40+ ReactJs Interview Questions & Answers – 2025

ReactJS is one of the most widely used JavaScript libraries for developing dynamic and responsive user interfaces. If you are preparing for a ReactJS interview, it’s important to understand the fundamentals of React, as many interview questions will focus on core concepts.

In this blog, we’ll cover some of the common ReactJS interview questions for beginners, helping you get ready for your next interview.

Why are ReactJS Skills in High Demand?

As mentioned, React is developed by Meta, so React skills are in high demand. React ensures the development of fast, scalable, and interactive web applications. Businesses building advanced web applications opt for it for its component-based architecture, virtual DOM, and strong community support. React because of its flexibility, compatibility, and efficiency enhances the popularity of frameworks like Next.js.

Basic ReactJS Interview Questions

Q1. What are the key features of React?

Here are some of the key features of React:

• JSX (JavaScript XML): JSX allows you to write HTML elements and components in JavaScript code, which React then compiles to create virtual DOM elements.
• Components: React centres the concept of components. A component is a reusable, self-contained unit that encapsulates its own structure, behavior, and appearance.
• Virtual DOM: React uses something known as a virtual DOM to enhance performance. Instead of updating the real DOM every time the state changes. To reduce modifications and increase speed, React initially modifies the virtual DOM before comparing it to the real DOM.
• State and Props: React components have state (data that can change) and props (data passed from parent to child components).
• Unidirectional Data Flow: React follows one-way data flow, meaning data is passed from parent to child components, making it easier to debug and manage.

Q2. What is JSX?

JSX stands for JavaScript XML. Writing HTML elements in JavaScript code is made possible by this syntax enhancement. JSX makes React code more readable and concise, as it closely resembles HTML. React components use JSX to define how the UI should look.

Here’s an example of JSX syntax:
const element = <h1>Hello, world!</h1>;
While JSX looks like HTML, it’s actually JavaScript, so it can be compiled into JavaScript functions.

Q3. What is the Virtual DOM?

The Virtual DOM (VDOM) is a lightweight copy of the actual DOM. It is a concept implemented by React to optimize UI updates. When the state of an object changes, React first updates the Virtual DOM. It then determines the most effective method of altering the real DOM by comparing the current Virtual DOM with an earlier version. This process, called reconciliation, makes React programs quick and effective.

Q4. Why use React instead of other frameworks, like Angular?

Businesses and developers’ go-to-choice is React rather than Angular.

• While code for JavaScript apps tends to become complex very fast, React requires less coding and more functionality, making it easier to build dynamic web applications.
• Web apps run quicker because of React’s use of virtual DOM. Unlike traditional web applications, which update every component in the real DOM, virtual DOM compares its prior state and only updates the components whose states have changed.
• Any React application’s building elements are called components, and each application often comprises several components. Development time can be significantly decreased by reusing these components, which have logic and controls.
• The data flow in React is unidirectional. Child components are frequently built in parent components while creating a React application. Debugging errors and identifying the issue in an application is made easy because the data flows in a single direction.
• A Chrome extension for debugging React apps has been made available by Facebook. This speeds up and simplifies the debugging process for React web applications.

Q5. What is the difference between the ES6 and ES5 standards?

EC5 (ECMAScript 5) and ES6 (ECMAScript 2015) are the two different versions of JavaScript with some key differences like:

• EC5 uses var , meanwhile ES6 uses let and const for better scoping.
• ES6 uses => syntax for shorter and readable function expressions.
• ES6 provides a class-based syntax (class and extends) for object-oriented programming.
• ES6 allows string interpolation using backticks (‘ ‘), unlike ES5’s string concatenation.
• ES6 uses import and export for modular code, meanwhile ES5 relies on libraries like CommonJS.
• ES6 allows extracting values from arrays/objects into variables in a concise way.
• ES6 adds Promise for handling asynchronous operations, improving readability over callbacks.

Q6. How do you create a React app?

If you want to create a React app, follow the steps below:

Install Node.js on your system.

Open a terminal and run:
npx create-react-app my-app

Or, if npx is not available:

npm install -g create-react-app
create-react-app my-app

Go to the project directory:
cd my-app
Start the development server:
npm start

Q7. What is an event in React?

An event is user interaction in React such as a click, key press, or form submission, that triggers a response in the application. React events are nearly similar to native DOM events. But are wrapped in SyntheticEvent, which provides cross-browser compatibility and better performance.

Q8. What are synthetic events in React?

Synthetic events are wrapper objects around native browser events, providing a consistent API across different browsers. They also help in improving performance and make sure that event’s handle work the same way in all environments.

Q9. Explain how lists work in React.

Lists usually display multiple items dynamically typically by mapping over an array and rendering components for each item in React.

React uses the .map() function to iterate over an array and generate UI elements.

Example of Rendering Lists

function NameList() {
const names = ["Alice", "Bob", "Charlie"]; // Fixed quotes
return (
<ul>
{names.map((name, index) => (
<li key={index}>{name}</li> // Using index as key (okay for static lists)
))}
</ul>
);
}
export default NameList;


Q10. Why is there a need to use keys in Lists?

Keys help lists in identifying elements uniquely and ensure to allow React to efficiently update and re-render components when the list changes. Some of the benefits of using keys are optimized rendering, avoids unnecessary re-renders, and maintains component state.

