Top React JS Interview Questions

Introduction

In the ever-evolving landscape of web development, React JS has emerged as one of the most popular JavaScript libraries for building user interfaces. Created by Facebook in 2013, React has transformed how developers approach front-end development, offering a component-based architecture that makes building complex, interactive UIs more manageable and efficient.

Whether you're preparing for an interview, starting a new project, or simply curious about this technology, this guide will provide you with a solid foundation in React JS.

What is React JS?

React JS (commonly referred to as just "React") is an open-source JavaScript library developed and maintained by Facebook (now Meta) for building user interfaces, particularly single-page applications. Unlike full frameworks like Angular, React focuses specifically on the view layer of applications, making it more flexible and easier to integrate with other technologies.

Key Characteristics of React

• Component-Based Architecture: React applications are built using components—independent, reusable pieces of code that return HTML via a render function.
• Declarative Approach: You tell React what you want to achieve, and it handles the DOM updates to match that state.
• Virtual DOM: React creates a lightweight representation of the real DOM in memory, which helps optimize rendering performance.
• Unidirectional Data Flow: Data in React flows in one direction, from parent components to child components, making applications more predictable and easier to debug.
• JSX Syntax: React uses JSX, a syntax extension that allows you to write HTML-like code in your JavaScript.

Why Use React?

Performance

React's implementation of a virtual DOM significantly improves performance compared to directly manipulating the browser's DOM. When your data changes, React first updates its virtual DOM, compares it with the previous version (a process called "diffing"), and then efficiently updates only the necessary parts of the actual DOM.

Component Reusability

The component-based architecture of React promotes code reuse and separation of concerns. Components can be as small as a button or as large as an entire page, and they can be composed together to build complex UIs.

Strong Community Support

React has a vast and active community, resulting in extensive documentation, tutorials, and third-party libraries. This ecosystem makes learning and building with React more accessible and enjoyable.

Industry Adoption

Many leading companies use React in production, including Facebook, Instagram, Netflix, Airbnb, and Dropbox. This widespread adoption means there are plenty of job opportunities for React developers.

Core Concepts in React

Components

Components are the building blocks of any React application. They are self-contained pieces of code that encapsulate HTML, CSS, and JavaScript which can be reused throughout the application.

There are two types of components in React:

1. Functional Components: JavaScript functions that accept props and return React elements.

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

2. Class Components: ES6 classes that extend from React.Component and have a render method.

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

Since the introduction of Hooks in React 16.8, functional components can now handle state and lifecycle methods, making them the preferred approach for most new React code.

JSX

JSX is a syntax extension for JavaScript that looks similar to HTML. It allows you to write HTML-like code in your JavaScript files, making it easier to visualize the UI you're building.

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

This is neither a string nor HTML but JSX, which gets compiled to regular JavaScript function calls. Behind the scenes, the above JSX translates to:

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

Props

Props (short for "properties") are how components receive data from their parent. They are read-only and help make components reusable.

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

// Usage
<Welcome name="Sara" />;

State

State is a JavaScript object that stores data that may change over time and affects the component's rendering. Unlike props, state is managed within the component.

In class components:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

In functional components with Hooks:

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

Lifecycle Methods

React components have a lifecycle that includes mounting, updating, and unmounting phases. Each phase has associated methods that you can override to run code at specific times.

For class components, some key lifecycle methods include:

• componentDidMount(): Runs after the component is rendered to the DOM
• componentDidUpdate(): Runs after updates to the component
• componentWillUnmount(): Runs before the component is removed from the DOM

With Hooks, the useEffect Hook replaces most lifecycle methods:

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

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    document.title = `You clicked ${count} times`;

    // Similar to componentWillUnmount:
    return () => {
      document.title = 'React App';
    };
  }, [count]); // Only re-run if count changes

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

React Hooks

Introduced in React 16.8, Hooks allow you to use state and other React features without writing a class component. They provide a more direct API to React concepts like state, context, refs, and lifecycle.

Basic Hooks

1. useState: Adds state to functional components
2. useEffect: Performs side effects in functional components
3. useContext: Subscribes to React context without introducing nesting

Additional Hooks

1. useReducer: An alternative to useState for complex state logic
2. useCallback: Returns a memoized callback function
3. useMemo: Returns a memoized value
4. useRef: Returns a mutable ref object
5. useLayoutEffect: Similar to useEffect, but fires synchronously after all DOM mutations
6. useDebugValue: Displays a label for custom hooks in React DevTools

Virtual DOM

The Virtual DOM is a key concept that contributes to React's performance. Instead of updating the browser's DOM directly, React:

1. Creates a virtual representation of the UI in memory (Virtual DOM)
2. When state changes, it creates a new Virtual DOM and compares it with the previous one
3. Calculates the most efficient way to update the browser's DOM
4. Performs only the necessary updates to the real DOM

This process, known as reconciliation, minimizes expensive DOM operations and makes React applications fast and responsive.

