Top React JS Interview Questions and Answers for All Levels

TL;DR: Explore the most important React JS interview questions and answers for beginners, intermediate, and experienced developers. This guide walks you through fundamental concepts like components, props, and state management, then dives deeper into React Router, Redux, and best practices for large-scale applications.

React JS Interview Questions for Freshers

Are you starting your journey with React? This section covers the 25 essential ReactJS interview questions and answers for freshers that will help you build a strong foundation and confidently answer entry-level React interview topics.

1. What is ReactJS?

ReactJS is a JavaScript library for building user interfaces, primarily for web applications. It helps developers create reusable components that update efficiently when data changes, making websites faster and more interactive. 

Instead of reloading the entire page, ReactJS updates only the parts that need to change, improving performance and user experience.

ReactJS Syntax for Creating a Component (Function-based):

import React from 'react'
;function MyComponent() 
{return <h1>Hello, World!</h1>;
}
export default MyComponent;

2. Why is ReactJS used?

• ReactJS is used to build fast, interactive user interfaces for web applications
• It enables efficient updates by changing only the parts of the page that need to be updated (using the Virtual DOM)
• ReactJS enables the creation of reusable components, simplifying UI development and maintenance
• It improves performance by optimizing rendering with features like React Hooks and memoization
• ReactJS makes scalable web applications easier to develop by breaking the UI into smaller, manageable components

3. How Does ReactJS Work?

ReactJS uses a Virtual DOM, a lightweight copy of the real DOM (web page structure). When something changes on the page, React updates the Virtual DOM first, compares it to the previous version, and then updates only the changed parts in the real DOM quickly. This makes the app faster and more efficient.

Here's a breakdown:

• Defining UI segments as components using JSX
• Managing component state and props
• Leveraging the Virtual DOM
• Diffing algorithm to identify the changes
• Reconciliation by applying UI updates

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4. What are the features of ReactJS?

Here are the key features of ReactJS:

• JSX (JavaScript XML): Allows you to write HTML-like code in JavaScript
• Virtual DOM: Efficiently updates and renders only the changed parts of the web page
• Component-Based Architecture: Encourages the development of reusable, self-contained components that can be easily integrated and maintained
• One-Way Data Binding: Data flows in one direction, making the app easier to understand and debug
• Hooks: Functions like useState and useEffect that allow state and lifecycle management in functional components
• Declarative UI: You describe the UI in terms of its state, and React handles the rendering
• React Router: Allows navigation between different views or components in single-page applications
• Performance Optimization: ReactJS utilizes techniques such as shouldComponentUpdate and React.memo to optimize re-renders
• React DevTools: A browser extension for debugging and inspecting React applications
• Cross-Platform Development: With React Native, React can be used to build mobile applications for iOS and Android

5. What is JSX?

JSX (JavaScript XML) is a syntax in React that lets you write HTML-like code in JavaScript. It makes it easier to create and understand the structure of your UI components.

JSX is

• easier to write and visualize UI structure
• helpful in combining HTML and JavaScript logic in one place
• converted behind the scenes into JavaScript using tools like Babel

Example Code: Inside a Component

function Welcome() 
{return (
<div>
<h1>Welcome to React!</h1>
<p>This is written using JSX.</p>
</div>
);
}

6. How to create components in ReactJS?

In ReactJS, components are created in two main ways:

• Using Functional Component
• Using Class Component

Example #1: Functional Component

import React from 'react';
function Greeting() {
return <h1>Hello from Functional Component!</h1>;
}
export default Greeting;

Usage:

<Greeting />

Example #2: Class Component

import React, { Component } from 'react';
class Greeting extends Component {
render() {
return <h1>Hello from Class Component!</h1>;
}
}
export default Greeting;

Usage:

<Greeting />

7. What are the advantages of ReactJS?

The key advantages of ReactJS are as follows:

• Fast performance with Virtual DOM
• Reusable components for easier maintenance
• Simple to learn and use (especially with JSX)
• Declarative UI makes code more readable
• One-way data binding for predictable state management
• Supports Hooks in functional components
• Large community and ecosystem
• Enables cross-platform development with React Native
• Provides powerful developer tools (React DevTools)
• Easy to integrate with other libraries and frameworks

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8. Differentiate between real DOM and virtual DOM.

