What Is React Complete Guide

Last Update:2025-06-22T00:00:00 .NET School AI Teacher - SELECT ANY TEXT TO EXPLANATION. 7 mins read Difficulty-Level: beginner

Understanding the Core Concepts of What is React

What is React: A Comprehensive Guide Under 700 Keywords

React is a popular, open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications where you need to manage the state across dynamic components efficiently. Its key focus lies in providing a declarative approach to UI development, making it easier to reason about the application's behavior and structure.

Key Features

Declarative: In React, developers declare what the UI should look like at any given point of time. React handles transitioning between these states.
Component-Based: React allows you to build encapsulated components that manage their own state, then compose them to make complex UIs. Since component logic is written in JavaScript instead of templates, you can easily pass rich data through your app and keep state out of the DOM.
Learn Once, Write Anywhere: React is not just about rendering web pages in HTML. It can also render on the server using Node and power mobile apps using React Native.

JSX (JavaScript XML)

JSX is a syntax extension for JavaScript that looks similar to XML or HTML. JSX helps to write cleaner and more efficient code by allowing you to use HTML tags inside your JavaScript code. These tags get transformed into functions that generate plain JavaScript objects.

Components

In React, everything is a component. Components are the building blocks of any React application. There are two types of components:

Functional Components: These are simple JavaScript functions that return JSX and help to compose other components that make up the application. They are stateless and only responsible for receiving props and returning an element tree describing part of the UI.
Class Components: Introduced with ES6 classes, this type of component holds some internal state and implements lifecycle methods which make them powerful but more complex than functional components.

State and Props

State: State is an object that holds information about a specific component. State changes cause the component to re-render, reflecting the new state in the UI. State is mutable within the same component using the setState() method. State is local to the component, meaning it cannot be accessed by any other component unless passed as props.
Props: Props (short for properties) are read-only components used to pass data from one component to another. Props can be objects, arrays, booleans, strings, numbers or even functions. Unlike state, props are immutable, so they cannot be modified. They help to communicate between components.

Virtual DOM

The Virtual DOM is a programming concept where an ideal, or “virtual”, representation of a UI is kept in memory and synced with the “real” DOM by a library such as React. This process is known as reconciliation. When the state of a component changes through the setState() method, React creates a new Virtual DOM tree. To find out the differences between the old and new Virtual DOM trees, React runs a diffing algorithm. Following the algorithm, the minimal required number of operations will be performed to update the real DOM.

Lifecycle Methods

React class components have several methods that allow you to run code before and after rendering the component. These methods are called lifecycle methods. Common lifecycle methods include:

componentDidMount(): Runs after the component output has been rendered to the DOM. Used for fetching data, subscribing to events, etc.
componentDidUpdate(prevProps, prevState): Invoked immediately after updating occurs. Not called for the initial render.
componentWillUnmount(): Invoked just before the component is unmounted and destroyed.

Hooks

Introduced in React 16.8, Hooks let you use state and other React features without writing a class. The most common Hook is useState. It lets you add React state to function components. Other popular hooks include useEffect for side effects handling, useContext for context API management, useReducer for handling complex state logic, and custom hooks for reusing logic between different components.

Fragments

React Fragments allow you to group a list of children without adding extra nodes to the DOM. This means that you can group multiple elements and return them from a component’s render method without wrapping them in an additional DOM node. Syntax: <></> or <>...</>

Context API

The Context API provides a way to share values between components without having to explicitly pass a prop through every level of the tree. It is designed to share data that can be considered “global” for a tree of React components, like the current authenticated user, theme, or preferred language.

Redux

While Redux is not built-in React, it often used alongside React to manage the application's state in a more predictable way. Redux maintains a global state of the application, which can be consumed from any component, making the state management much simpler. Redux follows predictable state management principles and uses actions, reducers, and middleware to manage state.

Performance Optimization

React provides several ways to optimize performance, such as:

ShouldComponentUpdate: This lifecycle method lets React know if a component's output is not affected by the current change in state or props. The default behavior of this method is true, causing React to always refresh the component when the state or props changes.
Pure Component: Similar to shouldComponentUpdate(), React automatically performs a shallow comparison of current props and state with next ones when rendering pure components.
React.memo: This function memoizes a component so React skips rendering it if its props haven’t changed.
Code-Splitting: Code splitting is a technique used to divide your code into smaller chunks which can then be loaded on demand or in parallel.
Lazy Loading: Lazy loading is a design pattern commonly used in conjunction with virtual doms. It involves deferring initialization of an object until the point at which it is needed. React.lazy and Suspense are tools for lazy loading components in React.
List and Keys: Lists in React can be created by mapping over an array and returning JSX for each item. Key prop helps React identify which items have changed, are added, or are removed.