Q11. What are forms in React?

Forms in React are used to collect user input. Unlike traditional HTML forms, React handles form data using controlled components, where form elements are controlled by the component’s state. There are two types of forms in React:

Controlled Form in React

import { useState } from "react"; // Fixed quotes
function ControlledForm() {
const [name, setName] = useState(""); // Fixed quotes
const handleChange = (event) => {
setName(event.target.value); // Updating state on input change
};
const handleSubmit = (event) => {
event.preventDefault();
alert(`Submitted Name: ${name}`);
};
return (
<form onSubmit={handleSubmit}>
<input type="text" value={name} onChange={handleChange} /> {/* Fixed quotes */}
<button type="submit">Submit</button> {/* Fixed quotes */}
</form>
);
}
export default ControlledForm;

Uncontrolled Component

import { useRef } from "react"; // Fixed quotes
function UncontrolledForm() {
const inputRef = useRef(); // Correct useRef initialization
const handleSubmit = (event) => {
event.preventDefault();
alert(`Submitted Name: ${inputRef.current.value}`);
};
return (
<form onSubmit={handleSubmit}>
<input type="text" ref={inputRef} /> {/* Fixed quotes */}
<button type="submit">Submit</button> {/* Fixed quotes */}
</form>
);
}
export default UncontrolledForm;


Q12. How do you create forms in React?

Forms in React are typically created using controlled components, where the form elements are controlled by React state.

Controlled Components

The form elements (input, textarea, select) are controlled by React state.
Changes in the form update the component’s state using useState.

import { useState } from "react"; // Fixed quotes
function ControlledForm() {
const [name, setName] = useState(""); // Fixed quotes
const [email, setEmail] = useState(""); // Fixed quotes
const handleSubmit = (event) => {
event.preventDefault();
alert(`Name: ${name}, Email: ${email}`);
};
return (
<form onSubmit={handleSubmit}>
<input
type="text"
placeholder="Enter Name"
value={name}
onChange={(e) => setName(e.target.value)}
/>
<input
type="email"
placeholder="Enter Email"
value={email}
onChange={(e) => setEmail(e.target.value)}
/>
<button type="submit">Submit</button>
</form>
);
}
export default ControlledForm;

Uncontrolled Components

Instead of storing the input value in state, we use useRef to access the form values.

import { useRef } from "react"; // Fixed quotes
function UncontrolledForm() {
const nameRef = useRef();
const emailRef = useRef();
const handleSubmit = (event) => {
event.preventDefault();
alert(`Name: ${nameRef.current.value}, Email: ${emailRef.current.value}`);
};
return (
<form onSubmit={handleSubmit}>
<input type="text" placeholder="Enter Name" ref={nameRef} />
<input type="email" placeholder="Enter Email" ref={emailRef} />
<button type="submit">Submit</button>
</form>
);
}
export default UncontrolledForm;


Q13. How do you write comments in React?

Developers can write comments in JSX and JavaScript using different syntaxes.

Single-Line Comments in JSX

Since JSX looks like HTML but is actually JavaScript, you must use {/* */} inside JSX.

function App() {
return (
<div>
{/* This is a single-line comment in JSX */}
<h1>Hello, React!</h1>
</div>
);
}
export default App;

Multi-Line Comments in JSX

For multi-line comments, wrap them inside {/* */} as well.

function App() {
return (
<div>
{/*
This is a multi-line comment in JSX
It spans multiple lines
*/}
<h1>Welcome to React </h1>
</div>
);
}
export default App;

Comments in JavaScript

Use // for single-line comments and /* */ for multi-line comments in JavaScript.

function App() {
// This is a single-line comment in JavaScript
/*
This is a multi-line comment in JavaScript
It works outside JSX
*/
return <h1>Hello, World!</h1>;
}
export default App;

Comments Inside Return Statements

If you need comments inside a return statement but outside JSX, use JavaScript syntax.

function App() {
const message = "Hello, React!"; // JavaScript comment
return (
<div>
{/* JSX comment inside return */}
<h1>{message}</h1>
</div>
);
}


Q14. What is an arrow function and how is it used in React?

An arrow function is a shorter syntax for writing functions in JavaScript. It was introduced in ES6 and is commonly used in React for event handling and functional components.

Syntax of an Arrow Function

const functionName = (parameters) => {
// Function body
};

If the function has only one expression, you can write it in a single line without {} and return:
const add = (a, b) => a + b; // Implicit return
How are Arrow functions used in React?

In functional components:
Arrow functions are commonly used to define React components.

const Greeting = () => {
return <h1>Hello, React!</h1>;
};
export default Greeting;

In event handling:
Arrow functions are used to define event handlers inside components.

const ButtonClick = () => {
const handleClick = () => alert("Button Clicked!"); // Fixed quotes
return <button onClick={handleClick}>Click Me</button>;
};
export default ButtonClick;

Using Arrow Functions Inside JSX:
You can pass arrow functions directly in event handlers.

const names = ["Alice", "Bob", "Charlie"]; // Fixed quotes
const NameList = () => (
<ul>
{names.map((name, index) => (
<li key={index}>{name}</li> // Key should be unique when possible
))}
</ul>
);
const App = () => (
<div>
<button onClick={() => alert("Clicked!")}>Click Me</button> {/* Fixed quotes */}
<NameList />
</div>
);
export default App;


Q15. How is React different from React Native?