9. What are forms in ReactJS?

Forms in ReactJS are used to handle user input, including text fields, checkboxes, radio buttons, and other elements. ReactJS uses controlled components, meaning form inputs are connected to the component's state, allowing you to easily manage and respond to user input.

Example: Simple React Form

import React, { useState } from 'react';
function SimpleForm() {
const [name, setName] = useState('');
const handleSubmit = (e) => {
e.preventDefault(); // prevents page reload
alert(`Hello, ${name}!`);
};
return (
<form onSubmit={handleSubmit}>
<label>
Enter your name: 
<input 
type="text" 
value={name} 
onChange={(e) => setName(e.target.value)} 
/>
</label>
<button type="submit">Submit</button>
</form>
);
}
export default SimpleForm;

Explanation:

• The input’s value is tied to the name state (value={name})
• On each keystroke, setName() updates the state
• When the form is submitted, an alert appears with the input value
• This is a controlled component—a standard way to work with forms

10. How is React different from React Native?

ReactJS is used for building web applications, while React Native is used for building mobile applications using React principles. Both utilize the same core ideas (components, props, state), but the environments and UI elements differ.

ReactJS and React Native are both frameworks developed by Facebook, but they serve different purposes and run in different environments.

About ReactJS

1. ReactJS (also known simply as React) is a JavaScript library used for building user interfaces for web applications. It runs in the browser and uses HTML, CSS, and JavaScript to create components and manage the interface.
2. ReactJS utilizes a concept called the Virtual DOM to update only the parts of a webpage that have changed, making web apps faster and more efficient. Developers can build single-page applications (SPAs) that dynamically update content without requiring a full page reload.

About React Native

1. React Native is a framework for building mobile applications for iOS and Android. Instead of using HTML and CSS, React Native uses native mobile components to create real mobile interfaces.
2. It allows developers to write JavaScript code (similar to ReactJS) but renders components, such as buttons, views, and text, using native platform APIs. This means that apps built with React Native have a look and feel that closely resembles truly native apps, rather than mobile websites.

11. Explain how lists work in React.

In React, lists are used to display multiple elements dynamically, usually by looping through an array of data. We use JavaScript's map() function to transform each array element into a React element (usually a component or JSX).

Each list item should have a unique key prop to help React identify which items have changed, been added, or been removed.

Example: Displaying a List of Names

import React from 'react';
function NameList() {
const names = ['Alice', 'Bob', 'Charlie'];
return (
<ul>
{names.map((name, index) => (
<li key={index}>{name}</li>
))}
</ul>
);
}
export default NameList;

Explanation:

• names is an array of strings
• map() loops through the array and returns a <li> for each name
• key={index} gives each item a unique key (in real apps, use a unique ID if available)

Output:

- Alice

- Bob

- Charlie

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

In React, keys are used to uniquely identify each item in a list. They help React track which items have changed, been added, or removed, so it can update the UI efficiently. Here are the best practices to use Keys:

• Use a unique and stable ID as the key (like a user ID or database ID)
• Avoid using the array index as a key, unless the list is static and won’t change

Example:

const users = [
{ id: 1, name: 'Alice' },
{ id: 2, name: 'Bob' }
];
<ul>
{users.map(user => (
<li key={user.id}>{user.name}</li>
))}
</ul>

Here, key={user.id} uniquely identifies each list item, so React can handle updates efficiently.

13. What are synthetic events in React?

Synthetic events in React are wrapper objects around the browser’s native events (such as clicks, key presses, etc.). They provide a consistent, cross-browser-compatible interface for handling events in React applications.

14. How do you create forms in React?

In React, we create forms using controlled components, where form inputs (like text fields) are connected to state using useState() (in functional components) or this.state (in class components). This allows React to manage form data and respond to user input.