Community and Ecosystem

React has a large community and boasts an ecosystem of third-party libraries and tools that can accelerate the development process. Libraries like Material-UI or Ant Design provide pre-designed, customizable UI components. React Router manages routing between views of the application efficiently. GraphQL libraries like Apollo Client help to fetch and modify data from a GraphQL server.

Future of React

Continuously evolving with frequent releases, React's future is looking bright. The core team at Facebook continues to invest heavily in development tools and features to improve performance and developer experience. React Concurrent Mode aims to enable smoother animations by breaking down work into smaller chunks and giving priority to more urgent updates without blocking the execution.

Conclusion

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Step-by-Step Guide: How to Implement What is React

Complete Examples, Step by Step for Beginners: Understanding React

Step 1: Setting Up Your Environment

First, you need to set up your environment. We'll use Create React App, a tool that sets up a new React project with sensible default settings.

Install Node.js and npm: React requires Node.js and npm (Node Package Manager). You can download it from nodejs.org.
Install Create React App: Open your terminal or command prompt and run the following command:
```
npx create-react-app react-examples
```
Navigate to the Project Directory:
```
cd react-examples
```
Start the Development Server:
```
npm start
```
This will start the development server and open a new browser window/tab with your new React app running at http://localhost:3000.

Step 2: Basic React Component

React is all about components. Components let you split the UI into independent, reusable pieces.

Create a Simple Functional Component: Open src/App.js and replace its content with the following code:

import React from 'react';

function App() {
  return (
    <div>
      <h1>Hello, React!</h1>
    </div>
  );
}

export default App;

Understanding the Component:
- import React from 'react'; - Imports the React library.
- function App() { ... } - Defines a functional component named App.
- return (...) - A functional component returns a JSX (JavaScript XML) code that describes the UI.
- export default App; - Exports the App component so that it can be imported and used elsewhere.

Custom Component: Create a new file src/Welcome.js and add the following code:

import React from 'react';

function Welcome() {
  return (
    <h2>Welcome to our React app!</h2>
  );
}

export default Welcome;

Using the Custom Component in App.js:

import React from 'react';
import Welcome from './Welcome';

function App() {
  return (
    <div>
      <h1>Hello, React!</h1>
      <Welcome />
    </div>
  );
}

export default App;

Step 3: Props

Props (properties) allow you to pass data from one component to another.

Passing Data via Props: Modify Welcome.js to accept a name prop and display it:

import React from 'react';

function Welcome(props) {
  return (
    <h2>Welcome to our React app, {props.name}!</h2>
  );
}

export default Welcome;

Passing the Prop from App.js:

import React from 'react';
import Welcome from './Welcome';

function App() {
  return (
    <div>
      <h1>Hello, React!</h1>
      <Welcome name="Alice" />
    </div>
  );
}

export default App;

Step 4: State

State allows React components to change their output over time in response to user actions, network responses, and anything else.

Adding State to App.js: Import useState and use it within the App component.

import React, { useState } from 'react';
import Welcome from './Welcome';

function App() {
  const [name, setName] = useState('Alice');

  function handleChange(event) {
    setName(event.target.value);
  }

  return (
    <div>
      <h1>Hello, React!</h1>
      <Welcome name={name} />
      <input type="text" value={name} onChange={handleChange} />
    </div>
  );
}

export default App;

Understanding State and Events:
- const [name, setName] = useState('Alice'); - Declares a name state variable with an initial value of 'Alice'. The setName function is used to update name.
- handleChange is a function that updates the name state based on the input value.
- <input ... /> - The value attribute is bound to the name state, and the onChange event updates the state.

Step 5: Handling Lists and Keys

React can render multiple components by using an array of data. It's important to provide a unique key to each element in the array to help React identify which items have changed, are added, or are removed.

