Top 45 React JS Interview Questions and Answers Latest 2024

Table of Content

Basic ReactJS Interview Questions

Q.1 What is React and why would you use it?

React is a JavaScript library for building user interfaces, developed by Facebook. You would use it to create dynamic, responsive, and efficient web applications with reusable components and a virtual DOM for optimized rendering.

Q.2 How do you create a React component?

To create a React component, you define a JavaScript function or class that extends React.Component. Here’s a basic example using a function component:

import React from 'react';

function MyComponent(props) {
  return (
    <div>
      {/* JSX content here */}
    </div>
  );
}

export default MyComponent;

And here’s an example using a class component:

import React, { Component } from 'react';

class MyComponent extends Component {
  render() {
    return (
      <div>
        {/* JSX content here */}
      </div>
    );
  }
}

export default MyComponent;

These components can then be imported and used within other parts of your application.

Q.3 Explain the difference between a class component and a functional component.

The main differences between class components and functional components in React:

Feature	Class Component	Functional Component
Definition	Defined using ES6 class syntax	Defined using JavaScript functions
State management	Can use this.state and setState()	Use useState hook for managing local state
Lifecycle methods	Has lifecycle methods (componentDidMount, etc.)	Uses useEffect hook for lifecycle operations
Code complexity	Typically more verbose	Generally more concise
Performance	Slightly slower due to overhead	Slightly faster due to no this binding
Reusability	Supports inheritance	No inheritance support
Usage of this	Uses this keyword for accessing props and state	No this keyword; props are passed as function arguments
Hooks support	Does not support hooks (prior to React 16.8)	Supports hooks from React 16.8 onwards

Q.4 What are props in React?

In React, props (short for properties) are used to pass data from one component to another. They are read-only and help make components reusable and modular.

Q.5 How do you handle state in a React component?

In React, state is managed using the useState hook or the this.state mechanism in class components. Use useState for functional components and this.state for class components to store and update component-specific data.

Q.6 What is JSX and why do we use it in React?

JSX (JavaScript XML) is a syntax extension for JavaScript used in React to write HTML-like code directly in your JavaScript files. It allows developers to write UI components more intuitively and efficiently, blending HTML structures with JavaScript logic.

Q.7 What is the virtual DOM and how does React use it?

The virtual DOM (Document Object Model) is a lightweight copy of the real DOM in memory. React uses it to improve performance by comparing changes in the virtual DOM and only updating what’s necessary in the real DOM, minimizing DOM manipulations and improving rendering speed.

Q.8 Can you describe the lifecycle of a React component?

The lifecycle of a React component refers to the series of phases a component goes through from its creation to its eventual removal from the DOM. Here’s a concise overview of the lifecycle phases for a class component:

Mounting Phase:
- constructor(): Called when the component is initialized.
- static getDerivedStateFromProps(): Used to update state based on props changes before rendering.
- render(): Returns the JSX to be rendered.
- componentDidMount(): Executed after the component is rendered for the first time, useful for side effects like fetching data.
Updating Phase:
- static getDerivedStateFromProps(): Again used for updating state based on props changes before re-rendering.
- shouldComponentUpdate(): Determines if the component should re-render, optimizing performance.
- render(): Re-renders the component with updated state and props.
- getSnapshotBeforeUpdate(): Captures current DOM state before changes are applied.
- componentDidUpdate(): Executes after the component updates and re-renders, useful for side effects after a render.
Unmounting Phase:
- componentWillUnmount(): Called before the component is removed from the DOM, used for cleanup tasks like removing event listeners.
Error Handling Phase:
- static getDerivedStateFromError(): Used to render a fallback UI if a component within a try…catch block throws an error.
- componentDidCatch(): Used to log error information after an error has been thrown by a child component.

These lifecycle methods provide hooks into different stages of a component’s existence, allowing us to manage state, perform side effects, optimize rendering, and handle errors effectively.

Q.9 How do you pass data between components in React?

In React, you can pass data between components by using props. Parent components can pass data to child components through props, which are like function arguments in JavaScript. This helps maintain a unidirectional data flow and keeps components isolated and reusable.

Q.10 What are keys in a React list and why are they important?

Keys in a React list are unique identifiers that help React efficiently update the UI by identifying changed, added, or removed items. They are crucial for performance and ensuring proper component state management.