Quick Steps:

• Create a form element with input fields
• Use useState() to manage the input's value
• Update the state using onChange event handlers
• Handle form submission using onSubmit

Example: Simple Controlled Form

import React, { useState } from 'react';
function ContactForm() {
const [email, setEmail] = useState('');
const handleSubmit = (e) => {
e.preventDefault();
// Prevent page reload
alert(`Submitted Email: ${email}`);
};
return (
<form onSubmit={handleSubmit}>
<label>
Email:
<input
type="email" 
value={email} 
onChange={(e) => setEmail(e.target.value)} 
/>
</label>
<button type="submit">Submit</button>
</form>
);
}
export default ContactForm;

Explanation:

• React's state controls the email input
• Every time the user types, setEmail updates the state
• On form submission, React handles the data (without reloading the page)

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15. How do you write comments in React?

Step 1: Identify where you want to add the comment

• Inside JSX (the HTML-like part inside return)
• Inside JavaScript code (outside JSX)

Step 2: Use the correct comment syntax

• Inside JSX: Use curly braces {} and wrap your comment with /* */
• Inside JavaScript: Use // for single-line or /* */ for multi-line comments

Step 3: Write the Comment

Example:

import React from 'react';
function MyComponent() {
// This is a JavaScript comment outside JSX
return (
<div>
{/* This is a comment inside JSX */}
<h1>Hello, React!</h1>
</div>
);
}
export default MyComponent;

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

An arrow function is a shorter syntax for writing JavaScript functions. It’s more concise and often easier to read compared to traditional function expressions.

Arrow functions also do not have their own this, which makes them handy in React for handling the context.

Syntax:

const add = (a, b) => a + b;

How is it used in React?

Defining functional components:

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

Event handlers inside components:

<button onClick={() => alert('Clicked!')}>Click Me</button>

Using arrow functions helps avoid binding this when you use class components (though hooks and functional components are now preferred):

class MyComponent extends React.Component {
handleClick = () => {
console.log(this); // Automatically bound
};
render() {
return <button onClick={this.handleClick}>Click</button>;
}
}

17. How is React different from Angular?

React and Angular are both popular tools for building web applications, but they differ significantly in their approaches, architectures, and ecosystems.

About React

• React is a JavaScript library focused primarily on building user interfaces. It follows a component-based architecture, where the UI is divided into reusable components
• React uses a virtual DOM to efficiently update the UI and emphasizes declarative programming
• It’s flexible and lightweight, allowing developers to choose their own libraries for routing, state management, and other needs
• React’s simplicity and performance have made it popular for building fast, scalable single-page applications

About Angular

• Angular is a full-fledged front-end framework developed by Google. It provides a complete solution with built-in tools for routing, state management, form handling, HTTP services, and more
• Angular uses a real DOM and a two-way data-binding system that automatically synchronizes the UI and the model
• It follows a more opinionated and structured approach, often requiring developers to adhere to its conventions and use its extensive feature set
• Angular applications are typically larger in size due to the framework’s comprehensive nature, but they offer a lot out of the box

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18. State the limitations of React.

The key limitations of React are:

• Only handles the UI layer
• Steep learning curve for beginners
• Fast-paced development
• JSX can be confusing
• Boilerplate code
• SEO limitations (without SSR)
• Performance issues in large apps

19. State the use of Webpack.

Webpack is a powerful module bundler for JavaScript applications. Its primary purpose is to take many different files, such as JavaScript, CSS, images, and more, and bundle them into one or more optimized files (called bundles) that browsers can load efficiently.

20. Differentiate between controlled and uncontrolled components.

21. Define Custom Hooks.

Custom Hooks are special JavaScript functions in React that let you reuse stateful logic across multiple components. They start with the word use and allow you to extract and share common logic, making your code cleaner and more maintainable.

22. What is a dispatcher?

A dispatcher is a central component in certain application architectures, especially in Flux (a design pattern used with React). Its main job is to manage data flow by receiving actions and dispatching them to the appropriate stores or handlers.

A dispatcher is like a traffic controller that ensures actions reach the right parts of the app to update data and the UI consistently.

