Top 50 React Important Interview Questions and Answers | For Fresher and Experienced | 2025

React important interview questions and answers, ReactJS Interview Questions and answers for experienced, React Hooks Interview Questions
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If you're preparing for a React.js interview, it's important to learn both basic and advanced topics. In this article, I will share the most frequent interview questions and give you easy-to-understand answers.


Important Topics You Must Prepare

Here are the main React topics that are frequently asked in interviews:

Core React Topics:

React Function Component Life-Cycle, Virtual DOM, Class components vs functional components, Pass the data between component, Keys in Lists, States and Props, React Rendering Behavior, HOCs,  Hooks ( useEffect , useCallback, useMemo) etc..

Performance & Optimization:

Link Available Soon Here..

State Management Based:

Architechture of Redux, Context API Vs Redux etc. (Link Available Soon Here..)

🎯 Interview Questions and Detailed Answers

What is React?

React is a JavaScript library used to build user interfaces (UIs), especially for single-page applications (SPAs). It was developed by Facebook (now Meta) and released in 2013.

React allows us to build reusable UI components that efficiently update and render when data changes. It follows a component-based architecture, meaning the UI is broken into small, manageable parts (components).

Why Use React?

  • Reusable Components – Write once, use anywhere.

  • Efficient Updates – Uses a Virtual DOM to make updates fast.

How React Works 

React works through a combination of JSX, Components, Props & State, and the Virtual DOM.

1. JSX (JavaScript XML)

React uses JSX, a syntax extension that looks like HTML but runs inside JavaScript.

Example:

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

Under the hood, this is compiled to:

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

So JSX makes it easier to write UI code.

2. Components (Functional or Class)

Everything in React is a component. Components are like JavaScript functions that return JSX.

Functional Component:

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

Usage:

jsx
<Welcome name="Alice" />

Each time this component is used, it renders with different data (via props).

3. Props and State

  • Props (short for properties) are read-only inputs passed to components.

  • State is local data that can change over time within a component.

Example of State (using React hooks):

jsx
import { useState } from 'react';

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

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

In this example, count is a piece of state. Each button click updates it, and React re-renders the UI.

 What is the Virtual DOM?

The Virtual DOM (V-DOM) is a lightweight copy of the real DOM that React uses to improve performance.

  • It’s a JavaScript object representation of the actual DOM elements.

  • React creates and updates this Virtual DOM instead of changing the real DOM directly.

  • After computing the changes, React updates only the parts of the real DOM that actually changed.

Why Use the Virtual DOM?

Updating the real DOM is slow. Even small changes can make the browser re-calculate layout, styles, and repaint elements.

The Virtual DOM helps by:

  • Reducing direct interaction with the DOM.

  • Batching updates.

  • Updating only what's necessary (diffing algorithm).

How Does the Virtual DOM Work?

Step 1: Create Virtual DOM

When you build a React component using JSX:

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

React transforms it into a Virtual DOM object:

js
{
  type: 'h1',
  props: { children: 'Hello, world!' }
}

This is not yet shown on screen — it’s just a virtual version.

Step 2: Render to Real DOM

React converts the Virtual DOM object into real DOM nodes and inserts them into the browser.

Step 3: State or Props Change

Suppose the component’s state changes:

jsx
const element = <h1>Hello, React!</h1>;

React creates a new Virtual DOM tree.

Step 4: Diffing (Comparison)

React compares the new Virtual DOM with the previous one using a diffing algorithm.

It finds that only the text 'world' changed to 'React'.

Step 5: Update the Real DOM

Instead of re-rendering the entire <h1>, React updates only the changed part (Hello, React!) in the real DOM.

This makes the update faster and more efficient.

What are Components in React?

Components are the building blocks of any React application. They return JSX to render UI.

There are two types of components:

  1. Class Components – Older way (uses ES6 classes).

  2. Functional Components – Modern way (uses JavaScript functions and Hooks).

1. Class Components

Class components use ES6 classes and extend React.Component. They can hold state and use lifecycle methods.