JSX in Depth

JSX is more powerful than it first appears. Here are some advanced JSX features:

Expressions in JSX

You can embed any JavaScript expression in JSX by wrapping it in curly braces.

const name = 'Josh Perez';
const element = <h1>Hello, {name}</h1>;

Conditional Rendering

You can use conditional operators or logical operators to conditionally render elements.

function Greeting({ isLoggedIn }) {
  return (
    <div>{isLoggedIn ? <h1>Welcome back!</h1> : <h1>Please sign in.</h1>}</div>
  );
}

Lists and Keys

When rendering multiple components from an array, you should include a "key" prop to help React identify which items have changed, been added, or been removed.

function NumberList({ numbers }) {
  return (
    <ul>
      {numbers.map((number) => (
        <li key={number.toString()}>{number}</li>
      ))}
    </ul>
  );
}

State Management in React

For simple applications, React's built-in state management through the useState hook or component state is often sufficient. However, as applications grow more complex, you might need more sophisticated state management solutions.

Context API

The Context API provides a way to share data that can be considered "global" for a tree of React components, without having to pass props down manually at every level.

// Create a context
const ThemeContext = React.createContext('light');

// Provider component
function App() {
  return (
    <ThemeContext.Provider value="dark">
      <Toolbar />
    </ThemeContext.Provider>
  );
}

// Consumer component
function ThemedButton() {
  const theme = useContext(ThemeContext);
  return <button className={theme}>Themed Button</button>;
}

Redux

Redux is a popular state management library often used with React for managing complex application state. It centralizes your application's state and logic, making state changes predictable and traceable.

// Action
const increment = () => {
  return {
    type: 'INCREMENT',
  };
};

// Reducer
const counterReducer = (state = 0, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1;
    default:
      return state;
  }
};

// Store
const store = createStore(counterReducer);

Routing in React

React doesn't have built-in routing, but the React Router library is commonly used to handle navigation in React applications.

import { BrowserRouter, Routes, Route, Link } from 'react-router-dom';

function App() {
  return (
    <BrowserRouter>
      <nav>
        <Link to="/">Home</Link>
        <Link to="/about">About</Link>
      </nav>

      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/about" element={<About />} />
      </Routes>
    </BrowserRouter>
  );
}

Forms in React

Handling forms in React typically involves controlled components, where form data is handled by React state.

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

  const handleSubmit = (event) => {
    event.preventDefault();
    alert('A name was submitted: ' + name);
  };

  return (
    <form onSubmit={handleSubmit}>
      <label>
        Name:
        <input
          type="text"
          value={name}
          onChange={(e) => setName(e.target.value)}
        />
      </label>
      <button type="submit">Submit</button>
    </form>
  );
}

Server-Side Rendering (SSR)

React can be rendered on the server using Node.js, which helps with performance and SEO. Frameworks like Next.js provide an easier way to implement SSR with React.

// server.js with Express
import express from 'express';
import React from 'react';
import { renderToString } from 'react-dom/server';
import App from './App';

const app = express();

app.get('/', (req, res) => {
  const html = renderToString(<App />);

  res.send(`
    <!DOCTYPE html>
    <html>
      <head>
        <title>React SSR</title>
      </head>
      <body>
        <div id="root">${html}</div>
        <script src="/bundle.js"></script>
      </body>
    </html>
  `);
});

app.listen(3000);

React Ecosystem

The React ecosystem is rich with tools and libraries that enhance development:

1. Create React App: A comfortable environment for learning and building React applications
2. Next.js: A framework for server-rendered React applications
3. Gatsby: A static site generator based on React
4. Material-UI: A popular UI framework implementing Google's Material Design
5. React Testing Library: A testing utility for React components
6. React Native: A framework for building native mobile applications using React
7. Redux Toolkit: The official, opinionated toolkit for Redux development
8. React Query: A library for fetching, caching, and updating data in React applications

Best Practices in React Development

Component Organization

• Keep components small and focused on a single responsibility
• Use a consistent naming convention (e.g., PascalCase for components)
• Organize related files together (e.g., component, styles, tests)

Performance Optimization

• Use the React DevTools Profiler to identify performance issues
• Implement memoization with React.memo, useMemo, and useCallback
• Avoid unnecessary re-renders by structuring state properly
• Use code-splitting to reduce bundle size