Q.11 How can you handle events in React?

In React, you handle events using JSX by passing event handlers as props to components. You typically define event handler functions and then reference them in JSX using syntax like onClick or onChange. This approach keeps your UI interactive and responsive.

Q.12 What is React Fragments and why would you use them?

React Fragments allow you to group multiple children elements without adding extra nodes to the DOM. They are useful when you need to return multiple elements from a component’s render method, but you don’t want to introduce unnecessary parent elements. This helps keep your JSX clean and improves code readability.

Intermediate ReactJS Interview Questions

Q.13 What are higher-order components in React?

Higher-order components (HOCs) in React are functions that take a component as an argument and return a new component. They are used to enhance the functionality of existing components by adding additional props or encapsulating common logic. HOCs facilitate code reusability and help in separating concerns by abstracting shared functionality into reusable components.

Q.14 Explain how context works in React.

Context in React provides a way to pass data through the component tree without having to pass props down manually at every level. It’s a mechanism to share values like themes, locale preferences, or authentication status across many components without explicitly passing props.

Here’s how it works:

Create Context: Define a context using React.createContext() with an initial value.
Provide Context: Use the Context.Provider component to wrap the part of the tree where you want to make the context available. Provide a value prop to pass the data.
Consume Context: Components that need the context can access it using Context.Consumer within their render method or by using the useContext hook in functional components.

Context helps in avoiding prop drilling (passing props through intermediate components), making the code cleaner and more maintainable, especially in large applications with deeply nested components.

Q.15 What is a pure component in React?

In React, a pure component is a class component that extends React.PureComponent instead of React.Component. It automatically implements shouldComponentUpdate with a shallow prop and state comparison to improve performance by avoiding unnecessary re-renders.

Q.16 Describe how conditional rendering works in React.

Conditional rendering in React involves rendering different components or elements based on certain conditions. Here’s how it works:

1. Using if Statements in JSX: You can use JavaScript if statements or ternary operators directly within JSX to conditionally render components or elements.

function ExampleComponent({ isLoggedIn }) {
  if (isLoggedIn) {
    return <LoggedInComponent />;
  } else {
    return <LoggedOutComponent />;
  }
}

2. Using Logical && Operator: You can conditionally render elements using the logical && operator. If the condition is true, the element following && is rendered.

function ExampleComponent({ isLoggedIn }) {
  return (
    <div>
      {isLoggedIn && <LoggedInComponent />}
      {!isLoggedIn && <LoggedOutComponent />}
    </div>
  );
}

3. Using Conditional (ternary) Operator: You can use a ternary operator to conditionally render different components or elements based on a condition.

function ExampleComponent({ isLoggedIn }) {
  return (
    <div>
      {isLoggedIn ? <LoggedInComponent /> : <LoggedOutComponent />}
    </div>
  );
}

4. Using Switch Case: For more complex conditions, you might use a switch statement outside of JSX to determine which component or element to render.

function ExampleComponent({ userType }) {
  let componentToRender;

  switch (userType) {
    case 'admin':
      componentToRender = <AdminDashboard />;
      break;
    case 'user':
      componentToRender = <UserDashboard />;
      break;
    default:
      componentToRender = <GuestDashboard />;
      break;
  }

  return <div>{componentToRender}</div>;
}

These methods allow React components to dynamically render different content based on changing data or user interaction, providing a flexible and responsive user interface.

Q.17 How do you implement error boundaries in React?

To implement error boundaries in React:

Create an Error Boundary Component: Define a component that uses componentDidCatch(error, info) to catch errors.
Wrap Components: Wrap the components that might throw errors with the Error Boundary component using <ErrorBoundary>.
Define Error Handling: Within componentDidCatch, set state or handle errors gracefully (e.g., show a fallback UI).

Here’s a basic example:

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  componentDidCatch(error, info) {
    this.setState({ hasError: true });
    // You can also log the error to an error reporting service
    console.error(error, info);
  }

  render() {
    if (this.state.hasError) {
      // Fallback UI when error occurs
      return <h1>Something went wrong.</h1>;
    }
    // Render children if no error
    return this.props.children;
  }
}

// Usage
<ErrorBoundary>
  <YourComponent />
</ErrorBoundary>

This setup catches errors that occur in YourComponent and displays a fallback UI when an error occurs. Adjust componentDidCatch to suit your error handling needs.