Example:

jsx
import React, { Component } from 'react';

class Welcome extends Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'Alice'
    };
  }

  render() {
    return <h1>Hello, {this.state.name}!</h1>;
  }
}

2. Functional Components

Functional components are plain JavaScript functions. Originally, they couldn’t hold state or lifecycle logic — but since React 16.8 introduced Hooks, they became powerful and preferred.

Example (with Hooks):

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

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

  return <h1>Hello, {name}!</h1>;
}

Differences 

FeatureClass ComponentFunctional Component
SyntaxUses class and thisUses functions and Hooks
State Managementthis.state, this.setState()useState() Hook
Lifecycle MethodscomponentDidMount(), etc.useEffect() Hook
Simpler Code❌ Often more verbose✅ Cleaner and shorter
PerformanceSlightly heavierSlightly faster due to less boilerplate
Preferred in modern React❌ No (legacy)✅ Yes (with Hooks)

 How and Why Pass Data Between Components?

In React, your app is made up of components. These components often need to share data — for example:

  • A parent wants to give data to a child.

  • A child wants to send data back to a parent.

  • Two sibling components want to communicate.

1. Parent to Child (via Props)

This is the most common way. The parent component passes data as props.

Example:

jsx
function Child(props) {
  return <h2>Hello, {props.name}!</h2>;
}

function Parent() {
  return <Child name="Alice" />;
}

Note: Child receives the name as a prop from Parent.

2. Child to Parent (via Callback Function) 

To send data back to the parent, the child calls a function passed from the parent.

Example:

jsx
function Child({ onSend }) {
  return <button onClick={() => onSend('Hello from child!')}>Send</button>;
}

function Parent() {
  const handleChildData = (data) => {
    console.log(data); // Output: Hello from child!
  };

  return <Child onSend={handleChildData} />;
}

The parent sends onSend as a prop, and the child calls it.

3. Sibling to Sibling (via Common Parent) 

Siblings can't talk directly, so they send data through the parent.

Example:

jsx
function Sibling1({ onMessage }) {
  return <button onClick={() => onMessage("Data from Sibling1")}>Send</button>;
}

function Sibling2({ data }) {
  return <p>Received: {data}</p>;
}

function Parent() {
  const [message, setMessage] = useState("");

  return (
    <>
      <Sibling1 onMessage={setMessage} />
      <Sibling2 data={message} />
    </>
  );
}

📌 Sibling1 sends data to Parent, and Parent passes it to Sibling2.

4. Global Data Sharing (Context API) 

If you want to pass data deeply through many components without drilling props, use React Context.

Example:

jsx
const UserContext = React.createContext();

function Parent() {
  return (
    <UserContext.Provider value="Alice">
      <Child />
    </UserContext.Provider>
  );
}

function Child() {
  const user = React.useContext(UserContext);
  return <h2>Hello, {user}!</h2>;
}

📌 Context avoids prop drilling when many nested components need access.

What Are Keys in React?

In React, a key is a special string attribute you must include when rendering lists of elements.

Purpose:

Keys help React identify which items changed, added, or removed. They make list rendering more efficient and correct.

Why Are Keys Important?

React uses a diffing algorithm to compare the Virtual DOM before and after an update. Keys help it do this faster.

Without keys:

  • React may re-render the whole list even if only one item changed.

  • Or worse, React may confuse one item for another — causing bugs.

Basic Example – Using Keys

❌ Incorrect (No key or index key in wrong place):

jsx
const items = ['Apple', 'Banana', 'Orange'];

return (
  <ul>
    {items.map(item => <li>{item}</li>)}  // ❌ Missing key
  </ul>
);

✅ Correct (Using a unique key):

jsx
const items = ['Apple', 'Banana', 'Orange'];

return (
  <ul>
    {items.map(item => <li key={item}>{item}</li>)}  // ✅ Unique key
  </ul>
);

Why Not Use Index as Key?

Using index is okay only if the list never changes (no reordering, adding, or deleting).

❌ Problem with index:

jsx
{items.map((item, index) => <li key={index}>{item}</li>)}

If an item is added or deleted, the keys change position, and React might:

  • Reuse old elements incorrectly.

  • Cause bugs like wrong input values, animations breaking, etc.

What Are Props in React?