• Purpose: Reacts are used to build web applications whereas react native are used to build mobile applications.
• Platform: React runs in a web browser and react native runs on mobile devices.
• Rendering: React used DOM for rendering UI. React native uses native components.
• Styling: React uses CSS, SCSS, Styled Components and React native uses StyleSheet API (similar to CSS but with JavaScript)
• Navigation: React uses React Router for navigation and React native uses React Navigation or other libraries for mobile navigation
• Components: React uses HTML elements (<div>, <h1>, <button>). React native uses native mobile UI components (<View>, <Text>, <TouchableOpacity>).
• APIs: React uses Web APIs (like Fetch, LocalStorage). React native uses APIs (like Camera, GPS, AsyncStorage)
• Performance: React performance depends on browser rendering whereas React native is Closer to native performance using bridge communication.

Q16. How is React different from Angular?

There have been key differences between React and Angular:

• Type- React is a library for UI components whereas Angular is a full fledged framework.
• Language- React uses the language of JavaScript (mostly with JSX). Angular uses TypeScript language.
• Architecture- React has component- based (only UI layer) architecture whereas Angular also has component based but with MVC structure.
• DOM handling- React uses Virtual DOM for fast updates. Angular uses Real DOM (but optimized with change detection).
• Data Binding- React has One-way data binding (unidirectional) whereas Angular has Two-way data binding (bidirectional).
• Performance- React performance has faster updates with Virtual DOM. Angular performance comparatively has slower due to Real DOM updates.
• Learning Curve- React is easier to learn, especially for beginners. Comparatively Angular has a steeper learning curve due to its complexity.
• State Management- React uses third-party tools like Redux, Context API. Angular has built-in state management with RxJS.
• Routing- React requires a React Router for navigation. Whereas Angular has a built- in @angular/router module.
• Size- React is lightweight and flexible. Angular is larger in size due to built-in features.

Q17. What are React hooks?

React hooks are functions that allow you to use React state and lifecycle features in functional components. They were introduced in React 16.8 to provide a simpler and more intuitive way to manage state and side effects in functional components.

Some common hooks include:

import { useState } from "react";
const Counter = () => {
const [count, setCount] = useState(0); // Initializes state with 0
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>Click Me</button>
</div>
);
};
export default Counter;


Q18. What is useState in React, & How Does It Work?

You can add state to functional components with the useState React hook. The current state value and a function to change that value are the two elements of the array that is returned.

Example:

import { useState } from "react";
function Counter() {
const [count, setCount] = useState(0);
function increment() {
setCount(count + 1);
}
return <button onClick={increment}>Clicked {count} times</button>;
}
export default Counter;

The state variable in this example is count, and the function is setCount. used to update its value.

Q19. What is Memoization in React?

Memoization is a React optimization technique that helps increase performance by reusing the results of costly function calls while the inputs stay the same and storing the results. Memoization helps React apps run faster and smoother by avoiding unnecessary computations and re-renders.

Q20. What is Prop Drilling and how do you avoid it?

Prop Drilling happens when props are passed down through multiple levels of components, even though only a deeply nested child component actually needs them. This makes the code hard to manage and maintain.

ReactJS Interview Questions For Freshers

Q1. What is React?

React is a JavaScript library created by Facebook for crafting user interfaces, especially single-page applications (SPAs). It allows developers to build reusable UI components that dynamically update based on changes in state or data, offering an efficient solution for developing interactive and responsive web applications.

Q2. What are props in React?

Props (short for “properties”) are the mechanism through which data is passed from a parent component to a child component. Props are immutable (i.e., they cannot be changed by the child component). They are used to configure a child component and allow for communication between components.

For example:

import React from "react";
import ReactDOM from "react-dom";
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}
// Rendering the component inside root
ReactDOM.createRoot(document.getElementById("root")).render(
<Greeting name="John" />
);

In this example, name is passed as a prop to the Greeting component, and it will be displayed as “Hello, John!”.

Q3. What is the difference between state and props in React?

• State: State refers to the internal data of a component that can change over time. It is mutable and is used to manage changes in the component’s behavior or appearance. When the state of a component changes, the component re-renders to reflect the updated state.
• Props:Props are immutable and are employed to transfer information between parent and child components. A component cannot modify its own props, but it can use props to render content.

In short, state is managed within a component,while components of the props are transmitted from parent to kid.

Q4. What is a component in React?

A component in React is a JavaScript function or class that accepts input (called props) and returns React elements describing what should appear on the screen. Components can be either:

Functional Components: These are simpler components written as JavaScript functions. They can use hooks like useState and useEffect to manage state and lifecycle events.

Example:

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

Class Components: These are written as ES6 classes and extend the React.Component class. They have lifecycle methods and can manage state.

Example:

import React from "react";
class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}
export default Welcome;


Q5. What is the difference between functional and class components in React?

Functional Components: These simple JavaScript functions take props as arguments and return React elements. They are stateless by default but can use hooks to manage state and lifecycle methods in modern React.

Example:

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

Class Components: These are ES6 classes that extend React.Component. They can have state and lifecycle methods. Class components were the traditional way to write React components, but with the introduction of hooks, functional components are now preferred.

