Top Most React JS Interview Questions That You Should Know

Updated on December 6, 2024

Article Outline

Basic-level ReactJS Interview Questions Intermediate-level ReactJS Interview Questions Advanced-level ReactJS Interview Questions Conclusion FAQs

ReactJS is one of the most famous libraries for building the user interface, mainly single-page applications. Its component-based architecture makes it so easy to develop UI components and then reuse them—generally, almost anything becomes much more maintainable and efficient to develop. There’s a demand for skilled React developers among companies throughout the world to improve their web applications.

This blog post describes some common ReactJS interview questions one needs to be prepared with to keep the interviewers in the loop of one’s overall ReactJS expertise and understanding of the overall web front end programming. And since simple questions are almost always the easiest, we will start with them, building a rock solid foundation that you can build on, and then to intermediate and advanced interview questions. This guide is for developers of all levels from beginners to veterans, so here should help you get ready for your next ReactJS interview.

Basic-level ReactJS Interview Questions

1. What is ReactJS?

Basically, ReactJS is a JavaScript framework to make the end parts of the project and create user interfaces, most frequently used for single page applications. It’s a Facebook-developed tool for our developers to create reusable UI components. They are used to construct complex user interfaces from isolated small pieces of code. React is using a virtual DOM from which they can efficiently update only the parts of the UI that are needed when the state changes.

2. What is MVC architecture in ReactJS?

MVC stands for Model-View-Controller, a design pattern used to separate concerns in application development. In MVC architecture:

Model: Represents the data and business logic.
View: Represents the UI components and presentation logic.
Controller: Handles user input and interacts with the Model and View to update the UI.

ReactJS does not strictly follow the MVC architecture. Instead, it focuses mainly on the View layer, providing a component-based approach to build UIs. The Model and Controller aspects can be handled using other libraries and patterns, such as Redux for state management.

3. What is JSX?

JSX stands for JavaScript XML. It is a syntax extension for JavaScript used in React to describe the UI structure. JSX looks similar to HTML but allows you to write HTML elements in JavaScript and place them in the DOM. It helps in creating React elements, making the code more readable and easier to debug. Here’s an example of JSX:

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

4. What are components and their types in React?

Components are the building blocks of a React application. They allow you to split the UI into independent, reusable pieces. Components can be broadly classified into two types:

Class Components: Defined using ES6 class syntax, they can maintain local state and lifecycle methods.

class Welcome extends React.Component {

render() {

return <h1>Hello, {this.props.name}</h1>;

}

}

Function Components: Defined using JavaScript functions, they don’t have state or lifecycle methods until the introduction of Hooks in React 16.8.

function Welcome(props) {

return <h1>Hello, {props.name}</h1>;

}

5. When should you use a class component over a function component?

Traditionally, you would use class components when you needed to manage state (or lifecycle methods). With the introduction of Hooks in React 16.8, function components can now properly handle state and lifecycle methods, and Function Components are now equally powerful. You might still use class components if:

You are working on an older codebase that uses class components.
You prefer the syntax and structure of class components.
You need to use certain lifecycle methods that are not easily replicated with Hooks (though this is rare).

6. How to create an event in React? Explain with basic code.

In React, you can handle events similarly to handling events in plain HTML. However, the syntax is slightly different.

Here is how you can create and handle an event in a React component:

import React, { useState } from 'react';

function ButtonClick() {

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

const handleClick = () => {

setCount(count + 1);

};

return (

<div>

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

<p>You clicked {count} times</p>

</div>

);

}

export default ButtonClick;

In this example, the handleClick function is called when the button is clicked, updating the state using setCount.

7. What are the key features of ReactJS?

ReactJS offers several key features that make it a popular choice for building user interfaces:

Component-Based Architecture: Allows for building encapsulated components that manage their own state.
Virtual DOM: Improves performance by updating only the parts of the DOM that have changed.
JSX: A syntax extension that combines JavaScript and HTML-like code for easier component creation.
Unidirectional Data Flow: This increases predictability and debugging of the code.
Hooks: Allow you to use state and other React features in function components.