Q.18 What is the purpose of useRef in React?

The useRef hook in React is used to persist values across renders without causing re-renders. Its primary purposes include:

Accessing DOM nodes or React elements: You can use useRef to get references to DOM elements or React components directly.
Storing mutable values: Unlike useState, updating a useRef value doesn’t trigger a re-render. It’s useful for storing mutable values that you want to persist across renders.
Caching values: It can be used to cache values that need to persist between renders without triggering re-renders.

Q.19 How does React implement re-rendering of components?

React implements re-rendering of components through its virtual DOM (VDOM) and reconciliation process. Here’s a simplified explanation of how it works:

Virtual DOM (VDOM):
- React maintains a lightweight representation of the actual DOM in memory, known as the Virtual DOM.
- When state or props of a component change, React first calculates the difference (diff) between the current Virtual DOM and the new Virtual DOM that reflects the updated state.
Reconciliation:
- React reconciles the Virtual DOM changes efficiently. It identifies what has changed and updates only those parts of the actual DOM that need to be updated.
- The process of comparing the old Virtual DOM with the new one and applying the differences to the real DOM is called reconciliation.
Component Re-rendering:
- When a component’s state or props change, React schedules a re-render of that component.
- React uses a diffing algorithm to determine the minimal set of changes needed to update the DOM efficiently.
- Components are re-rendered in a top-down manner starting from the root of the component tree.
Rendering Cycle:
- React uses a batched update strategy to perform re-renders. Multiple state updates within the same event handler or lifecycle method are batched together to optimize performance.
- After computing the changes to the Virtual DOM, React applies these changes in a single pass to update the actual DOM, reducing unnecessary reflows and repaints.

In summary, React’s efficient re-rendering is facilitated by its Virtual DOM and reconciliation mechanism, which ensures that updates to the UI are performed in a performant and optimal manner.

Q.20 What are controlled components in React?

Controlled components in React are components whose input elements like input, textarea, and select are controlled by React state. This means React handles the value of the input elements and updates it based on state changes, ensuring that React is the single source of truth for the input’s value. This approach allows for more control over form elements and facilitates handling user input in React applications.

Q.21 How would you integrate a third-party library in a React project?

Integrating a third-party library in a React project typically involves the following steps:

1. Install the Library: Use npm or yarn to install the library. For example:

npm install <library-name>

2. Import the Library: Import the necessary components, functions, or styles from the library into your React component where you intend to use them.

import { SomeComponent } from '<library-name>';

3. Use the Library: Incorporate the library’s components or functions into your JSX code.

function MyComponent() {
  return (
    <div>
      <SomeComponent />
    </div>
  );
}

4. Configure and Initialize: Follow any specific setup or initialization steps mentioned in the library’s documentation, such as providing API keys or setting up configuration options.

5. Handle Dependencies: Ensure any dependencies required by the library are also installed and configured properly.

6. Test: Verify that the integration works as expected by testing your React component where the library is used.

By following these steps, you can effectively integrate third-party libraries into your React project to extend its functionality.

Q.22 Explain the use of useEffect in React.

In React, useEffect is used to perform side effects in functional components. It runs after every render and replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount. This hook takes two arguments: a function containing the side effect, and an optional array of dependencies to control when the side effect runs.

Q.23 What are hooks in React?

Hooks in React are functions that allow functional components to “hook into” React state and lifecycle features without writing a class. They include useState for managing state, useEffect for performing side effects, useContext for accessing context, and more. Hooks enable developers to reuse stateful logic across components and simplify complex component logic.

Q.24 How do you handle forms in React?

In React, handling forms typically involves using controlled components where form elements like <input>, <textarea>, and <select> maintain their state in the component’s state using useState hooks. Here’s a concise approach:

State Setup: Initialize state variables for each form input.
Event Handlers: Write event handlers (onChange, onSubmit) to update state as users interact with the form.
Form Submission: Handle form submission using the onSubmit handler, where you can process the form data or trigger further actions.