Example:

import React from "react";
class Hello extends React.Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}
export default Hello;


Q6. What is the difference between componentDidMount() and useEffect()?

componentDidMount(): This is a lifecycle method in class components. It is called after a component is mounted (added to the DOM) and is used for one-time setup like fetching data or subscribing to events.

Example:

import React from "react";
class MyComponent extends React.Component {
componentDidMount() {
console.log("Component mounted");
}
render() {
return <h1>Hello</h1>;
}
}
export default MyComponent;

useEffect(): This is the hook used in functional components to handle side effects like fetching data or DOM manipulation. It serves a similar purpose to componentDidMount() but in a more flexible way.

Example:

import { useEffect } from "react";
function MyComponent() {
useEffect(() => {
console.log("Component mounted");
}, []); // Runs only once when the component mounts

return <h1>Hello</h1>;
}
export default MyComponent;


Q7. What are keys in React, and why are they important?

Keys are distinct identifiers that assist React in monitoring of components and efficiently updating the UI. They are especially important when rendering lists of elements. React uses keys to optimize re-rendering and reconciliation of the Virtual DOM with the actual DOM.

Example:

const items = ["Apple", "Banana", "Orange"];
function ItemList() {
return (
<ul>
{items.map((item, index) => (
<li key={index}>{item}</li>
))}
</ul>
);
}
export default ItemList;

In this case, each <li> element needs a key to help React distinguish between elements in the list.

Q8. What is a controlled component in React?

A component that contains form data is said to be controlled (such as input fields) is controlled by React state. In a controlled component, the value of the input is bound to a state variable, and any changes to the input are handled by a function that updates the state.

Example:

import { useState } from "react";
function MyForm() {
const [inputValue, setInputValue] = useState("");
const handleChange = (event) => {
setInputValue(event.target.value);
};
return (
<form>
<input type="text" value={inputValue} onChange={handleChange} />
</form>
);
}
export default MyForm;

Here, the value of the input field is controlled by the inputValue state.

Q9. What are React fragments?

React fragments allow you to group a list of children without adding extra nodes to the DOM. This is useful when you want to return several components’ elements without enclosing them in a parent element like a <div>.

Example:

function List() {
return (
<>
<h1>Item List</h1>
<ul>
<li>Item 1</li>
<li>Item 2</li>
</ul>

);
}
export default List;

Here, we are returning two elements without adding an extra wrapping element to the DOM.

Q10. What are defaultProps in React?

defaultProps is used to specify default values for props if they are not passed by the parent component. This is useful to ensure that a component has default values for props, preventing errors when the parent doesn’t provide them.

Example:

function Greeting({ name }) {
return <h1>Hello, {name}!</h1>;
}
Greeting.defaultProps = {
name: "Guest",
};
export default Greeting;

If the name prop is not passed, it defaults to ‘Guest’.

Q11. What is prop drilling in React?

The technique of using intermediary components to transfer information from a parent component to a child component that is deeply nested is known as prop drilling.   When multiple levels of components are involved, this might make maintaining the code challenging:

Example:

function Parent() {
const name = "John";
return ;
}
function Child({ name }) {
return ;
}
function GrandChild({ name }) {
return <h1>Hello, {name}!</h1>;
}
export default Parent;

To avoid prop drilling, you can use tools like React Context API or state management libraries like Redux to share data without passing props through many layers.

Q12. What is the Context API in React?

The React Context API is a way to share data across the component tree without having to pass props down manually at every level. It’s useful for global data like themes, user authentication, or language preferences.

To use Context:

Create a Context:

import React from "react";
const ThemeContext = React.createContext("light");
export default ThemeContext;

Provider:

function App() {return (
<ThemeContext.Provider value=”dark”>
<Child />
</ThemeContext.Provider>
);
}

Consume Context:

import React, { useContext } from "react";
import ThemeContext from "./ThemeContext";
function Child() {
const theme = useContext(ThemeContext);
return <h1>Current theme: {theme}</h1>;
}
export default Child;

The Context API is a great alternative to prop drilling for passing data through deeply nested components.

Q13. What is useState in React?

You can add state to functional components with the useState React hook. The current state value and a function to change that value are the two elements of the array that is returned.

Example:

import React, { useState } from "react";
function Counter() {
const [count, setCount] = useState(0);
function increment() {
setCount(count + 1);
}
return <button onClick={increment}>Clicked {count} times</button>;
}
export default Counter;

The state variable in this example is count, and the function is setCount. used to update its value.

Q14. How Would You Describe React JS to Someone Without any Programming Background?

Imagine that a restaurant’s website menu changes every time the chef makes changes in one dish. While with React only that particular dish information will be changed, the rest of the menu remains untouched. Similarly, React JS works the same for web applications. It helps the developers build web pages using small, reusable components instead of writing everything from scratch every time.

Q15. List the Drawbacks and Limitations of React?

React is still the first choice among the developers for modern web development due to its flexibility and performance. However, as everything has limitations, React too has some of the drawbacks and limitations:

• Freshers may face issues in learning about JSX, state management, and hooks.
• With the rapidly growing technology, React also updates quickly. So it is somehow difficult for the developers to stay updated.
• React affects SEO unless SSR (server-side rendering) is used with Next.js.
• React is not a full fledged framework, developers have to rely on third-party tools for routing, state management, and form handling.
• React has complex state management as managing large-scale app states can be quite difficult. One can require tools like Redux, Zustand, or Recoil.
• Excessive re-renders or poor handling of components can impact performance, requiring optimizations like React.memo(), useMemo(), and useCallback().
• JSX can be confusing for developers who are used to traditional HTML and JS.
• React is for building web apps. But to build mobile apps, developers need React Native, which has a different learning.