8. What is the difference between Virtual DOM and Real DOM?

The Virtual DOM and Real DOM differ in several ways:

Feature	Virtual DOM	Real DOM
Definition	A lightweight copy of the Real DOM	The actual Document Object Model
Updates	Updates are batched and optimised	Direct updates on the elements
Performance	Faster updates due to less re-rendering	Slower updates due to re-rendering
Usage	Used by React to improve performance	Used by the browser to render the page

9. Why is React widely used today?

React is widely used today for several reasons:

Component-Based Architecture: Promotes reusability and easier management of the UI.
Performance: The Virtual DOM optimises updates and improves rendering performance.
Flexibility: It can be used with other libraries or frameworks and for both client-side and server-side rendering.
Developer Tools: It has excellent tools for debugging and development.
Strong Community: A large and active community that contributes to a rich ecosystem of libraries, tools, and resources.

10. Explain the difference between Angular and React.

Angular and React are both popular front-end frameworks/libraries, but they have key differences:

Feature	Angular	React
Type	Full-fledged framework	Library focusing on the view layer
Language	TypeScript (a superset of JavaScript)	JavaScript and JSX
Data Binding	Two-way data binding	One-way data binding
DOM	Real DOM	Virtual DOM
Learning Curve	Steeper due to more features	Easier with a simpler API
Architecture	Component-based with MVVM	Component-based
State Management	Built-in (RxJS, NgRx)	External libraries (Redux, MobX)

11. What are Hooks in React?

Hooks are functions that let you use state and other React features in function components.

Common hooks include:

useState: Manages state.
useEffect: Handles side effects.
useContext: Accesses context values.

12. How do you write comments in React?

In JSX, use curly braces with JavaScript comment syntax:

{/* This is a comment in JSX */}

Outside of JSX, use regular JavaScript comments:

// Single-line comment

/* Multi-line comment */

13. How to use styles in React?

You can style React components using:

Inline Styles: <div style={{ color: ‘blue’ }}>Hello</div>
CSS Classes: <div className=”myClass”>Hello</div>
CSS Modules:

import styles from './App.module.css';

<div className={styles.myClass}>Hello</div>

- Styled-Components:

import styled from ‘styled-components’; const StyledDiv = styled.divcolor: blue; <StyledDiv>Hello</StyledDiv>

14. How to use InnerHtml in React?

To use innerHTML in React, use dangerouslySetInnerHTML. Ensure the content is from a trusted source to avoid security risks.

<div dangerouslySetInnerHTML={{ __html: ‘<h1>Hello, world!</h1>’ }} />

15. How to loop inside JSX?

You can loop inside JSX using JavaScript’s map() function. Here’s an example:

const numbers = [1, 2, 3, 4, 5];

const listItems = numbers.map((number) =>

<li key={number.toString()}>{number}</li>

);

return (

<ul>{listItems}</ul>

);

16. What is a React Router?

This is a library installed in React Application for Routing. It enables you to describe several routes in your app and navigate between them. It has components like <BrowserRouter>, <Route> and <Link> to take care of routing.

17. What is Redux?

It’s a state management library for JavaScript apps called Redux. It makes you manage the application state in a predictable way: using a single source of truth (the store). It does things with the state by way of actions and reducers and middlewares like Redux Thunk for asynchronous operations.

18. Explain how to create a list in React.

To create a list in React, use the map() function to iterate over an array and return a list of elements. Here’s a simple example:

const items = ['Apple', 'Banana', 'Cherry'];

const itemList = items.map((item) =>

<li key={item}>{item}</li>

);

return (

<ul>{itemList}</ul>

);

19. What are the limitations of React?

Some limitations of React include:

Learning Curve: This can be steep for beginners.
Complex Configuration: Requires setup and configuration for advanced features.
Boilerplate Code: This may require additional boilerplate for state management.
Performance Issues: This can occur with large applications if not optimised properly.

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Intermediate-level ReactJS Interview Questions

20. What are Pure Components?

Pure Components in React are components that do not re-render if their props and state have not changed. They perform a shallow comparison of props and state to determine if a re-render is necessary, optimising performance. You can create a Pure Component by extending React.PureComponent.

21. What is a state in React?

State in React is an object that holds data that may change over the component’s lifecycle. It allows components to create and manage their own data, which can trigger re-rendering when updated. State is typically managed within class components using this.state and this.setState, or in function components using the useState hook.

Example using the class component:

class MyComponent extends React.Component {

constructor(props) {

super(props);

this.state = { count: 0 };

}

increment = () => {

this.setState({ count: this.state.count + 1 });

};

render() {

return (

<div>

<p>Count: {this.state.count}</p>

<button onClick={this.increment}>Increment</button>

</div>

);

}

}

Example using function component:

import React, { useState } from 'react';

function MyComponent() {

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

const increment = () => {

setCount(count + 1);

};

return (

<div>

<p>Count: {count}</p>

<button onClick={increment}>Increment</button>

</div>

);

}

22. What are props in React?

Props (short for properties) are read-only attributes passed from a parent component to a child component. They allow components to communicate with each other and pass data down the component tree. Props are accessed in a component via this.props in class components or directly in function components as function arguments.