Example (assuming functional component with hooks):

import React, { useState } from 'react';

function MyForm() {
  const [formData, setFormData] = useState({
    username: '',
    password: ''
  });

  const handleInputChange = (event) => {
    const { name, value } = event.target;
    setFormData({ ...formData, [name]: value });
  };

  const handleSubmit = (event) => {
    event.preventDefault();
    // Process form data, e.g., send it to a server
    console.log(formData);
  };

  return (
    <form onSubmit={handleSubmit}>
      <label>
        Username:
        <input
          type="text"
          name="username"
          value={formData.username}
          onChange={handleInputChange}
        />
      </label>
      <br />
      <label>
        Password:
        <input
          type="password"
          name="password"
          value={formData.password}
          onChange={handleInputChange}
        />
      </label>
      <br />
      <button type="submit">Submit</button>
    </form>
  );
}

export default MyForm;

In this example:

State (formData) keeps track of input values.
handleInputChange updates state on input change.
handleSubmit prevents default form submission and logs form data.

This approach ensures that React manages the form state and allows for easy validation, submission handling, and interaction with other React features.

Q.25 Explain how to use the React Router for navigation.

To use React Router for navigation:

1. Install React Router:

npm install react-router-dom

2. Set up Router in App.js:

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

function App() {
  return (
    <Router>
      <Switch>
        <Route exact path="/">
          {/* Your home component */}
        </Route>
        <Route path="/about">
          {/* About component */}
        </Route>
        <Route path="/contact">
          {/* Contact component */}
        </Route>
      </Switch>
    </Router>
  );
}

3. Link to Routes:

import { Link } from 'react-router-dom';

function Navbar() {
  return (
    <nav>
      <ul>
        <li>
          <Link to="/">Home</Link>
        </li>
        <li>
          <Link to="/about">About</Link>
        </li>
        <li>
          <Link to="/contact">Contact</Link>
        </li>
      </ul>
    </nav>
  );
}

4. Use Route Parameters (Optional):

<Route path="/user/:id">
  {/* User component using useParams() */}
</Route>

5. Navigate Programmatically:

import { useHistory } from 'react-router-dom';

function Component() {
  let history = useHistory();

  function handleClick() {
    history.push('/path'); // Navigate to /path
  }

  return (
    <button onClick={handleClick}>Go to Path</button>
  );
}

6. SEO Best Practices:

Use descriptive <title> and <meta> tags.
Ensure each route has unique, relevant content.
Optimize images and other media for performance.

Implementing these steps effectively helps in optimizing your React application for Google’s featured snippet.

Q.26 What is server-side rendering with React and why might you use it?

Server-side rendering (SSR) with React involves rendering React components on the server before sending the HTML to the client. Here’s why you might use it:

Improved SEO: Search engines can crawl and index content more effectively because they receive fully rendered HTML from the server.
Faster Initial Page Load: Users see content faster because the server sends pre-rendered HTML, reducing the time needed to render on the client.
Better Performance on Low-Powered Devices: Devices with limited processing power benefit from receiving pre-rendered content, as it reduces client-side rendering overhead.
Social Media Sharing: Platforms like Facebook and Twitter typically scrape HTML for previews; SSR ensures that previews show accurate content.
Progressive Enhancement: SSR provides a base experience for all users, while client-side JavaScript enhances interactivity.

To implement SSR with React, frameworks like Next.js or custom setups using Node.js and libraries like ReactDOMServer are commonly used.

Q.27 How can you improve the performance of a React application?

Improving the performance of a React application involves several strategies:

Minimize Renders: Use React.memo and useMemo to prevent unnecessary re-renders.
Virtualize Lists: Use react-virtualized or react-window for long lists to render only what’s visible.
Code Splitting: Lazy load components with React.lazy and Suspense to reduce initial bundle size.
Optimize Images: Compress and lazy-load images to reduce load times.
Bundle Size: Use tools to analyze and reduce bundle size, splitting large bundles.
Production Build: Use React’s production build (react.production.min.js) for optimized performance.
Memoization: Cache expensive computations with useMemo and useCallback.
Network: Implement SSR or PWA techniques for faster initial load and offline capabilities.
Efficient Practices: Avoid forceUpdate, deep nesting, and refactor inefficient code.
Monitoring: Use tools like Chrome DevTools and React Profiler to find and fix performance issues.

