Management: We use the useState hook to define and manage the books data.

  useState: This is a React Hook that allows functional components to have state variables. It returns a pair: the current state value and a function that allows you to update it.

  const [books, setBooks] = useState([ /* initial book data */ ]);

  This line initializes a state variable called books with an array of book objects. setBooks is the function used to update the books state.

• Context Provider: The core of the provider component is the BookContext.Provider component.

  Provider: In the Context API, the Provider component makes context available to its descendant components. It accepts a value prop, which is the data you want to share.

  <BookContext.Provider value={{ books }}>
    {props.children}
  </BookContext.Provider>

  • value Prop: The value prop of the Provider is set to an object { books }. This is the data that will be accessible to components consuming this context. Here, we are passing the books state.

  • props.children: The props.children are rendered within the Provider. This ensures that any components wrapped by BookContextProvider will have access to the context value.

  Children Prop: In React, props.children represents the components that are rendered between the opening and closing tags of a component. It is a way to pass components as data to other components.

• Exporting the Provider: We need to export BookContextProvider so it can be used to wrap other components in our application.

export default BookContextProvider;

Consuming the Context in a Functional Component

Now that we have created the context and the provider, let’s see how to consume this context in another functional component, for example, a BookList component. We will use the useContext hook for this.

import React, { useContext } from 'react';
import { BookContext } from './BookContext';

const BookList = () => {
  const { books } = useContext(BookContext);

  return (
    <ul>
      {books.map(book => (
        <li key={book.id}>
          {book.title}
        </li>
      ))}
    </ul>
  );
};

Let’s break down the BookList component:

• Importing useContext and BookContext: We import the useContext hook and the BookContext we created earlier.

  useContext: This is a React Hook that allows functional components to consume values from a context object. It takes a context object as an argument and returns the current context value for that context.

• Consuming the Context with useContext: The useContext hook is used to access the context value.

  const { books } = useContext(BookContext);

  We pass BookContext to useContext, and it returns the value we provided in the BookContextProvider. We then destructure books from this value.

• Rendering the Book List: We use the books array obtained from the context to render a list of book titles.

  <ul>
    {books.map(book => (
      <li key={book.id}>
        {book.title}
      </li>
    ))}
  </ul>

  • .map() Function: The .map() function is used to iterate over the books array and transform each book object into an <li> element.

    .map(): In JavaScript, the .map() method is used to iterate over an array and apply a function to each element, returning a new array of the results.

  • key Prop: The key prop is essential when rendering lists in React. It helps React identify which items have changed, are added, or are removed. It should be a unique and stable identifier for each item in the list. In this case, we use book.id as the key.

    key Prop: In React, when rendering lists of components, each item should have a unique key prop. This prop helps React efficiently update and re-render list items by providing a stable identity for each item.

Wrapping Components with the Provider

To make the context available to the BookList component (and any other component that needs it), we need to wrap BookList with the BookContextProvider in our main application component (e.g., App.js or index.js).

import React from 'react';
import BookList from './BookList';
import BookContextProvider from './BookContext';

function App() {
  return (
    <BookContextProvider>
      <BookList />
    </BookContextProvider>
  );
}

export default App;

By wrapping BookList within BookContextProvider, the BookList component and all of its children now have access to the books data provided by the BookContext.

Advantages of Functional Components for Context

Using functional components with hooks for context management offers several advantages:

• Conciseness: Functional components with hooks often lead to more concise and readable code compared to class components.
• Readability: The logic for state management and context consumption is often easier to follow in functional components with hooks.
• Modern React Practices: Functional components and hooks are considered the modern and recommended approach for building React applications.

While class components are still perfectly valid for creating context providers, functional components with hooks provide a compelling alternative that aligns with modern React development practices and often results in cleaner, more maintainable code.

Building a Reading List Application with React Context and Hooks

This chapter will guide you through the process of building a simple reading list application using React Context and Hooks. This project will consolidate your understanding of these concepts by demonstrating their practical application in a cohesive project. We will be creating a new React application from scratch and incrementally building its features.

Project Setup

Creating a New React Application

To begin, we will create a brand new React project. Ensure you have Node.js and npm (Node Package Manager) or yarn installed on your system. We will use create-react-app, a widely used tool for setting up React projects with a sensible default configuration.

Open your terminal and execute the following command:

npx create-react-app book-list

npx create-react-app is a command-line tool that bootstraps a new React project. It sets up the basic project structure, build scripts, and development server, allowing you to quickly start building React applications.

This command will create a new directory named book-list containing the basic structure of a React application. Once the process is complete, navigate into the newly created directory:

cd book-list

cd (Change Directory) is a command used in the terminal to navigate between directories in the file system. In this case, it moves you into the book-list directory.

To verify that the project has been set up correctly, start the development server using:

npm start

npm start is a command that runs the start script defined in the package.json file of a Node.js project. For projects created with create-react-app, this command typically starts a development server that hosts your React application, making it accessible in your web browser.

This command will compile your React application and open it in your default web browser. You should see the default React starter page, confirming that your project is set up and running correctly.

Setting up Context

Creating a Context File

The first step in utilizing React Context is to create a context file. In the src directory of your project, create a new folder named contexts. Inside this folder, create a new file named BookContext.js. This file will house the logic for our book context.

Implementing the Book Context

Open BookContext.js and begin by importing the necessary modules from React:

import React, { createContext, useState } from 'react';

Here, we are importing React itself, createContext, and useState.

createContext is a React Hook that creates a Context object. This object provides a Provider component to supply values to the context and a Consumer component (or useContext Hook) to access those values.

useState is a React Hook that allows functional components to have state. It returns a state variable and a function to update that variable.

Next, create the context itself using createContext:

export const BookContext = createContext();

This line exports a constant BookContext which holds the context object. This context will be used to share book-related data throughout our application.