23. State the different side effects of the React component.

Common side effects in React components include:

1. Data Fetching: Making API calls to load data from a server
2. Subscriptions: Setting up subscriptions or event listeners
3. Manipulating the DOM: Directly interacting with the DOM outside of React’s rendering
4. Timers: Using setTimeout or setInterval for delayed or repeated actions
5. Logging: Console logs or analytics tracking
6. Updating External Systems: Writing to local storage, sending data to an analytics service, or interacting with browser APIs

24. What are the lifecycle steps in React?

React components undergo a series of lifecycle phases: Mounting, Updating, and Unmounting. These phases enable code to be run at specific points.

25. What are Error Boundaries in React?

Error boundaries are special React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the entire app.

Error boundaries catch errors during Rendering, Lifecycle methods, and Constructors of the whole tree below them.

Example:

class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
// Update state so next render shows fallback UI
return { hasError: true };
}
componentDidCatch(error, info) {
// You can log the error to an error reporting service
console.error(error, info);
}
render() {
if (this.state.hasError) {
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}

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React JS Interview Questions for Intermediate

Explore the diverse set of React JS interview questions and answers that test your practical knowledge and problem-solving skills. These Q&As are ideal for software developers with hands-on experience.

26. What are React Hooks?

React Hooks are special functions that let you use state and other React features inside functional components. Before hooks, only class components could have state and lifecycle methods, but hooks bring these capabilities to functional components.

Example: Using the useState Hook

import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0); // Declare state variable 'count' with initial value 0
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
export default Counter;

Explanation:

• useState(0) creates a state variable count initialized to 0
• setCount is a function to update the count
• Clicking the button updates the state and re-renders the component with the new count

27. State the rules to follow when using React Hooks.

React Hooks have some important rules to ensure they work correctly and predictably:

• Don’t call hooks inside loops, conditions, or nested functions. Always call hooks at the top level of your React function to maintain consistent hook order between renders
• Call hooks only inside React functional components or custom hooks. Don’t call them from regular JavaScript functions, class components, or event handlers directly
• Hooks are designed for functional components and custom hooks; class components don’t support them
• Custom hooks should start with the word 'use' (e.g., useFetch, useAuth) so that React can identify them as hooks

28. What is useState, and how does it work?

useState is a React Hook that lets you add state variables to functional components. It allows your component to track data that changes over time, such as user input, toggles, and counters.

You call useState inside a functional component and pass an initial value. It returns an array with two things:

• The current state value
• A function to update that state

When you update the state using the setter function, React re-renders the component to reflect the new state.

29. What is useEffect?

useEffect is a React Hook that lets you perform side effects in functional components. Side effects include things like:

• Fetching data from an API
• Subscribing to events
• Manually manipulating the DOM
• Setting timers

useEffect runs after the component renders and can also clean up resources when the component unmounts or before the effect runs again.

Example:

import React, { useState, useEffect } from 'react';
function Timer() {
const [count, setCount] = useState(0);
useEffect(() => {
const interval = setInterval(() => {
setCount(c => c + 1);
}, 1000);
// Cleanup on unmount
return () => clearInterval(interval);
}, []); // Empty dependency array means it runs once on mount
return <h1>Seconds: {count}</h1>;
}
export default Timer;

30. What is Memoization in React?

Memoization in React is a technique that optimizes performance by caching the results of expensive function calls or component renders so they don’t have to be recalculated on every render.

React provides hooks like React.memo and useMemo to help with this. By using memoization, React can skip unnecessary re-renders and recalculations, making your app faster.

Example: Using React.memo:

const ExpensiveComponent = React.memo(function({ value }) {
console.log('Rendering ExpensiveComponent');
return <div>{value}</div>;
});
function App() {
const [count, setCount] = React.useState(0);
return (
<>
<button onClick={() => setCount(count + 1)}>Increment</button>
<ExpensiveComponent value="Hello" />
</>
);
}

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

Prop Drilling is the process of passing data (props) from a parent component down through multiple nested child components, even if only the most deeply nested component actually needs the data. This can make the code:

• Hard to maintain
• Difficult to read
• Prone to bugs

How to Avoid Prop Drilling?