Q16. State the use of webpack

Webpack is a module bundler for JavaScript applications. It helps manage and optimize project files like JavaScript, CSS, images, and more. There have been key uses in React:

• Combines bundles of files efficiently.
• Webpack improves React’s performance by minimizing JavaScript & CSS. It removes unused code (tree shaking), and optimizes assets.
• Webpack supports ES6 imports that allows developers to split code efficiently.

Define Custom Hooks

Custom Hooks is a function beginning with ‘use’. It allows React to extract and share logic between components without duplicating code.

Q17. State the different side effects of the React component

A side effect in React refers to anything that affects something outside the component’s scope

(e.g., fetching data, modifying the DOM, or interacting with APIs).

Certainly there are common types of side effects in React listed below:

• Fetching Data (API Calls)
• Updating the DOM Manually
• Setting Up Subscriptions
• Timers & Intervals
• Event Listeners
• Local Storage or Session Storage Updates
• Side Effects in Animations

Q18. What are the lifecycle steps in React?

React’s lifecycle consists of initialization, property or state changes, and destruction. Creating components is the first stage in initialization; re-rendering them during a state or property change is the second. Destroying entails taking elements out of the user interface and cleaning up as needed.

Q19. What do you understand by error boundaries?

An Error Boundary is a special React component that catches JavaScript errors in its child components and prevents the entire app from crashing. Instead, it displays a fallback UI to users when an error occurs.

Q20. State the rules to follow when using React Hooks

React Hooks have specific rules to ensure they work correctly and avoid bugs. Here are the two main rules:

• Only Call Hooks at the Top Level
  This means that calling Hooks at the top level. Don’t call hooks inside loops, conditions, or nested functions.
• Only Call Hooks from React Functions
  This means that using hooks in functional components and custom hooks. Don’t use hooks inside regular JavaScript functions or class components.

ReactJS Interview Questions for Intermediate

Q1. What are the components in React?

A component in React is a reusable piece of UI that helps break a webpage into smaller, manageable parts. This makes the code organized, easy to reuse, and simple to maintain.

There are two types of components:

• Functional Components- Functional components are written as JavaScript functions are more concise and easier to test. Functional components use React hooks like useState, useEffect, etc.
• Class Components- Class components are written as JavaScript classes are More complex, replaced by hooks in modern React. Class components uses this.state and lifecycle methods (componentDidMount, componentDidUpdate).

Q2. What is the use of render() in React?

The render() method in React is used to display UI on the screen. It returns JSX (HTML-like code) that tells React what to show. In class components, render() is required and must return a single JSX element. Functional components, don’t use render(). Instead, they return JSX directly.

Q3. How do you implement state in React?

State in React is used to store and manage dynamic data. You can implement state in two ways:

• Using useState in Functional Components.
• Using this.state in Class Components.

Q4. How do you update the state of a component?

You can update the state in React using:

• setState() in class components.
• useState setter function in functional components.


class App extends React.Component {
constructor() { super();
}
this.state = {
}:
message: "Welcome to Akal"
this.button Press = this.button Press.bind(this);
button Press() {
this.setState({
});
}
message:"The best place to learn"
render() {
return ( <div>
<h1>{this.state.msg)</h1>
<button onClick={this.button Press}>Click Me!</button>
</div>


Q5. How do you pass props between components?

Props is short for properties that allow passing data from one component to another in React. Props make components reusable and dynamic.

• Props are passed as attributes in JSX. The child component receives props as a function parameter.
• Props can hold different data types like strings, numbers, objects, or functions.
• Instead of props.name, we can destructure props for better readability.
• Props can also be functions, allowing a child to send data back to the parent.

Q6. What is a higher-order component in React?

Higher Order Component [HOC] in React is a function that takes a component and returns a new enhanced component. This allows us to reuse component logic and add functionality without modifying the original component.

Q7. How can you embed two or more components into one?

In React, you can combine multiple components into a single component by nesting them inside a parent component.

• You can place multiple components inside a parent component and return them together.
• React requires components to return a single parent element, but you can avoid unnecessary
  tags using fragments (<>…).
• You can pass components as props to make them more reusable.


class App extends React.Component {
render (){
return ( <div>
<h1>Hello</h1>
</div>
)
class Simple extends React.Component {
render (){
return (
<h1>Akal</h1>
>
}
ReactDOM.render(
):
<app/>, document.getElementById('index')


Q8. What are the differences between class and functional components?

• Definition- Class components have ES6 classes that extend React.Component. Functional components are simple JavaScript functions.
• State Management- Class components uses this.state and this.setState(). Whereas functional components uses useState hook.
• Lifecycle Methods- Class components uses lifecycle methods (componentDidMount, etc). Comparatively functional components uses useEffect hook.
• Syntax Complexity- Class components are more complex (needs this). Functional components are simpler and easier to read.
• Performance- Class components are slightly heavier than due to class overhead. Functional components are faster and optimized.
• Hooks Support- Class components support no hooks whereas functional components support hooks.
• Code Reusability- Class components are less reusable due to lifecycle methods. Whereas functional components are more reusable with Hooks & HOCs.