Q.28 Discuss the concept of prop drilling and how to avoid it.

Prop drilling refers to the process where props (properties) are passed through multiple levels of components in a React application, even though some intermediate components do not directly use these props. This can lead to code that is harder to maintain and refactor, as changes to props may require modifications across many components.

To avoid prop drilling, you can employ the following strategies:

Context API: Use React’s Context API to provide a way to share data across the component tree without explicitly passing props manually at every level. Context allows you to pass data through the component tree without having to pass props down manually at every level.
State Management Libraries: Utilize state management libraries like Redux or MobX. These libraries provide centralized stores where data can be stored and accessed by any component without prop drilling.
Component Composition: Instead of passing props through multiple layers, consider composing components in a way that each component only receives the props it needs directly from its parent or from a global state.
Higher-Order Components (HOCs) and Render Props: Use HOCs or render props to encapsulate logic and pass props down to the components that need them, reducing the need to pass props through intermediate components.
Use of Hooks: With the introduction of hooks in React, such as useContext and useReducer, you can manage state and context more effectively within functional components, reducing the need for prop drilling.

Q.29 What is lazy loading in React?

Lazy loading in React is a technique used to optimize performance by loading components or assets only when they are needed, typically when they are about to become visible on the screen or when required by the user interaction. This helps reduce the initial bundle size and speeds up the initial load time of the application.

Q.30 What are React Portals and where would you use them?

React Portals provide a way to render children into a DOM node that exists outside the DOM hierarchy of the parent component. They are useful for scenarios where you need to render content in a different part of the DOM tree, such as modals, tooltips, or any overlay that should not be constrained by the parent’s styles or layout.

Q.31 How does React handle dependencies in useEffect?

In React’s useEffect hook, dependencies are specified as an array that determines when the effect should re-run. React compares the current values of these dependencies with the previous ones on every render. If any of the dependencies have changed (based on a shallow comparison), the effect callback function is executed again. This allows developers to control when side effects should be re-triggered based on changes in state or props.

Q.32 Describe how to use context API with hooks in React.

To use the Context API with hooks in React:

Create a Context: Define your context using createContext() from react.
Provide Context Values: Wrap your components with <MyContext.Provider> to provide the context values.
Consume Context Values: Use the useContext hook in your functional components to access the context values.
Access Context in Class Components: Use <MyContext.Consumer> to access context in class components.
Update Context Values: To update context values, modify the state within the provider component and pass down the updated value through context.

These steps allow you to effectively use the Context API with hooks in React to manage and share state across your application.

Q.33 How can you use memoization in React components?

You can use memoization in React components using the React.memo() higher-order component or the useMemo() hook.

1. Using React.memo(): React.memo() memoizes the component based on its props, preventing unnecessary re-renders when props don’t change.

import React from 'react';

const MyComponent = React.memo(function MyComponent(props) {
  // Component logic here
});

export default MyComponent;

2. Using useMemo() hook: useMemo() memoizes the result of a function and re-computes it only if the dependencies (like data in this example) change.

import React, { useMemo } from 'react';

function MyComponent({ data }) {
  const processedData = useMemo(() => {
    // Expensive data processing logic
    return processData(data);
  }, [data]);

  return <div>{processedData}</div>;
}

export default MyComponent;

These techniques help optimize React components by avoiding unnecessary renders and computations, improving performance.

Q.34 What are the common ways to handle state management in React applications beyond setState?

Context API: Use React.Context to share state across components without prop drilling.
Redux: Centralize state management with predictable state containers and reducers.
MobX: Observable state management with minimal boilerplate using decorators.
useState Hook: Local component state management, particularly useful for smaller-scale needs.
useReducer Hook: More complex state logic using a reducer function and dispatch.

Q.35 Explain how to use the useCallback hook.

The useCallback hook in React is used to memoize functions so that they are only recreated when their dependencies change. This optimization can be particularly useful in scenarios where passing callbacks to child components that rely on reference equality or performance improvements.