I. React Context API

Create a context to share data globally without passing it through every component.

const UserContext = React.createContext();
function App() {
const user = { name: 'John' };
return (
<UserContext.Provider value={user}>
<Parent />
</UserContext.Provider>
);
}
function GrandChild() {
const user = React.useContext(UserContext);
return <p>Hello, {user.name}</p>;
}

II. State Management Libraries

Use tools like Redux, Zustand, or Recoil for larger applications where multiple components need to access shared state.

III. Component Composition

Sometimes restructuring your components can reduce the need for deeply nested props.

32. What are the components in React?

Components in React are the building blocks of a React application. They are reusable UI components that manage their own structure, logic, and behavior. Each component can be as simple as a button or as complex as an entire page.

Types of Components in React:

• Functional Components: written as JavaScript functions, use hooks (like useState, useEffect) for state and side effects, and are used in modern React apps
• Class Components: use ES6 classes, use this.state and lifecycle methods, and mostly replaced by functional components + hooks

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

In class components, the render() method defines the UI. It returns JSX, which React then uses to update the DOM.

• It is required in every class component
• It must return a single React element
• Called every time the component's state or props change

Example:

class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}

34. What is a state in React?

In React, state refers to a built-in object that allows components to store and manage dynamic data. It represents values that can change over time, such as user input, API responses, or UI conditions (like whether a modal is open). When the state of a component changes, React automatically re-renders the component to reflect the updated data in the user interface.

State is local to the component in which it is defined and can only be updated using specific functions, such as setState in class components or the useState hook in functional components. By effectively managing the state, React makes it easy to build interactive and responsive web applications.

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35. How do you implement state in React?

• Import the useState hook from React
• Initialize state with useState(initialValue)
• Update state using the state updater function
• Use the state in your JSX to reflect dynamic data

Example: A Simple Counter

import React, { useState } from 'react';
function Counter() {
// Step 2: Initialize state
const [count, setCount] = useState(0);
// Step 3: Function to update state
const increaseCount = () => {
setCount(count + 1);
};
return (
<div>
{/* Step 4: Use state in JSX */}
<p>Count: {count}</p>
<button onClick={increaseCount}>Increment</button>
</div>
);
}
export default Counter;

Explanation:

• useState(0) initializes the count state to 0
• setCount updates the state
• When the button is clicked, increaseCount is triggered, changing the state and causing React to re-render the component

Did You Know?

Many popular, high-traffic websites rely on React for their front-end development. Among the top sites that utilize React are Facebook, Instagram, and Airbnb, each known for its user-friendly interfaces. (Source: Intelivita)

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

Updating the state in React depends on whether you're using a functional component (with useState) or a class component (with this.setState). Here's how it works in both:

In Functional Components (Using useState)

• You declare a state with useState
• You update it using the state updater function

Example:

import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0); // declare state
const handleClick = () => {
setCount(count + 1); // update state
};
return (
<div>
<p>Count: {count}</p>
<button onClick={handleClick}>Increment</button>
</div>
);
}

In Class Components (Using this.setState)

• State is declared in a constructor
• You update it using this.setState()

Example:

import React, { Component } from 'react';
class Counter extends Component {
constructor() {
super();
this.state = {
count: 0,
};
}
handleClick = () => {
this.setState({ count: this.state.count + 1 }); // update state
};
render() {
return (
<div>
<p>Count: {this.state.count}</p>
<button onClick={this.handleClick}>Increment</button>
</div>
);
}
}

37. What are props in React?

Props (short for properties) are read-only inputs passed from one component to another in React. They allow parent components to send data to child components, helping make components reusable and dynamic.

Example:

function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}
// Using the component
<Greeting name="Alice" />

38. How do you pass props between components in React?

To pass props between components,

• Define the data in a parent component
• Pass it down to a child component using JSX attributes
• Access it in the child component via props

Step-by-Step Example:

1. Parent Component – Passing the prop

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

2. Child Component – Receiving and using the prop

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

Output:

Hello, Alice!

