React Interview Questions and Answers

This section covers frequently asked React interview questions, categorized for clarity.

General React Interview Questions

1. What is React?

React is a free and open-source front-end JavaScript library for building user interfaces (UIs), particularly for single-page applications. Developed by Facebook, it uses a component-based architecture and the Virtual DOM for efficient updates. It's used for web and mobile app development.

2. What are the Key Features of React?

• JSX: A syntax extension that allows you to write HTML-like code within JavaScript.
• Components: Reusable building blocks of the UI.
• One-way Data Binding: Data flows in a single direction, making applications easier to understand and debug.
• Virtual DOM: A lightweight representation of the real DOM, improving performance.

3. Advantages of Using React.

• Easy to Learn: React has a relatively gentle learning curve, especially for developers with JavaScript experience.
• Excellent Resources: Abundant documentation, tutorials, and community support are available.
• MVC Architecture (View Part): React fits well within the Model-View-Controller architecture, handling the View layer effectively.
• Virtual DOM for Efficiency: Significantly improves performance by only updating parts of the real DOM that have changed.
• Easy to Build Dynamic Web Apps: JSX simplifies the creation of dynamic UIs.
• SEO-Friendly: React allows server-side rendering, which is beneficial for search engine optimization.
• Reusable Components: Promote code reusability and easier maintenance.
• Handy Developer Tools: Chrome and Firefox extensions simplify debugging and inspection of React components.
• Rich Ecosystem: A vast collection of libraries expands functionality and flexibility.
• Testable Code: React applications are relatively easy to test.

4. Limitations of React.

• Just a Library, Not a Full Framework: React handles only the view layer; you need other libraries and tools for other aspects of application development.
• Steep Learning Curve (for some): While generally considered easy to learn, JSX and the overall ecosystem can be initially challenging for new developers.
• JSX Can Be Complex: While beneficial, embedding HTML-like syntax within JavaScript can feel cumbersome to some.

5. What is JSX?

JSX (JavaScript XML) is a syntax extension that allows you to write HTML-like code within JavaScript. It's not JavaScript itself but gets compiled down to regular JavaScript code before the browser runs it. It's designed to enhance readability when building React components.

class App extends React.Component {
  render() {
    return (
      
Hello JavaTpoint
    );
  }
}

6. Why Can't Browsers Read JSX?

Browsers only understand JavaScript. JSX is a syntax extension that needs to be transpiled (converted) into plain JavaScript using tools like Babel before being used by a browser.

7. Why Use JSX?

• Performance: JSX code often compiles into more optimized JavaScript.
• Conciseness: Combines markup and logic in one place, improving readability.
• Type Safety: Catches errors during compilation.
• Template Creation: Simplifies the creation of templates.

8. What is the Virtual DOM?

The Virtual DOM is a lightweight in-memory representation of the real DOM. React uses it to efficiently update the actual DOM. Only the necessary changes are applied, improving performance.

9. How Does the Virtual DOM Work?

1. When data changes, React creates a new Virtual DOM.
2. React compares the new Virtual DOM with the old one to find differences.
3. Only the changed parts of the real DOM are updated, minimizing browser re-renders.

10. React vs. Angular.

11. React's ES6 vs. ES5 Syntax.

React commonly utilizes ES6 (ECMAScript 2015) syntax features, such as import/export statements, classes instead of createClass, and concise syntax for defining properties and state, which differ significantly from ES5 (ECMAScript 5) equivalents.

12. ReactJS vs. React Native.

13. Real DOM vs. Virtual DOM.

14. "In React, Everything is a Component." Explain.

In React, components are the fundamental building blocks. They break down the UI into small, independent, and reusable pieces of code. Each component manages its own state and rendering, promoting modularity and maintainability.

15. Purpose of render() in React.

The render() method is essential for every React component. It's responsible for returning the JSX (JavaScript XML) that represents the component's UI. It should return a single root element (e.g., a div, section, etc.) that encloses other elements.

import React from 'react';
class App extends React.Component {
  render() {
    return (
      
Hello World
    );
  }
}
export default App;

16. Embedding Multiple Components.

You embed components by using one component within another's JSX. The parent component's render() method includes the child components as elements.

import React from 'react';
const ChildComponent = () => 
Child Component
;
const ParentComponent = () => (
  

    
Parent Component
    
  
);
export default ParentComponent;

17. What are Props?

Props (short for "properties") are read-only data passed to a React component. They're similar to function arguments and are used to configure and customize components. Props are immutable within the component itself.