Q9. Explain the lifecycle methods of components.

React components have different lifecycle phases, each with specific lifecycle methods that run at different stages of a component’s life. Lifecycle methods exist only in class components. Lets understand below the different lifecycle phase and methods:

• getInitialState(): This initial state is set up before the component is created.
• componentDidMount(): In this state is executed after the component is added to the DOM. Used for API calls, event listeners, or setting up subscriptions.
• shouldComponentUpdate(): This phase decides if a component should re-render. Returns true (re-render) or false (prevent re-render) based on conditions.
• componentDidUpdate(): This phase runs after component updates. Used for updating the DOM or making API calls after state/props change.
• componentWillUnmount(): This phase runs before a component is removed from the DOM. Used for cleaning up timers, event listeners, or subscriptions.

Q10. What are React Hooks and why were they introduced?

React hooks were introduced in React 16.8 to allow functional components to use state and lifecycle features without converting them into class components. Hooks like useState, useEffect, useReducer, and useContext provide a simpler and more reusable way of managing state, side effects, and context in functional components.

Q11. What is the difference between useEffect and componentDidMount?

• componentDidMount is a lifecycle method in class components that runs once after the component mounts.
• useEffect is the functional component equivalent. By providing an empty dependency array ([]), you can simulate the behavior of componentDidMount because it will run once after the initial render.

Q12. What is useCallback in React?

useCallback is a hook that returns a memoized version of the callback function. It is used to prevent unnecessary re-creations of functions, which can improve performance in some cases, especially when passing functions as props to child components.

Example:

const memoizedCallback = useCallback(() => {// callback logic}, [dependencies]);


Q13. What is useMemo in React?

useMemo is a hook that returns a memoized value. It helps optimize performance by preventing expensive calculations from being re-executed unless specific dependencies change. It’s useful when you need to optimize rendering performance, especially for calculations that depend on expensive logic.

Example:

const memoizedValue = useMemo(() => expensiveCalculation(), [dependencies]);


Q14. What are Higher-Order Components (HOCs) in React?

A Higher-Order Component (HOC) is a function that takes a component and returns a new component with enhanced functionality. HOCs are used to share common logic between components without repeating code.

Example:

function withLoading(Component) {return function WrappedComponent({ isLoading, ...props }) {if (isLoading) {
return <div>Loading...</div>;
}
return <Component {...props} />;
};
}


Q15. In React, what are the controlled and uncontrolled components?

Controlled Components: In controlled components, the form element’s value is controlled by React state. The value is set via the state, and updates are handled by event handlers.

Example:

function ControlledComponent() {const [value, setValue] = useState('');const handleChange = (event) => {
setValue(event.target.value);
};
return <input type="text" value={value} onChange={handleChange} />;
}

Uncontrolled Components: In uncontrolled components, the form elements manage their own state, and you access their values using a ref instead of React state.

Example:

function UncontrolledComponent() {const inputRef = useRef();const handleSubmit = () => {
alert(inputRef.current.value);
};
return (
<div>
<input ref={inputRef} />
<button onClick={handleSubmit}>Submit</button>
</div>
);
}


Q16. What is React Router and how do you implement it in a React application?

React Router is a library used for handling routing in React applications. It allows for navigation between different views or pages within the app without reloading the entire page (client-side routing).

Basic implementation:

Install React Router: npm install react-router-dom

Define routes and navigation in your app:

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';function App() {return (
<Router>
<nav>
<Link to="/">Home</Link>
<Link to="/about">About</Link>
</nav>
<Switch>
<Route path="/" exact component={Home} />
<Route path="/about" component={About} />
</Switch>
</Router>
);
}


Q17. What is React Context API and how do you use it?

The React Context API is a way to exchange information throughout the component tree without requiring manual prop passing down at each level.  It’s useful for global data like themes, user authentication, or language preferences.

Basic usage:

Create Context:
const MyContext = React.createContext();
Provider:

function App() {return (<MyContext.Provider value="someValue">
<Child />
</MyContext.Provider>
);
}

Consumer:

function Child() {const value = useContext(MyContext);return <h1>{value}</h1>;
}


Q18. What is the shouldComponentUpdate lifecycle method and how does it relate to performance?

The shouldComponentUpdate method in class components is used to optimize performance. It allows you to prevent unnecessary re-renders by returning false when React determines that the component does not need to be updated.

Example:

class MyComponent extends React.Component {shouldComponentUpdate(nextProps, nextState) {
// Return false if props or state hasn't changed
return nextProps.value !== this.props.value;
}
}

In functional components, React.memo is used to achieve similar functionality.

React JS Interview Questions for Experienced

Q1. Describe the lifting state up in React

Lifting state up is a technique in React. This means moving state from a child component to a common parent component to share data between multiple child components.

Q2. How do we avoid binding in React JS?

In React JS, event handlers need to be bound to this, or they won’t have access to the components properties. However, binding in every render can affect performance. Ways to avoid binding in React JS:

• Use arrow functions in class properties.
• Use arrow functions in the render() method.
• Bind in the constructor.
• Use Functional Components with Hooks.

Q3. What are the different types of Hooks in React?

There are three types of Hooks in React:

• Basic Hooks which are commonly used.
• Additionally Hooks are for advanced cases.
• Custom Hooks extract and reuse logic across multiple components.