Props = "Properties"

  • Passed from parent to child

  • Read-only

  • Help make components reusable

Example:

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

function App() {
  return <Welcome name="Alice" />;
}

📌 Here, name="Alice" is a prop passed from App to Welcome.

What Is State in React?

📌 State = "Component's own memory"

  • Managed inside a component

  • Can change over time

  • Used for interactive features

Example:

jsx
import { useState } from 'react';

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

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

📌 Here, count is the component’s state. It updates each time the button is clicked.

What is a Higher-Order Component (HOC)?

A Higher-Order Component is a function that takes a component as input and returns a new component with added features or behavior.

Definition:

“A Higher-Order Component is a pattern in React for reusing component logic.”

HOC Concepts

Think of an HOC like a wrapper or a decorator.

Example:
You have a plain coffee. Wrapping it with milk and sugar makes it a cappuccino.
The base is still coffee, but the wrapped version has extra features.

Syntax of an HOC

jsx
const EnhancedComponent = higherOrderFunction(WrappedComponent);

Basic Example of HOC

Let’s build an HOC that adds loading behavior.

Step 1: The HOC Function

jsx
function withLoading(Component) {
  return function EnhancedComponent({ isLoading, ...props }) {
    if (isLoading) return <p>Loading...</p>;
    return <Component {...props} />;
  };
}

Step 2: The Wrapped Component

jsx
function UserList({ users }) {
  return (
    <ul>
      {users.map(u => <li key={u.id}>{u.name}</li>)}
    </ul>
  );
}

Step 3: Apply the HOC

jsx
const UserListWithLoading = withLoading(UserList);

Step 4: Use It

jsx
<UserListWithLoading isLoading={true} users={[]} />

📌 Output: "Loading..." is shown if isLoading is true.


What Are React Hooks?

Hooks are special functions in React that let you "hook into" React features (like state, lifecycle, etc.) in functional components.

✅ Hooks removed the need for class components.

What is useState?

useState is a React Hook that allows you to create and manage local state in a functional component.

Before Hooks, only class components had state. Now, with useState, functional components can also store and update data.

Syntax

jsx
const [state, setState] = useState(initialValue);

  • state = current value

  • setState = function to update the value

  • initialValue = starting value (e.g., 0, "", false, [], {})

Simple Example

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

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

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

📌 Each click calls setCount(count + 1), which updates state and re-renders the component.

Example with Strings

jsx
const [name, setName] = useState('Alice');

<input value={name} onChange={(e) => setName(e.target.value)} />

📌 React tracks the input and updates the name state every time you type.

Example with Arrays

jsx
const [items, setItems] = useState([]);

const addItem = () => {
  setItems([...items, "New Item"]);
}; 

📌 Always use a new array/object to trigger a re-render.


What is useEffect()?

useEffect() is a React Hook that lets you run side effects in functional components.

A side effect is anything that affects something outside the component — like an API call, logging, or setting up an event listener.

Basic Syntax

jsx
useEffect(() => {
  // Side effect code here

  return () => {
    // Cleanup code (optional)
  };
}, [dependencies]);

How useEffect() Works

React runs the effect:

  1. After the first render (if no dependencies)

  2. After every re-render, if dependencies changed

Example 1: Run on Mount (ComponentDidMount)

jsx
useEffect(() => {
  console.log("Component mounted");
}, []);

📌 Empty dependency array [] = run only once (on first mount)

Example 2: Run on State Change

jsx
useEffect(() => {
  console.log("Count changed");
}, [count]);

📌 Runs every time count changes.

Example 3: Cleanup (ComponentWillUnmount)

jsx
useEffect(() => {
  const timer = setInterval(() => {
    console.log("Tick");
  }, 1000);

  return () => {
    clearInterval(timer); // Cleanup on unmount
  };
}, []); 

📌 The function inside return is the cleanup — runs on unmount or before re-running the effect.


2. useCallback() – Memoize Functions

What it does:

Returns a memoized version of a function that only changes if dependencies change.

Why use it?