Example using the class component:

class Greeting extends React.Component {

render() {

return <h1>Hello, {this.props.name}</h1>;

}

}

// Usage

<Greeting name="John" />

<strong>Example using the function component:</strong>

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

<Greeting name="John" />

23. What is the difference between state and props?

Aspect	State	Props
Mutability	Mutable (can be changed)	Immutable (read-only)
Ownership	Owned by the component	Passed from parent to child component
Usage	Manages dynamic data	Passes static or dynamic data to the child

24. Differentiate HTML and React event handling.

Feature	HTML	React
Syntax	onclick=”handleClick()”	onClick={handleClick}
Event Binding	Inline string in HTML	JavaScript function in JSX
Event Names	Lowercase (onclick)	CamelCase (onClick)
Event Object	Uses the global event object	Uses the synthetic event object
This Binding	Need to manually bind this	Automatically binds this

25. Why does React use className over class attribute?

React uses className instead of class to avoid conflicts with the reserved keyword class in JavaScript. Using className ensures that the JSX syntax remains compatible with JavaScript, preventing any syntax errors or ambiguities.

26. What is the React memo function?

React.memo is a higher-order component that memorises the rendered output of a function component, preventing unnecessary re-renders when the component’s props have not changed. This optimisation can improve performance, especially for components that do not need to re-render frequently.

Example:

const MyComponent = React.memo(function MyComponent(props) {

return <div>{props.name}</div>;

});

27. What is the React lazy function?

React.lazy is a function that allows you to load components lazily through code splitting. It helps reduce the initial bundle size by loading components only when they are needed. This function works with React’s Suspense component to provide a fallback while the lazy component is loading.

Example:

const OtherComponent = React.lazy(() => import('./OtherComponent'));

function MyComponent() {

return (

<React.Suspense fallback={<div>Loading...</div>}>

<OtherComponent />

</React.Suspense>

);

}

28. What are default props?

Props are values passed to the parent component when the parent component never assigns values. This allows you to check that if there is no props value, then it has a fallback value.

Example:

class MyComponent extends React.Component {

render() {

return <div>{this.props.name}</div>;

}

}

MyComponent.defaultProps = {

name: 'Guest',

};

// Usage

<MyComponent /> // Renders: <div>Guest</div>

29. When do you need to use refs?

Refs in React are used to directly access and manipulate DOM elements or React elements. They are useful in scenarios such as:

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

Example:

class MyComponent extends React.Component {

constructor(props) {

super(props);

this.myRef = React.createRef();

}

componentDidMount() {

this.myRef.current.focus();

}

render() {

return <input type="text" ref={this.myRef} />;

}

}

30. How do you print falsy values in JSX?

By default, Falsy values like 0 or false aren’t rendered in JSX. You can display the falsy values to print if you are using logical operators or using conditional statements.

Example:

function MyComponent({ value }) {

return (

<div>

{value === 0 ? 0 : value}

</div>

);

}

31. Explain the conditional rendering in React.

Conditional rendering in React allows you to render different components or elements based on certain conditions. You can use JavaScript operators like if, ternary operator, and logical &&` to achieve conditional rendering.

Example:

function MyComponent({ isLoggedIn }) {

return (

<div>

{isLoggedIn ? <h1>Welcome back!</h1> : <h1>Please sign in</h1>}

</div>

);

}

32. What are the components of a react-router?

The main components of React Router are:

Component	Description
BrowserRouter	Uses HTML5 history API to keep your UI in sync with the URL.
Route	Renders a specific component based on the URL path.
Link	Creates hyperlinks to navigate between routes.
Switch	Renders the first child Route or Redirect that matches the location.
Redirect	Redirects to a different route.
NavLink	A special version of Link that adds styling attributes when active.

33. What are the lifecycle methods of components?

React class components have several lifecycle methods, categorised by the phases of the component lifecycle:

Phase	Lifecycle Method	Description
Mounting	constructor	Initialises component state and binds methods.
	static getDerivedStateFromProps	Updates state based on props before rendering.
	render	Returns the JSX to render to the DOM.
	componentDidMount	Executes after the component is inserted into the DOM.
Updating	static getDerivedStateFromProps	Updates state based on props before rendering.
	shouldComponentUpdate	Determines whether the component should re-render.
	render	Returns the JSX to render to the DOM.
	getSnapshotBeforeUpdate	Captures some information before the DOM is updated.
	componentDidUpdate	Executes after the component updates are flushed to the DOM.
Unmounting	componentWillUnmount	Executes just before the component is removed from the DOM.