Code Style and Quality

• Follow a style guide (e.g., Airbnb React Style Guide)
• Use ESLint and Prettier for consistent code formatting
• Write unit tests for components using Jest and React Testing Library

Accessibility

• Use semantic HTML elements
• Include proper aria attributes
• Ensure keyboard navigation works correctly
• Test with screen readers

React JS complete Roadmap and Resources

Interview questions

React Fundamentals

1. What is React?
2. When was React first released?
3. Does React use HTML?
4. What is JSX?
5. Can you write React without JSX?
6. How can a browser read JSX files?
7. What are the key features of React?
8. What are the limitations of React?
9. What is the difference between React and Angular?
10. What's the difference between Element and Component in React?
11. What is the significance of keys in React?
12. Explain React's one-way data binding
13. What are the advantages of using React?
14. What is the virtual DOM and how does it work?
15. How would you optimize a React application's performance?

Virtual DOM

16. What is the difference between the virtual DOM and the real DOM?
17. Is the virtual DOM the same as the shadow DOM?
18. What is reconciliation in React?
19. How does the diffing algorithm work in React?
20. Why is the virtual DOM faster than the real DOM?
21. What happens when you call setState in React?
22. Explain how React updates the DOM efficiently
23. What are the limitations of the virtual DOM approach?
24. How does React batch DOM updates?
25. What is the fiber architecture in React?

Components

26. What is the difference between an element and a component?
27. What are the types of React components?
28. When should you create class-based components versus function components?
29. What are stateless components?
30. What are Pure Components?
31. What is a higher-order component (HOC)?
32. What is the main difference between createElement and cloneElement?
33. What are render props in React?
34. What is the difference between a Container and a Presentational component?
35. How do you create a component that renders different elements based on props?
36. Explain how to implement code-splitting in React
37. What are React.lazy() and Suspense?
38. What is the component composition pattern?
39. How do you test React components?
40. What are the advantages of using functional components over class components?

Props

41. What are Props in React?
42. How do you pass a value from parent to child?
43. How do you pass a value from child to parent?
44. Can a child component modify its own props?
45. What is prop drilling?
46. What are children props?
47. Why do you need to use props.children?
48. What is the use of the prop-types library?
49. How do you handle prop validation in React?
50. How would you implement default props?
51. What are the alternatives to prop drilling?
52. How do you handle conditional rendering based on props?
53. How can you spread props in JSX?
54. How do you handle required props?
55. What is the difference between props and attributes?

State

56. What is React State?

57. What is the difference between props and state?
58. How does state in a class component differ from state in a functional component?
59. How can you update state in React?
60. What is the difference between getInitialState() and constructor()?
61. What kind of information controls a component in React?
62. What happens when you update state directly instead of using setState()?
63. How do you update state that depends on the previous state?
64. What is the second argument to setState function?
65. What is state lifting in React?
66. How do you share state between components?
67. What are controlled vs uncontrolled components?
68. How do you initialize state in a functional component?
69. What is the useState hook and how does it work?
70. How do you implement global state management without Redux?

Lifecycle and Effects

71. What is the component lifecycle?
72. What are the lifecycle methods in React?
73. How do you update lifecycle in functional components?
74. What is ComponentWillMount()?
75. Explain the meaning of Mounting and Demounting
76. What is the replacement for componentWillReceiveProps()?
77. What is the purpose of shouldComponentUpdate()?
78. How do useEffect and componentDidMount differ?
79. How do you implement componentDidUpdate functionality with hooks?
80. How do you replicate componentWillUnmount with useEffect?
81. What are the common mistakes when using useEffect?
82. How do you optimize renders with shouldComponentUpdate and React.memo?
83. What is the difference between componentDidMount and useEffect with empty dependencies?
84. How do you handle errors in lifecycle methods?
85. What is the execution order of lifecycle methods in React?

Hooks

86. What are React Hooks?
87. What arguments does useEffect take?
88. When does the useEffect function run?
89. What is the useEffect function's return value?
90. What is meant by callback function in React?
91. What is the useState hook and how does it differ from this.setState?
92. What is the useContext hook?
93. What is the useReducer hook?
94. What is the useRef hook?
95. What is the useCallback hook?
96. What is the useMemo hook?
97. What are custom hooks?
98. What are the rules of hooks?
99. How do you implement your own custom hook?
100. What is the difference between useMemo and useCallback?
101. How do you test components that use hooks?
102. What is the useLayoutEffect hook?
103. How does useImperativeHandle work?
104. When would you use useDebugValue?
105. How can hooks help with code organization and reuse?