To prevent re-creating the same function on every render, especially useful when:

  • Passing functions to child components (with React.memo)

  • Avoiding unnecessary re-renders

Syntax:

jsx
const memoizedFn = useCallback(() => {
  // logic
}, [dependencies]);

Example:

jsx
const handleClick = useCallback(() => {
  console.log("Button clicked");
}, []);

3. useMemo() – Memoize Values

What it does:

Returns a memoized value. Only re-computes if dependencies change.

Why use it?

To optimize expensive calculations or avoid recalculating derived data.

Syntax:

jsx
const memoizedValue = useMemo(() => {
  return computeExpensiveValue(a, b);
}, [a, b]);

Example:

jsx
const sortedList = useMemo(() => {
  return list.sort((a, b) => a - b); 

}, [list]);


 1. useRef – Reference DOM or Persist Values

What is it?

useRef() returns a mutable object that:

  • Doesn’t trigger re-render when changed.

  • Can hold references to DOM elements or store values across renders.

Syntax

js
const myRef = useRef(initialValue);

  • myRef.current holds the actual value or DOM node.

Example 1: Access a DOM element

jsx
import { useRef } from 'react';

function InputFocus() {
  const inputRef = useRef(null);

  const focusInput = () => {
    inputRef.current.focus();
  };

  return (
    <>
      <input ref={inputRef} />
      <button onClick={focusInput}>Focus Input</button>
    </>
  );
}

📌 useRef(null) gives access to the input element using inputRef.current.

 Example 2: Persist values (like a previous state)

jsx
const count = useRef(0);

useEffect(() => {
  count.current += 1;
});

📌 This value persists across renders without causing re-renders.

When to Use useRef?

  • Access DOM elements directly

  • Store timeout/interval IDs

  • Track previous values without re-render

  • Avoid triggering state updates

2. useContext – Access Shared/Global State

What is it?

useContext() allows a component to consume values from a React Context, without passing props manually through every level (prop drilling).

Steps to Use

  1. Create Context:

js
const ThemeContext = React.createContext();

  1. Provide Context (top-level):

jsx
<ThemeContext.Provider value="dark">
  <App />
</ThemeContext.Provider>

  1. Consume with useContext:

jsx
const theme = useContext(ThemeContext);

When to Use useContext?

  • Share global data like:

    • Theme (dark / light)

    • Auth user

    • Language

    • App settings

  • Avoid prop drilling

 

What is Rendering in React?

Rendering means React builds and updates the UI based on the component’s return value (usually JSX).

  • On the first render, React builds the initial UI.

  • On updates, React re-renders affected components when state or props change.

When Does React Re-render?

React re-renders a component when:

  1. Its state changes (via useState, useReducer, etc.)

  2. Its props change (passed from parent)

  3. Its parent re-renders, causing all children to re-evaluate

Example: State Change Triggers Re-render

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

  return (
    <div>
      <p>You clicked {count} times</p> {/* re-renders when count changes */}
      <button onClick={() => setCount(count + 1)}>Click</button>
    </div>
  );
}

Virtual DOM Optimization

React uses a Virtual DOM to make rendering efficient.

How it works:

  1. React renders a Virtual DOM tree.

  2. When something changes, it creates a new tree.

  3. React compares the old vs new tree (diffing).

  4. It updates only the changed parts of the real DOM.

This prevents unnecessary full page reloads.

Child Components and Re-renders

If a parent re-renders, all its children will re-render by default — unless optimized.

Example:

jsx
function Parent({ count }) {
  return (
    <>
      <Child />
      <p>{count}</p>
    </>
  );
}

function Child() {
  console.log("Child rendered");
  return <p>I am child</p>;
}

Even if Child has no props or state, it will re-render every time the parent does.

How to Avoid Unwanted Re-renders

✅ 1. React.memo() (for functional components)

Wrap the child component to skip re-renders if props didn’t change.

jsx
const Child = React.memo(function Child({ value }) {
  console.log("Child rendered");
  return <p>{value}</p>;
});

2. useCallback() and useMemo()

Prevent passing new functions/values every time, which triggers re-renders.

jsx
const handleClick = useCallback(() => {
  // function logic
}, []);

3. shouldComponentUpdate() (class components)

Control re-rendering manually.

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ReactJS Interview Questions and answers for experienced
React important interview questions and answers
React important interview questions for freshers
React Hooks Interview Questions