Now, we need to create a Context Provider component. This component will be responsible for holding the data we want to share and making it available to consuming components.

const BookContextProvider = (props) => {
  const [books, setBooks] = useState([
    { title: 'Name of the Wind', author: 'Patrick Rothfuss', id: 1 },
    { title: 'The Way of Kings', author: 'Brandon Sanderson', id: 2 },
  ]);

  const addBook = (title, author) => {
    setBooks([...books, { title, author, id: Math.random() }]);
  };

  const removeBook = (id) => {
    setBooks(books.filter(book => book.id !== id));
  };

  return (
    <BookContext.Provider value={{ books, addBook, removeBook }}>
      {props.children}
    </BookContext.Provider>
  );
}

export default BookContextProvider;

Let’s break down this code:

• BookContextProvider Functional Component: This is a functional component that will act as our context provider. It takes props as an argument, which is standard for React components.

  Functional Components: In React, functional components are JavaScript functions that return JSX. They are a simpler way to define components, especially when combined with Hooks for managing state and side effects.

  props (Properties): props are inputs to React components. They are objects containing data passed down from parent components to child components, allowing for dynamic and reusable components.

• useState Hook: We use useState to initialize and manage the state for our book data.

  const [books, setBooks] = useState([
    { title: 'Name of the Wind', author: 'Patrick Rothfuss', id: 1 },
    { title: 'The Way of Kings', author: 'Brandon Sanderson', id: 2 },
  ]);

  This initializes the books state variable with an array containing two book objects. setBooks is the function we’ll use to update this state.

  state: In React, state is a mechanism for components to manage and hold data that can change over time. When state changes, React re-renders the component and its children to reflect the updated data.

• addBook Function: This function is designed to add a new book to the books array.

  const addBook = (title, author) => {
    setBooks([...books, { title, author, id: Math.random() }]);
  };

  It takes title and author as arguments. Inside, setBooks is called to update the state. We use the spread operator (...) to create a new array containing all existing books plus a new book object.

  Arrow function: An arrow function is a concise syntax for writing function expressions in JavaScript. They are often used for short, simple functions.

  Spread operator (…): The spread operator in JavaScript is used here to expand the existing books array into individual elements and create a new array. This ensures we are not directly modifying the original state array, which is important in React for triggering re-renders.

• removeBook Function: This function removes a book from the books array based on its id.

  const removeBook = (id) => {
    setBooks(books.filter(book => book.id !== id));
  };

  It takes an id as an argument and uses the filter method to create a new array containing only the books whose id does not match the provided id.

  filter method (array): The filter method is a JavaScript array method that creates a new array containing only the elements that pass a certain condition provided by a callback function.

  Callback function: A callback function is a function passed as an argument to another function, which is then invoked or “called back” at a later point. In the context of filter, the callback function is executed for each element in the array.

• BookContext.Provider: This is the core of the Context API.

  return (
    <BookContext.Provider value={{ books, addBook, removeBook }}>
      {props.children}
    </BookContext.Provider>
  );

  It wraps the components that need to access the context value. The value prop is used to pass the data we want to share. Here, we are passing an object containing the books array and the addBook and removeBook functions.

  JSX (JavaScript XML): JSX is a syntax extension for JavaScript that resembles HTML. It is used in React to describe the structure of user interfaces and is transformed into regular JavaScript at runtime.

  props.children: props.children represents the components that are nested within the BookContextProvider component in the component tree. By rendering props.children, the provider makes its context available to all of its child components.

• export default BookContextProvider;: This line exports the BookContextProvider component so it can be used in other parts of our application.

Wrapping the App with the Context Provider

To make the context available to our application, we need to wrap the root component (App.js) with our BookContextProvider. Open src/App.js and modify it as follows:

import BookContextProvider from './contexts/BookContext';
import Navbar from './components/Navbar';

function App() {
  return (
    <div className="App">
      <BookContextProvider>
        <Navbar />
      </BookContextProvider>
    </div>
  );
}

export default App;

Here, we import BookContextProvider and wrap the Navbar component (which we will create next) with it. This makes the values provided by BookContextProvider accessible to the Navbar component and any other components nested within BookContextProvider. We also remove the boilerplate content from the App component to start with a clean slate. You might also want to remove the default CSS files (App.css, logo.svg, reportWebVitals.js, setupTests.js) and update index.js to remove imports of these files if you want a completely clean setup.

Creating Components

Navbar Component

Now, let’s create the Navbar component to display the title of our application and the number of books in the reading list. Create a new folder named components in the src directory, and inside it, create a file named Navbar.js.

Open Navbar.js and add the following code:

import React, { useContext } from 'react';
import { BookContext } from '../contexts/BookContext';

const Navbar = () => {
  const { books } = useContext(BookContext);
  return (
    <div className="navbar">
      <h1>Ninja Reading List</h1>
      <p>Currently you have {books.length} books to get through...</p>
    </div>
  );
}

export default Navbar;

Let’s analyze this component:

• Import Statements: We import React and useContext from React, and BookContext from our context file.

  useContext is a React Hook that allows functional components to consume values from a context. It takes a Context object as an argument and returns the current context value for that context.

• useContext(BookContext): We use the useContext Hook to access the values from BookContext.

  const { books } = useContext(BookContext);

  This line uses object destructuring to extract the books array from the context value.

  Destructuring: Destructuring is a JavaScript feature that allows you to extract values from arrays or properties from objects and assign them to distinct variables in a concise way.

• JSX Structure: The component returns JSX that represents the navbar. It displays a title “Ninja Reading List” and a paragraph indicating the number of books currently in the list, using books.length.

• Class Name navbar: The div has a className of navbar. This is used for styling the component using CSS.