18. What is State in React?

State is an internal, mutable data structure within a component. It's used to manage dynamic data that changes over time in response to user interactions or other events. The component re-renders when the state changes.

import React from 'react';
class User extends React.Component {
  constructor(props) {
    super(props);
    this.state = { message: 'Welcome!' };
  }
  render() {
    return 
{this.state.message}
;
  }
}
export default User;

19. Props vs. State.

20. Updating Component State.

Use the this.setState() method to update a component's state. This triggers a re-render of the component.

import React from 'react';
class App extends React.Component {
  constructor() {
    super();
    this.state = { msg: 'Initial message' };
    this.handleClick = this.handleClick.bind(this);
  }
  handleClick() {
    this.setState({ msg: 'Updated message' });
  }
  render() {
    return (
      

        
{this.state.msg}
        Update
      
    );
  }
}
export default App;

21. Stateless vs. Stateful Components.

22. Arrow Functions in React.

Arrow functions provide a concise syntax for defining functions. They automatically bind this, simplifying event handling in React components.

// Regular function (requires binding)
handleClick() {
  // ...
}

// Arrow function (no binding needed)
handleClick = () => {
  // ...
};

23. Events in React.

Events in React are similar to DOM events but use camelCase naming (e.g., onClick). They're handled by passing functions as event handlers to JSX elements.

24. Creating Events in React.

You create events by assigning functions to the appropriate event handler props in JSX (e.g., onClick, onChange).

Click Me

25. Synthetic Events in React.

Synthetic events are cross-browser wrappers around native browser events. They provide a consistent API for event handling across different browsers.

26. Controlled vs. Uncontrolled Components.

27. Lists in React.

Lists in React are rendered using JavaScript's array methods like map(). Each item in the list should have a unique key prop for efficient updates.

const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
  
{number}
);

28. Significance of Keys in React.

Keys are unique identifiers for elements in a list. React uses them to efficiently update lists when items are added, removed, or reordered. They improve performance and prevent unexpected UI behavior.

29. Creating Forms in React.

Forms in React are created using controlled components. The component's state manages the form data, and changes are handled using the onChange event handler.

30. Phases of a React Component's Lifecycle.

React components go through several phases:

1. Initial Phase: The component is created with default props and state.
2. Mounting Phase: The component is added to the DOM (Document Object Model).
3. Updating Phase: The component re-renders in response to changes in props or state.
4. Unmounting Phase: The component is removed from the DOM.

31. React Component Lifecycle Methods.

Key lifecycle methods:

• getInitialState() (Deprecated): Sets initial state (use constructor instead).
• componentWillMount() (Deprecated): Before rendering (use useEffect hook).
• componentDidMount(): After rendering, suitable for side effects (e.g., data fetching).
• componentWillReceiveProps() (Deprecated): Before receiving new props (use useEffect hook with prop dependencies).
• shouldComponentUpdate(): A performance optimization; returns true to update, false to skip updates (use useMemo or React.memo for similar optimizations).
• componentWillUpdate() (Deprecated): Before updating (use useEffect hook).
• componentDidUpdate(): After updating, for side effects that depend on the updated state.
• componentWillUnmount(): Before unmounting, for cleanup actions (e.g., clearing timers).

Note: Several lifecycle methods are deprecated in newer React versions. The useEffect hook provides a more flexible and modern approach to handling side effects.

32. What are Pure Components?

React.PureComponent is a base class that optimizes rendering by comparing props and state. If they haven't changed, it avoids re-rendering, improving performance. It's a simpler alternative to manually implementing shouldComponentUpdate.

33. What are Higher-Order Components (HOCs)?

HOCs are advanced techniques for reusing component logic. An HOC is a function that takes a component and returns a new enhanced component. It's a pattern, not a React API feature, leveraging React's composition.