Here’s how you use useCallback:

import React, { useCallback } from 'react';

function MyComponent() {
  // Define a function that you want to memoize
  const memoizedCallback = useCallback(
    () => {
      // Your function logic here
      console.log('Callback function executed');
    },
    [] // Dependency array, empty means no dependencies
  );

  return (
    <div>
      <button onClick={memoizedCallback}>Click me</button>
    </div>
  );
}

export default MyComponent;

Explanation:

Define the Callback: Start by defining the function you want to memoize inside the useCallback hook.
Dependency Array: Pass an array of dependencies as the second argument to useCallback. If any of these dependencies change, the callback function will be recreated. If the array is empty ([]), the callback will be created once and will not change.
Usage: Use the memoized callback function (memoizedCallback in this example) as you would normally use any callback in your components.

Advanced ReactJS Interview Questions

Q.36 What are the benefits of server-side rendering vs client-side rendering in React?

Here are the benefits of server-side rendering (SSR) vs client-side rendering (CSR) in React:

Performance: SSR generally provides faster initial page loads because the server sends fully rendered HTML to the client, reducing the time required for JavaScript execution.
SEO: SSR improves search engine optimization (SEO) because search engines can easily crawl and index content from server-rendered pages.
First Contentful Paint (FCP): SSR can lead to quicker FCP, improving perceived performance and user experience.
Accessibility: SSR ensures that content is available even if the client’s browser does not support JavaScript or has it disabled.
Security: SSR can be more secure for certain types of applications by keeping sensitive business logic on the server and reducing client-side exposure.

In contrast, client-side rendering (CSR) provides benefits such as interactivity after the initial load, easier implementation of complex UI transitions, and potentially reduced server load for subsequent page views once assets are cached.

Q.37 How do you handle global state management in React without third-party libraries?

Handling global state management in React without third-party libraries typically involves using React’s built-in capabilities effectively. Here’s a straightforward approach:

Context API: Utilize React’s Context API to create a context object that holds your global state. This context can be accessed by any component in the component tree.
Props Drilling: For simpler applications, you can pass down state and functions as props through multiple levels of components, though this approach can become cumbersome with deeply nested components.
Reducer Hook: Use the useReducer hook to manage complex state transitions in a predictable way, similar to how Redux handles actions and reducers.

These methods allow you to manage global state effectively within React applications without relying on third-party libraries like Redux or MobX.

Q.38 Discuss the implications of shouldComponentUpdate lifecycle method.

Purpose: Controls whether a React component should re-render or not based on certain conditions.
Performance Optimization: Helps in improving performance by preventing unnecessary renders.
Implementation: Compares current props and state with next props and state to decide.
Benefits: Reduces computational load and optimizes user experience.
Caution: Incorrect implementation can lead to bugs or unintended behavior.

Q.39 How would you handle side effects in server-rendered React components?

Use Effect Hooks: Leverage useEffect hook to manage side effects in server-rendered React components.
Server-Side Logic: Ensure any server-side logic or operations are appropriately managed.
Data Fetching: Implement data fetching using useEffect combined with useState or useReducer for managing fetched data.
Conditional Rendering: Conditionally render components based on server-side state or data fetched.
Lifecycle Management: Consider server-rendered lifecycle implications like hydration and initial data loading.
Error Handling: Implement error handling for server-side operations and data fetching to ensure robustness.

Q.40 Explain the Fiber architecture of React 16+.

React Fiber, introduced in React 16, is a complete rewrite of React’s core algorithm, designed to enable better handling of asynchronous rendering and component updates. It breaks down work into units called fibers, allowing React to prioritize and re-prioritize updates for better perceived performance and responsiveness. This architecture enhances React’s ability to manage complex component trees and perform updates more efficiently.

Q.41 What is the use of static type checking in React, and how do you implement it?

Static type checking in React helps catch errors early by verifying that variables, props, and state are used correctly based on their types. It enhances code reliability and developer productivity by reducing runtime errors.

To implement static type checking in React, you can use TypeScript or Flow:

1. TypeScript: TypeScript is a superset of JavaScript that adds static types to your code. You can start by renaming your .js files to .tsx (for TypeScript with JSX) and then define types for props, state, and any variables you use.

Example:

interface Props {
    name: string;
    age: number;
}

const MyComponent: React.FC<Props> = ({ name, age }) => {
    // Component logic
    return <div>{name}, {age}</div>;
};

2. Flow: Flow is a static type checker for JavaScript. You annotate your code with type annotations using comments.