39. What are the differences between state and props?

40. What is a higher‑order component in React?

A Higher-Order Component (HOC) in React is a function that takes a component as an argument and returns a new component with enhanced behavior or additional features. It’s a pattern for reusing component logic, particularly for tasks such as authentication, theming, or data fetching.

HOCs don’t modify the original component; instead, they wrap it with extra functionality. A common example is adding loading or error handling to components without repeating code across multiple places. An HOC is not a feature of React itself, but rather a design pattern built upon React’s compositional nature.

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

In React, you can embed multiple components inside a single component by nesting them in the JSX return block of a parent component. This allows you to compose complex UIs from smaller, reusable components.

Example:

function Header() {
return <h1>Welcome to My App</h1>;
}
function Footer() {
return <p>&copy; 2026 My Company</p>;
}
function App() {
return (
<div>
<Header />
<p>This is the main content.</p>
<Footer />
</div>
);
}

Explanation:

• The App component embeds the Header and Footer components
• You can include as many components as needed, just like HTML elements
• They must be wrapped inside one parent element, such as a <div> or <>

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

Class and functional components are two ways to create components in React, but they differ in structure, syntax, and capabilities, especially before the introduction of Hooks.

About Class Components

Class components are written using ES6 classes and require you to extend React.Component. They use this.state to manage internal state and lifecycle methods, such as componentDidMount() or componentDidUpdate(), to handle side effects. Class components were traditionally used when components needed state or lifecycle methods.

About Functional Components

Functional components are simpler JavaScript functions that return JSX. Originally, they were used only for presentational purposes (i.e., components without state). Still, with the introduction of React Hooks (such as useState and useEffect), functional components can now do everything class components can, often in a more concise and readable way.

43. Describe the lifting state up in React.

Lifting state up in React is the process of moving state from a child component to its closest common parent, allowing multiple components to share and sync the same state. This is useful when:

• Two or more sibling components need to access or update the same data
• You want to keep a single source of truth to avoid inconsistencies

44. How do we avoid binding in ReactJS?

In class components, you often need to bind event handler methods to the correct context. However, binding in the constructor or inline can be repetitive or inefficient. Here are 4 common ways to avoid binding manually in React:

• Use arrow functions as class methods
• Use arrow functions in JSX
• Bind in the constructor
• Use functional components + hooks

45. Explain React fragments.

React Fragments are a feature that lets you group multiple elements without adding extra nodes to the DOM. Normally, JSX requires a single parent element, so developers often wrap elements in a <div>, which can lead to unnecessary nesting and affect styling or layout.

React Fragments solve this by acting as invisible wrappers that don’t appear in the final HTML. This helps keep the DOM tree clean and efficient, especially when rendering lists or grouped elements that don’t need a container.

Example:

import React from 'react';
function MyComponent() {
return (
<React.Fragment>
<h1>Title</h1>
<p>This is a paragraph.</p>
</React.Fragment>
);
}

46. When should you use useMemo() in React?

You should use React's useMemo() to optimize performance by memoizing expensive computations or preventing unnecessary re-renders of components that depend on derived data. You can use the useMemo() function when,

• you have a slow/expensive calculation that doesn’t need to run on every render
• you want to avoid recalculating values unless certain dependencies change
• you are passing computed values to child components, and want to prevent unnecessary re-renders due to changed references

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

The different types of Hooks in React are:

• useState
• useEffect
• useContext
• useRef
• useMemo
• useCallback
• useReducer
• useLayoutEffect
• useImperativeHandle
• useDebugValue
• useDeferredValue
• useId
• useTransition
• useSyncExternalStore
• useInsertionEffect

48. State the different lifecycle methods in the updating phase.

• static getDerivedStateFromProps()
• shouldComponentUpdate()
• render()
• getSnapshotBeforeUpdate()
• componentDidUpdate()

49. Enlist the functions of high‑order components.

• Code Reusability: Share common logic between multiple components
• Enhance Components: Add extra features or behaviors without modifying original components
• Conditional Rendering: Control when and how a component should render
• Props Manipulation: Inject, filter, or modify props before passing them to wrapped components
• Abstraction of Logic: Separate UI from business logic for better code organization
• State and Side-Effect Management: Add state or side-effect logic to stateless components
• Cross-Cutting Concerns: Address concerns such as authentication, logging, or theming across multiple components

50. What Do the Three Dots (...) Mean in React? (Example: <Image {...aspects} source="img_source" />)

The three dots (...) in React syntax are known as the spread syntax. In the context of JSX (as in your example), it spreads the properties (props) of an object into a component.