34. What Can You Do with HOCs?

• Code Reusability: Extract common logic across multiple components.
• Props Manipulation: Modify or enhance props before passing them to a component.
• State Manipulation: Manage state for multiple components.
• Render Highjacking: Control how a component renders.

35. Difference Between Element and Component.

36. How to Write Comments in React.

Use standard JavaScript comment syntax within JSX:

{/* Single-line comment */}
{ /*
Multi-line
comment
*/ }

37. Why Start Component Names with a Capital Letter?

In JSX, uppercase component names are interpreted as React components, while lowercase names are treated as HTML tags. This distinction helps React differentiate between custom components and standard HTML elements.

38. What are Fragments?

Fragments (introduced in React 16.2) let components return multiple elements without adding extra nodes to the DOM. They're useful for grouping elements without introducing unnecessary DOM hierarchy.

  
Title
  
Paragraph

{/* Shorthand: <> ...  */}

39. Fragments vs. Container Divs.

Fragments are preferred over wrapping divs because they don't add extra nodes to the DOM, resulting in better performance and cleaner code.

40. Prop Validation in React.

Use propTypes to define data types and validation rules for component props. This helps catch errors early in development.

MyComponent.propTypes = {
  name: PropTypes.string.isRequired,
  age: PropTypes.number
};

41. What is Create React App?

Create React App (CRA) is a tool for quickly setting up a new React project. It handles the configuration of tools like Webpack and Babel, simplifying the development process.

42. How to Create a Component in React?

Two ways to define components:

• Functional Components: Simple functions that return JSX.
• Class Components: ES6 classes that extend React.Component; provide state and lifecycle methods.

function MyComponent(props) {
  return 
Hello {props.name}
;
}

43. Class Components vs. Functional Components.

Use class components when you need state or lifecycle methods. For simpler components without state, functional components are preferred. Hooks (introduced in React 16.8) largely bridge the gap, allowing state and other features within functional components.

44. Can Web Browsers Read JSX Directly?

No, browsers can't directly understand JSX. It must be transpiled into JavaScript using tools like Babel before running in a browser.

45. What is State in React?

State is an object that holds data for a component. When the state changes, React updates the UI accordingly.

46. React's ES6 vs. ES5 Syntax Differences.

React commonly uses ES6 (ECMAScript 2015) syntax. Here's a comparison with ES5 (ECMAScript 5):

47. What are Props in React?

Props (properties) are data passed down from a parent component to its children. They're used to customize and configure child components.

React Refs Interview Questions

48. What are Refs in React?

Refs provide a way to access DOM elements or React components directly. They're useful for situations where you need direct manipulation of the DOM or component instances, although this is generally less preferred than declarative approaches.

49. How to Create Refs.

Create refs using React.createRef() and attach them to elements via the ref attribute.

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.myRef = React.createRef();
  }
  render() {
    return ;
  }
}

50. What are Forward Refs?

Forward refs allow you to pass a ref through a component to one of its children. This is useful when you need to access a child component's DOM element or instance from a parent component, even if the child is a reusable component from a library.

51. Callback Refs vs. findDOMNode().

Callback refs are preferred over findDOMNode(). Callback refs give more control over when refs are set and unset, and findDOMNode() can interfere with React's optimization strategies.

52. Uses of Refs.

• Accessing DOM elements directly.
• Managing focus, text selection, or media playback.
• Triggering imperative animations.
• Integrating with third-party DOM libraries.

React Router Interview Questions

53. What is React Router?

React Router is a popular library for adding client-side routing to React applications. It allows you to create single-page applications with multiple views, managing navigation and URL updates.

54. Why Use React Router?

React Router allows you to build single-page applications with multiple views, handling navigation between different parts of your app without full page reloads. It keeps the URL in sync with the current view.

55. Advantages of React Router.

• Simplified URL management.
• Declarative routing.
• Easy navigation.
• Clean and organized code structure.

56. React Router vs. Conventional Routing.

57. "Router May Have Only One Child Element" Warning.