Q4. State the different lifecycle methods in the updating phase

Lifecycle methods in updating phase:

• shouldComponentUpdate()
• render()
• getSnapshotBeforeUpdate()
• componentDidUpdate()

Q5. Outline the Functions of Higher-Order Components

The functions of Higher-Order Components are:

• Code Reusability
• Props Manipulation
• Conditional Rendering
• State Management
• Enhancing Component Behavior

Q6. What is the strict mode in React?

React’s Strict Mode is a development tool that helps in identifying possible issues with an application. It allows for additional checks and warnings to enhance code quality without affecting the final product.

Q7. What do you understand by three dots in React in the syntax

The three dots (…) in represent the spread operator (…) in JavaScript.

Q8. What is (SSR) server-side rendering in React, and how is it different from client-side rendering (CSR)?

• Server-Side Rendering (SSR): React components are rendered to HTML on the server and can be sent to the client as a fully developed HTML page. The client then hydrates the application to make it interactive.
• Advantages:
  • Faster initial load time, especially for SEO.
  • Better performance on slower devices.
• Client-Side Rendering (CSR): The browser downloads a minimal HTML page and then loads the JavaScript bundle to render the application on the client-side.
  Advantages:
  • After the initial load, subsequent interactions are faster.
  • Less load on the server.

Example Tools: Next.js is a popular framework for SSR with React.

Q9. When would you use React Portals and what are they?

React Portals allow you to render components outside the main DOM hierarchy. This is helpful in situations such as popovers, tooltips, and modals, where the component should not be constrained by the parent component’s CSS or DOM structure.

Example:

ReactDOM.createPortal(<div>Modal Content</div>,
document.getElementById('modal-root')
);


Q10. What is React’s Reconciliation process?

Reconciliation is React’s algorithm for updating the DOM. React:

1. Generates a new Virtual DOM tree whenever the state or props of a component change.
2. Compare the new Virtual DOM with the previous one.
3. Calculates the minimal set of changes required.
4. Updates the real DOM efficiently.

Key concepts:

• Keys: Help React identify which items in a list have changed.
• Diffing Algorithm: Optimized algorithm for comparing trees by breaking them into smaller pieces.

4. What is code-splitting in React, and how do you implement it?

Code-splitting allows you to split your application into more manageable packages that may be loaded whenever needed, reducing the initial load time.

Implementation:

Use dynamic import():const LazyComponent = React.lazy(() => import('./LazyComponent'));function App() {
return (
<React.Suspense fallback={<div>Loading...</div>}>
<LazyComponent />
</React.Suspense>
);
}

Use a bundler like Webpack with tools like React’s React.lazy or libraries like loadable-components.

Q11. What are React Fiber and its goals?

React Fiber is the reimplementation of React’s reconciliation algorithm. It introduced incremental rendering, allowing React to split rendering work into chunks and prioritize updates, improving performance for complex applications.

Key Goals:

• Concurrency: Breaking rendering work into smaller units.
• Better responsiveness: Prioritizing updates like user interactions over less critical tasks.
• Pausing and resuming rendering: Allowing React to pause rendering for higher-priority updates.

Q12. How do you optimize performance in React applications?

Techniques:

1. Memoization: Use React.memo, useMemo, and useCallback to prevent unnecessary re-renders or recomputations.
2. Lazy Loading: Implement code-splitting and dynamic imports.
3. Virtualization: Use libraries like react-window or react-virtualized for efficiently rendering large lists.
4. Avoid Inline Functions: Define functions outside of JSX to avoid re-creation.
5. Use Key Properly: Ensure unique and stable keys for list elements.
6. State Management: Keep state local when possible and avoid unnecessary global state.

Q13. What is React Concurrent Mode?

Concurrent Mode is an experimental feature that allows React to work on multiple tasks at the same time. It improves the user experience by making React apps more responsive.

Key Features:

1. Suspense: Pause rendering until data is loaded.
2. Transitions: Mark updates as non-urgent to prioritize user interactions.

Example:

const [isPending, startTransition] = useTransition();startTransition(() => {// Non-urgent state update
setFilter(value);
});


Q14. What is the difference between React.memo and useMemo?

React.memo: Prevents a component from re-rendering if its props have not changed. It’s a higher-order component.

Example:
const MemoizedComponent = React.memo(MyComponent);
useMemo: Memoizes a value to avoid re-calculating it unless its dependencies change. It’s a hook used inside functional components.

Example:
const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

Q15. How do you handle state management in large React applications?

State management approaches:

1. React Context API: Suitable for moderate state sharing.
2. Third-party libraries:
   • Redux: Centralized state management with strict state update rules.
   • MobX: More flexible, uses observables for state management.
   • Recoil: A React-specific library that offers fine-grained state management.
3. Hooks: Local state management using useState and useReducer.
4. Query Libraries: For server-state management, libraries like React Query or SWR are ideal.

Q16. What is hydration in React?

Attaching event listeners is the process of hydration. It makes a server-rendered HTML page interactive. It is used in SSR applications where the server sends pre-rendered HTML to the client, and React “hydrates” it by attaching the JavaScript logic.