Example:

import React from 'react';
function MyComponent() {
return (
<React.Fragment>
<h1>Title</h1>
<p>This is a paragraph.</p>
</React.Fragment>
);
}

This is equivalent to

<Image width={100} height={200} resizeMode="contain" source="img_source" />

So, the spread syntax ...aspects takes each key-value pair from the aspects object and passes them as individual props to the Image component.

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51. What is the strict mode in React?

React Strict Mode is a development-only tool that highlights potential problems in a React application. It doesn't render anything to the DOM and has no impact in production builds.

Instead, it helps developers write safer and more robust code by identifying unsafe lifecycle methods, deprecated APIs, and unexpected side effects.

How to Use Strict Mode?

import React from 'react';
import ReactDOM from 'react-dom/client';
import App from './App';
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
<React.StrictMode>
<App />
</React.StrictMode>
);

52. State the components of the React router.

• BrowserRouter: Uses the HTML5 history API (pushState, popState) for clean URLs
• HashRouter: Uses the URL hash (#) portion to keep UI in sync (useful for static file servers)
• Routes: A container for all the Route definitions
• Link/NavLink: Link creates a basic hyperlink, and NavLink allows you to apply styles to the active link
• useNavigate: A hook that lets you navigate programmatically (e.g., after form submission)
• useParams: Retrieves dynamic parameters from the URL
• useLocation: Returns the current URL location object (pathname, search, hash)
• useSearchParams: Allows you to read and modify query parameters in the URL
• Outlet: Used in nested routes; it renders child routes inside a parent component

53. What is Redux?

Redux is a state management library for JavaScript apps, commonly used with React to manage and centralize application state. It helps to predictably manage data flow using three core principles:

• Single Source of Truth – One store holds the entire app state
• State is Read-Only – You can’t change it directly; use actions
• Changes via Pure Functions – Reducers describe how the state updates

Example:

// store.js
import { createStore } from 'redux';
// Initial state
const initialState = { count: 0 };
// Reducer
function counterReducer(state = initialState, action) {
switch (action.type) {
case 'INCREMENT':
return { count: state.count + 1 };
case 'DECREMENT':
return { count: state.count - 1 };
default:
return state;
}
}
// Create Store
const store = createStore(counterReducer);
// Dispatch Actions
store.dispatch({ type: 'INCREMENT' });
console.log(store.getState()); // { count: 1 }

54. What are the components of Redux?

• Store: Holds the entire state of the application
• Actions: Describe what changes need to occur
• Reducers: Specify how the state changes in response to actions
• Dispatch: Sends actions to the store
• Subscribers: Listen for state updates and re-render UI

55. What is Flux?

Flux is an architecture pattern used by React for unidirectional data flow. It organizes the application's structure around actions, a dispatcher, stores, and views (components) to make data management more predictable and easier to debug.

Data flows in one direction → Action → Dispatcher → Store → View

Example:

// Action
const addTodo = (text) => ({
type: 'ADD_TODO',
payload: text
});
// Dispatcher
import { Dispatcher } from 'flux';
const dispatcher = new Dispatcher();
// Store
let todos = [];
dispatcher.register((action) => {
if (action.type === 'ADD_TODO') {
todos.push(action.payload);
console.log("Updated Todos:", todos);
}
});
// View (Component)
addTodo("Learn Flux");
dispatcher.dispatch(addTodo("Learn Redux"));

56. How is Redux different from Flux?

Redux simplifies Flux by using a single store, pure reducers, and no dispatcher, making state management more predictable and maintainable. Here’s a detailed comparison between Redux and Flux.