Refs

106. What is the difference between refs and state variables?
107. When is the best time to use refs?
108. What is the proper way to update a ref in a function component?
109. What are refs in React?
110. What is the difference between React.createRef() and useRef()?
111. How do you access DOM elements using refs?
112. What are forward refs in React?
113. What are string refs and why are they deprecated?
114. What are callback refs?
115. How can refs be used for animations?

Context

116. What is Context in React?
117. What is the difference between the Context API and prop drilling?
118. When shouldn't you use the Context API?
119. How do you create a context in React?
120. What are context providers and consumers?
121. How do you update context from a nested component?
122. What is the useContext hook?
123. What are the performance implications of using Context?
124. How do you structure an application with multiple contexts?
125. How does Context API work under the hood?

State Management

126. What is Redux?
127. What is the 'Store' feature in Redux?
128. What is an action in Redux?
129. What is Flux Concept in React?
130. What is a dispatcher?
131. How to dispatch the data in store?
132. What is the purpose of using bindActionCreators?
133. How can you handle more actions using Redux?
134. How can you split reducers?
135. What are important features of Redux workflow?
136. What is the Redux middleware?
137. What is redux-thunk?
138. What is redux-saga?
139. What is the difference between Redux and Context API?
140. How do you implement optimistic updates in Redux?
141. What is Redux DevTools?
142. What is Redux Toolkit and why would you use it?
143. What are selectors in Redux?
144. How do you handle asynchronous operations in Redux?
145. What is immutability and why is it important in Redux?

Fragments and Portals

146. What is a Fragment?
147. What is the use of empty tags?
148. What is portal in React?
149. What are React portals?
150. When would you use a Fragment versus a regular DOM element?
151. What are the benefits of using Fragments?
152. How do you pass key props to Fragments?
153. What problems do portals solve?
154. How do you implement a modal using portals?
155. How do events propagate through portals?

Event Handling

156. What is a synthetic event in React?
157. Explain the use of the arrow function in React
158. How is event handling different in React compared to HTML?
159. How do you pass arguments to event handlers?
160. What is event pooling in React?
161. How do you stop event propagation in React?
162. How do you handle form submission in React?
163. How do you create custom events in React?
164. What is the difference between onChange and onInput events?
165. How do you optimize event handlers in React?

Forms and Controlled Components

166. What are controlled and uncontrolled components?
167. How do you handle forms in React?
168. What is the difference between controlled and uncontrolled inputs?
169. How do you handle multiple inputs in a form?
170. How do you handle form validation in React?
171. What are the advantages of controlled components?
172. How do you reset a form in React?
173. How do you submit a form without a page reload?
174. How do you implement file uploads in React?
175. What form libraries work well with React?

Advanced Concepts

176. What is strict mode in React?
177. What are error boundaries?
178. What is server-side rendering in React?
179. How can we do server-side rendering in React?
180. What is the create-react-app tool?
181. What can be done when there is more than one line of expression?
182. When should you use the top-class elements for the function element?
183. How can you share an element in the parsing?
184. How can you re-render a component without using setState()?
185. What is React presentational component?
186. Explain the term "restructuring" in React
187. What is React Concurrent Mode?
188. What are React Profiler and how do you use it?
189. What is React Suspense?
190. What is code-splitting in React?
191. How do you implement authentication in React?
192. What are React hooks for data fetching?
193. What is the Context + useReducer pattern?
194. How do you handle side effects in React?
195. How do you implement infinite scrolling in React?

Tools and Ecosystem

196. What is React Router?
197. What are popular animation packages in React ecosystem?
198. What is Jest?
199. What is Webpack in React?
200. What is Babel in React?
201. What is the use of the key prop in React lists?
202. List some methods in the react-dom package
203. What are the major issues of using MVC architecture in React?
204. What is reduction in React?
205. Name five predefined PropTypes in React
206. What is the "yield catchphrase" in JavaScript?
207. What is the use of the super keyword in React?
208. What is React Testing Library?
209. What is Styled Components?
210. What is Storybook?
211. What is Next.js?
212. What is Gatsby?
213. What is React Native Web?
214. What is Reselect in Redux?
215. What is Redux Persist?

React Native

216. What is the difference between React JS and React Native?
217. How does React Native work?
218. What are the core components in React Native?
219. How do you handle platform-specific code in React Native?
220. What is Expo in React Native?
221. How do you debug React Native applications?
222. What are the limitations of React Native?
223. How do you handle navigation in React Native?
224. How do you optimize performance in React Native?
225. What is the difference between React Native and Flutter?

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