Rendering a List: Create a new file src/UserList.js and add the following code:

import React from 'react';

function UserList(props) {
  return (
    <ul>
      {props.users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
}

export default UserList;

Using UserList in App.js:

Top 10 Interview Questions & Answers on What is React

Top 10 Questions and Answers on "What is React?"

2. Is React a full-stack framework? No, React is not a full-stack framework. It is specifically a front-end library focused on building user interfaces. For back-end tasks, developers often integrate React with other technologies like Node.js or Django.

3. What are the main features of React?

Components: The foundation of building UIs in React. They are reusable pieces of code.
JSX: A syntax extension for JavaScript that looks similar to HTML. It allows you to write HTML-like elements in JavaScript and embed expressions within those elements.
Virtual DOM: React uses a virtual DOM to improve performance by minimizing direct manipulations of the browser’s DOM.
Unidirectional Data Flow: In React, data generally flows in one direction from parent components to child components.
State Management: React helps manage states efficiently, allowing components to update when a piece of data changes, improving reusability and scalability.

4. What is JSX in React? JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows you to write HTML-like structures in your JavaScript files. JSX makes it easier to write and understand the structure of UI components because it combines HTML with JavaScript logic inside the same file. Here’s a simple example:

const element = <h1>Hello, world!</h1>;

This JSX code is syntactic sugar for:

const element = React.createElement(
  'h1',
  null,
  'Hello, world!'
);

5. Explain the main difference between a regular DOM and a Virtual DOM in React. The Virtual DOM is an in-memory copy of the real DOM. When a component's state changes, React creates a new Virtual DOM representation of the component’s updated UI. This updated Virtual DOM then gets compared with the previous Virtual DOM representation to determine what exactly changed in the UI. Only the changed parts are then updated in the Real DOM, which is a computationally expensive process. By only updating the necessary parts, React ensures that applications are more efficient and run faster.

6. How does React use the Virtual DOM to improve performance? React improves performance by using the Virtual DOM through a process known as reconciliation. Here's how it works:

Initial State: When a component's state changes, React will first construct a Virtual DOM tree representing the component’s state at time A.
New State: When a user interacts with the app and triggers a state change, React constructs another Virtual DOM tree representing the UI at time B.
Diffing Algorithm: React runs a diffing algorithm that compares the two Virtual DOM trees (time A and time B) and figures out the minimal set of operations required to transform the Real DOM from time A to time B.
Batch Update: React batches these operations together and updates the Real DOM all at once.

Updating the Real DOM only when necessary saves significant processing time and reduces flicker and repaint, making the web application faster and more responsive.

7. What are the main types of components in React? There are two main types of components in React:

Functional Components: These components are essentially plain JavaScript functions that take props and return a React node (typically JSX). They don't have their own state; they can only receive data via props and render UI.

Example:
```
function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}
```

Class Components: Class components have their own state and lifecycle methods. They inherit from React.Component class.

Example:

class Welcome extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'World'
    };
  }

  render() {
    return <h1>Hello, {this.state.name}</h1>;
  }
}

8. What are some common hooks used in React functional components? Hooks are a way to use state and other React features without writing a class. Here are some commonly used React hooks:

useState: For adding state management to functional components.
```
const [count, setCount] = useState(0);
```
useEffect: For handling side effects in function components, such as fetching data, subscriptions, or manually changing the DOM.
```
useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]);
```
useContext: Accesses context values from a provider component without passing props explicitly.
```
const value = useContext(Context);
```
useReducer: Often used for managing complex state logics, especially for large state objects.
```
const [state, dispatch] = useReducer(reducer, initialState);
```

9. Can React be used for mobile development? Yes, React can be used for mobile development through libraries like React Native. React Native allows developers to build truly native apps by combining the power of React with the native capabilities of iOS and Android. With React Native, you write your code in JavaScript or TypeScript and React renders it into actual native views.

10. How does React handle events? In React, event handling is very similar to handling DOM events. However, there are some differences like naming conventions (onClick instead of onclick) and the way event handlers are passed (using camelCase and passing a function as the value). React events do not work exactly like native events, but they follow a consistent cross-browser behavior and use a synthetic event system.

Example of an event handler in React:

function handleClick(event) {
  alert('Button was clicked!');
}

<button onClick={handleClick}>
  Click Me
</button>

In this example, handleClick is an event handler function, which gets triggered when the user clicks the button, displaying a browser alert.