34. Explain this.setState function in React.

The this.setState function in React is used to update the state of a component. It merges the new state with the existing state and triggers a re-render of the component. setState can take an object or a function as its argument.

Example:

this.setState({ count: this.state.count + 1 });

Or using a function:

this.setState((prevState) => ({ count: prevState.count + 1 }));

35.What is the useEffect hook in react?

The useEffect hook is used to perform side effects in function components. It runs after the initial render and after every update. You can control when useEffect runs by providing a dependency array as the second argument.

Example:

import React, { useEffect } from 'react';

function MyComponent() {

useEffect(() => {

document.title = 'My Component';

}, []); // Runs only once after initial render

return <div>My Component</div>;

}

36. What is prop drilling and its disadvantages?

Prop drilling is the process of passing data from a parent component to a deeply nested child component through multiple intermediary components. This can make the code harder to maintain and understand, as many intermediate components need to pass the props without using them.

37. How can you create a React application and print Hello World?

To create a React application and print “Hello World,” follow these steps:

Install Create React App:

npx create-react-app my-app

cd my-app

Edit the App Component: Open src/App.js and modify it:

function App() {

return (

<div className="App">

<h1>Hello World</h1>

</div>

);

}

export default App;

Run the Application:

npm start

This will start the development server and open the app in the browser, displaying “Hello World”.

38. Explain the difference between controlled and uncontrolled components.

Feature	Controlled Component	Uncontrolled Component
Data Management	Managed by React state	Managed by the DOM
Control	Easier to control and validate	Less control over form data
Complexity	Requires more setup with state handlers	Simpler, less setup needed

Advanced-level ReactJS Interview Questions

39. What are Higher-Order components?

Higher-order components (HOCs) are functions that take a component and return a new component with additional props or behaviour. They are used for reusing component logic. HOCs do not modify the original component but wrap it, providing enhanced functionality.

Example:

function withEnhancement(WrappedComponent) {

return function EnhancedComponent(props) {

return <WrappedComponent {...props} additionalProp="value" />;

};

}

40. What are fragments?

Fragments are a way to hold many elements without adding extra nodes to the DOM. If you have a component that would like to return multiple elements but don’t want to wrap them inside a div or any other kind of container, then these components are useful.

Example:

function MyComponent() {

return (

<React.Fragment>

<h1>Title</h1>

<p>Description</p>

</React.Fragment>

);

}

41. Why are fragments better than container divs?

Fragments are better than container divs because they:

Avoid Extra Nodes: Do not add unnecessary DOM elements, keeping the DOM cleaner.
Improved Performance: Reduce the amount of DOM manipulation and rendering.
Simplify Styling: Prevent CSS issues related to extra div wrappers.

42. Differentiate stateful and stateless components.

Feature	Stateful Component	Stateless Component
State Management	Manages its own state	Does not manage state
Component Type	Usually, class components (or function components with hooks)	Usually function components
Use Case	Dynamic behaviour that changes over time	Static content or UI without dynamic behaviour

43. What are the core principles of Redux?

The core principles of Redux are:

Single Source of Truth: The entire application’s state is stored in a single object tree within a single store.
State is Read-Only: The only way to change the state is to dispatch an action, an object that describes what happened.
Changes are Made with Pure Functions: Reducers specify how the state changes in response to actions. Reducers must be pure functions that return the new state based on the previous state and action.

44. How to reset the state in Redux?

To reset the state in Redux, you can handle a reset action in your reducer. Here’s how:

Define the Reset Action:

const RESET_STATE = ‘RESET_STATE’;

Create the Reset Action Creator:

const resetState = () => ({

type: RESET_STATE,

});

Handle the Reset Action in the Reducer:

const initialState = { count: 0 };

function rootReducer(state = initialState, action) {

switch (action.type) {

case RESET_STATE:

return initialState;

// other cases

default:

return state;

}

}

Dispatch the Reset Action:

store.dispatch(resetState());

45. What is Redux DevTools?

Redux DevTools is a powerful tool for debugging Redux applications. It allows developers to:

Inspect every state and action payload.
Travel through the state’s history.
Revert or replay actions.
Persist state across sessions.
Integrate with Chrome and Firefox browser extensions for an enhanced debugging experience.