Example:
ReactDOM.hydrate(<App />, document.getElementById('root'));

Q17. Bonus Topics for Further Exploration:

• Custom Hooks: Writing reusable logic encapsulated in a function.
• Error Boundaries: Handling errors in components using componentDidCatch.
• React Suspense for Data Fetching: Simplifying asynchronous data fetching with Suspense.
• Dynamic Theming: Implementing themes using Context API or CSS-in-JS libraries.
• Testing in React: Unit testing with Jest and React Testing Library.

ReactJS Redux Interview Questions

Q1. What is Redux?

Commonly used in React, Redux is a state management library for JavaScript applications. It helps manage and centralize application state, making state updates predictable, maintainable, and easier to debug.

Q2. What are the components of Redux?

Redux has three core components that help manage state in a predictable way. They are:

• Store- This holds the entire application state
• Actions- Describes what should change in the state.
• Reducers- This defines how the state changes in response to actions.

Q3. What is the Flux?

Flux is a pattern for managing data flow in React applications. It ensures that data moves in one direction, making state changes predictable and easy to manage.

Q4. How is Redux different from Flux?

• Feature- Flux architecture pattern has multiple stores whereas Redux has single stores.
• Dispatcher- Flux uses a centralised dispatcher and Redux has no separate dispatcher.
• State Management- Flux stores modify their own state. Redux reducers return a new state.
• Data Flow- Flux and Redux data flow is Unidirectional.
• Complexity- Flux is more boilerplate code. Redux is simpler and more structured.
• Predictability- Flux can be harder to track state changes whereas Redux is more predictable with pure functions (reducers).
• Ease of Debugging- Flux is harder to debug. And Redux DevTools makes debugging easier.
• Middleware Support- Flux has no built-in middleware. Redux supports middleware like Redux Thunk for async operations.

Q5. What is the difference between Context API and Redux?

Context API manages prop drilling i.e., passing data through components, is simple, and has a direct data flow. Whereas Redux manages global state across the app, is more complex, optimized with selective state updates.

ReactJS Router Questions

Q1. What is a React Router?

The React Router library enables navigation and routing to a single page React applications. This allows users to switch between different pages or views without refreshing the page, creating a single-page application (SPA) experience.

Q2. Why do we need to React Router?

React Router is required for the single-page application (SPA) framework which doesn’t reload pages like traditional websites. It gives uninterrupted navigation between different views without refreshing the page, improving user experience and performance.

Q3. How is React routing different from conventional routing?

React Routing updates the page without reloading, making navigation faster. Conventional routing reloads the page on every click, which is slower. React Router is great for speed but needs extra setup for SEO.

Q4. How do you implement React routing?

To implement React routing, install react-router-dom, define routes using Route and Switch components, and use Link or NavLink to navigate between them without reloading the page.

Considering we have the components App, service, and Blogs in our application:

const routing = ( <Router> <div>
>
<h1>React Router formula</h1>
<Route path="/" component={App}/>
<Route path="/service" component={Service} />
<Route path="/blogs" component={Blogs} />
</div>
</Router>


ReactJS Styling Interview Questions

Q1. How do you style React components?

You can style React components using inline styles, CSS files, CSS Modules, or Styled Components. Each method helps customize the design based on project needs.

Inline styles

const style = { color: "blue", fontSize: "20px" };
<p style={style}>Hello, React!</p>;

CSS files

import "./App.css"; // Import CSS file
<p className="text">Hello, React!</p>;
/* App.css */
.text {
color: blue;
font-size: 20px;
}

CSS Modules

import styles from "./App.module.css";
<p className={styles.text}>Hello, React!</p>;

Styled Components

import styled from "styled-components";
const StyledText = styled.p`color: blue; font-size: 20px;`;
<StyledText>Hello, React!</StyledText>;


Q2. Explain the use of CSS modules in React.

CSS Modules in React scope styles to specific components, preventing conflicts with other styles. You create a .module.css file, import it into a component, and apply styles using className={styles.className}.

Q3. Tips for Success in a React JS Interview

To achieve success in a React JS interview it is highly recommended to practice coding, understand core concepts and enhance your communication skills. Given below are some significant things to keep in mind to achieve success:

• Learn thoroughly about the fundamentals
• Focus on learning React Hooks
• Comprehensively understand State Management
• Aim to practice React Router
• Optimize Performance
• Work with APIs
• Know Style Components
• Learn how to debug & test
• Have an experience with React apps
• Stay Updated with latest features and best practices.

Q4. Choose The Right Software Development Program

Let’s understand how to choose the right software development programme. However, selecting the right software development program depends on your goals, experience level, and preferred learning style.

• Know your goals
• Understand your experience level as beginners may benefit from structured bootcamps, while experienced developers can opt for specialized courses.
• Ensure the course covers the fundamentals.
• Choose an ideal learning mode based on your preference.
• Ensure the program includes real world projects.
• Opt for a reliable course
• Pick one that fits your budget

Conclusion

ReactJS is a very powerful library used to create dynamic and efficient user interfaces with ease. The only thing for beginners would be very important to know core concepts which are components, state, props, hooks, the Virtual DOM. So before going to interviews with such primary concepts, it would make a huge difference, better prepared for any question at all they throw at you.

From simple user interface development to complex single-page applications, React’s flexibility, performance optimization, and reusable component structure make it a go-to tool for developers. Get ready confidently for your next ReactJS interview with these fundamental concepts and show mastery of this popular library.

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