This warning occurs if you don't wrap your routes within a single parent element (like

import { 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
      
    

64. Rules for Using Hooks.

1. Call hooks only at the top level of functional components.
2. Call hooks only from React functions.

65. What are Forms in React?

Forms in React are typically built using controlled components, where the component's state manages the form data. Changes in the form are reflected in the state through event handlers like onChange.

65. Uses of Forms in React.

Forms in React are used for user input and interaction. They provide a way to gather information from users, often used for things like authentication, data entry, and filtering.

66. What are Error Boundaries?

Error boundaries (introduced in React 16) are React components that catch JavaScript errors in their child component tree, preventing the entire app from crashing. They display a fallback UI instead of the crashed component.

Error boundaries catch errors in:

• The render() method
• Lifecycle methods
• Constructors

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  static getDerivedStateFromError(error) {
    return { hasError: true };
  }
  componentDidCatch(error, errorInfo) {
    console.error("Error:", error, errorInfo);
  }
  render() {
    if (this.state.hasError) {
      return 
Something went wrong.
;
    }
    return this.props.children;
  }
}

67. When Error Boundaries Don't Catch Errors.

• Errors in event handlers
• Errors during server-side rendering
• Errors within the error boundary itself
• Errors in asynchronous code (e.g., setTimeout)

React Redux Interview Questions

68. Problems with the MVC Framework.

Traditional MVC frameworks often had performance issues due to expensive DOM manipulations, leading to slow and inefficient applications.

69. Explain the Flux Architecture.

Flux is an application architecture designed to complement React. It promotes a unidirectional data flow model, improving predictability and making application state easier to manage.

70. What is Redux?

Redux is a predictable state container for JavaScript apps. It's often used with React to manage application state efficiently, making it easier to test and debug.

71. Three Principles of Redux.

1. Single source of truth: The entire application state is stored in a single object (the store).
2. State is read-only: The only way to change the state is by dispatching an action.
3. Changes are made with pure functions: Reducers (pure functions) specify how actions transform the state.

72. Components of Redux.

• Store: Holds the application state.
• Actions: Describe what happened.
• Reducers: Determine how the state changes based on actions.

73. Role of Reducers.

Reducers are pure functions that take the current state and an action and return a new state. They're responsible for updating the state based on dispatched actions.

74. Significance of the Store in Redux.

The store is the single source of truth for the application's state. It provides methods to access the state, dispatch actions, and subscribe to state changes.

75. Redux vs. Flux.

76. Advantages of Redux.

• Improved application architecture.
• Enhanced performance optimizations.
• Easier code maintenance.
• Testable code (pure functions).

77. Accessing the Redux Store Outside a Component.

Export the store from the module where it's created. Ensure it doesn't pollute the global scope.

React Multiple Choice Questions (MCQs)

1. What is Babel in React?

Babel is a tool that converts JSX and other modern JavaScript features into code that older browsers can understand. It's both a compiler and a transpiler.

2. What is the Reconciliation Process in React?

React's reconciliation process is how it efficiently updates the DOM. It uses a diffing algorithm to determine the minimal changes needed, improving performance.

3. Passing Data to a Component from Outside.

Props are used to pass data to React components from their parent components.

4. Rendering React Content on an HTML Page.

ReactDOM.render() renders a React component into a specified DOM element.

5. Phases of the Component Lifecycle.

The correct order is generally: Mounting, Updating, Unmounting. The specific lifecycle methods depend on the React version and whether you use class components or functional components with Hooks.

6. Role of React in the MVC Model.

In the Model-View-Controller (MVC) architecture, React primarily handles the View and a portion of the Controller logic.

7. Controlled vs. Uncontrolled Components.

The key difference lies in how component state is managed. In controlled components, the parent component manages the component's state, and changes are handled through event handlers. In uncontrolled components, the component itself manages its state, often using refs to access DOM values.

8. Arbitrary Inputs to React Components.

The arbitrary inputs to React components are called props.

9. What is the "key" Prop in React?

The key prop is a unique identifier for elements within an array of components. React uses keys to efficiently update lists when items are added, removed, or reordered, improving performance by allowing React to identify elements more efficiently.

10. Data Flow Sequence in Flux.

In the Flux architecture, the data flows unidirectionally: Action -> Dispatcher -> Store -> View.