46. What are the differences between redux-saga and redux-thunk?

Feature	redux-saga	redux-thunk
Purpose	Handles complex asynchronous tasks using sagas	Handles simple asynchronous tasks using functions
Implementation	Uses generator functions to yield effects	Uses plain functions returning functions
Complexity	Suitable for complex logic and side effects	Suitable for simple async logic
Middleware	Requires redux-saga middleware	Requires redux-thunk middleware
Readability	More declarative and easier to test	More imperative and straightforward

47. Explain the custom hooks in React.

Custom hooks in React are functions that allow you to extract and reuse stateful logic across multiple components. They enable you to encapsulate common behaviour and side effects into a single reusable hook.

Example of a Custom Hook:

import { useState, useEffect } from 'react';

function useFetch(url) {

const [data, setData] = useState(null);

const [loading, setLoading] = useState(true);

useEffect(() => {

fetch(url)

.then((response) => response.json())

.then((data) => {

setData(data);

setLoading(false);

});

}, [url]);

return { data, loading };

}

// Usage

function MyComponent() {

const { data, loading } = useFetch('https://api.example.com/data');

if (loading) return <div>Loading...</div>;

return <div>{JSON.stringify(data)}</div>;

}

48. Explain the difference between useState and useReducer hooks.

Feature	useState	useReducer
Purpose	Manages simple state	Manages more complex state logic
Syntax	const [state, setState] = useState(initialState);	const [state, dispatch] = useReducer(reducer, initialState);
Updates	State updated using setState	State updated using dispatch with actions
Best For	Simple, local state	Complex state logic or multiple sub-values

49.Compare Redux and Context API for state management.

Feature	Redux	Context API
Purpose	Manages complex state and side effects	Passes data through the component tree
API	Provides a full-featured state management library	Part of React’s built-in API
Boilerplate	Requires more setup and boilerplate code	Less setup, simpler API
Middleware	Supports middleware like redux-thunk and redux-saga for async actions	No built-in support for middleware
Scalability	Suitable for large-scale applications	Best for smaller applications or limited state sharing
Performance	Better for performance with complex state logic	Can lead to performance issues if not used carefully in large apps
Learning Curve	Steeper learning curve due to its complexity	Easier to learn and implement

50. Explain Strict Mode in React.

Strict Mode in React is a development tool that helps identify potential issues in an application. It activates additional checks and warnings for its descendants. Strict Mode does not render any visible UI and does not affect the production build. It helps in:

Identifying components with unsafe lifecycle methods.
Warning about legacy string ref API usage.
Detecting unexpected side effects.
Checking for deprecated API usage.

51. What is React Material UI?

React Material UI is the popular React UI framework that complies with Google’s Material Design guidelines. It provides pre-built, customisable UI components that will allow developers to build the modern, responsive user interfaces in simple ways. The components are well-designed and offer a solid look and feel across different devices and platforms.

Key Features:

Pre-Built Components: It includes components like buttons, cards, dialogs, and more.
Theming: It allows for easy customisation of the overall look and feel through themes.
Responsive Design: It ensures components work well on different screen sizes.

Conclusion

In this blog, we have gone through the most wide-ranging set of ReactJS interview questions as basic as they can get up to the advanced levels. We explored topics of the utmost importance like JSX, components, state management, and hooks to become a solid basis for beginners and serious developers alike. Then we dived deeper into more complex topics like Redux, custom hooks, and performance optimisation techniques. Understanding and mastering these will ensure you ace any ReactJS interview. If you want to explore ReactJS in detail, try the Certificate Program in DevOps & Cloud Engineering offered by Hero Vired in collaboration with Microsoft.

FAQs

What is React?

React is a JavaScript library for building user interfaces, particularly single-page applications. This allows developers to create reusable UI components.

What is JSX?

JSX is a syntax extension that allows mixing HTML with JavaScript. It makes writing React components more intuitive and easier to read.

What is Virtual DOM?

The Virtual DOM is a lightweight copy of the actual DOM. React updates the Virtual DOM first, calculates changes, and then efficiently updates the real DOM, improving performance.

What is the use of key prop in React?

The key prop uniquely identifies elements in a list. It helps react and optimise rendering by tracking elements when they are added, removed, or reordered.

What are error boundaries in React?

The error boundaries are components that catch JavaScript errors in their child components, log those errors, and display a fallback UI instead of crashing the entire app.

Updated on December 6, 2024

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