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

Both Context API and Redux are used for state management in React, but they differ in purpose, complexity, and scalability.

About Context API

The Context API, built into React, is primarily designed to avoid prop drilling. It allows you to share data (such as theme, user info, or language settings) across multiple components without manually passing props at every level. It’s lightweight, easy to set up, and ideal for small to medium-sized applications where state changes are relatively simple and localized.

However, it lacks features like middleware, dev tools, and time-travel debugging, which makes it less suitable for complex state management scenarios.

About Redux

Redux is a predictable state container that follows a strict architecture pattern with a single store, actions, and reducers. It’s best suited for large-scale applications with complex data flows and frequent updates.

Redux provides tools for debugging, performance optimization, and middleware integration (such as Redux Thunk or Redux Saga) to handle asynchronous logic effectively.

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58. Explain React Fiber.

React Fiber is the reconciliation engine introduced in React 16 that improves how React updates and renders components. It enables React to break rendering into small units, pause and resume tasks, and prioritize updates (such as animations or user input) for a smoother user experience.

Unlike the older stack-based algorithm, Fiber allows React to handle updates asynchronously, making the UI more responsive even during heavy computations.

Example:

function App() {
const [count, setCount] = React.useState(0);
const handleClick = () => {
// React Fiber prioritizes this update efficiently
setCount(count + 1);
};
return (
<div>
<h1>Count: {count}</h1>
<button onClick={handleClick}>Increment</button>
</div>
);
}

59. How to structure a large‑scale React app?

A large React app should be organized by features, with separation of concerns for components, state, routing, and utilities. Use code splitting for performance and a centralized store (e.g., Redux or React Query) for shared state.

Example Folder Structure:

Example: Lazy Loading & Routing

const Dashboard = React.lazy(() => import('./features/dashboard/Dashboard'));
<Routes>
<Route path="/" element={<Home />} />
<Route path="/dashboard" element={
<Suspense fallback={<Loader />}>
<Dashboard />
</Suspense>
} />
</Routes>

60. What are Server‑Side Rendering (SSR), Client‑Side Rendering (CSR), and React Server Components (RSC)?

1. Server-Side Rendering (SSR)

• Definition: The HTML for a page is generated on the server and sent to the browser in its fully formed state
• How it works: The server runs React code, renders components to HTML, and sends the result to the client. Then React “hydrates” it to make it interactive
• Pros: Faster first paint, better SEO, good for dynamic content
• Cons: Higher server load and slower page navigation

Example (Next.js):

export async function getServerSideProps() {
const data = await fetchData();
return { props: { data } };
}
export default function Page({ data }) {
return <div>{data.title}</div>;
}

2. Client-Side Rendering (CSR)

• Definition: The browser downloads a bare HTML shell and uses JavaScript to build and render the UI on the client
• How it works: The app loads once, and React handles routing and client-side rendering
• Pros: Smooth navigation, reduced server work
• Cons: Slower initial load and weaker SEO (unless prerendered)

Example:

// index.js
import ReactDOM from "react-dom/client";
import App from "./App";
ReactDOM.createRoot(document.getElementById("root")).render(<App />);

3. React Server Components (RSC)

• Definition: A newer React feature (introduced in React 18) that allows components to run on the server and stream UI to the client
• How it works: Server components handle heavy logic or data fetching and send lightweight UI updates; client components handle interactivity
• Pros: Smaller bundle sizes, improved performance, automatic data fetching
• Cons: Still maturing; limited interactivity in server-only components

Example:

// Server Component
export default async function ProductList() {
const products = await fetch('https://api.example.com/products').then(res => res.json());
return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>;
}

61. What is lazy loading in React, and how to implement it?

Lazy Loading in React means loading components only when they’re needed, rather than loading them all at once. It improves performance by reducing the initial bundle size and speeding up page load times.

Example: Use React’s built-in React.lazy() and Suspense components.

import React, { Suspense } from "react";
const Dashboard = React.lazy(() => import("./Dashboard"));
function App() {
return (
<Suspense fallback={<div>Loading...</div>}>
<Dashboard />
</Suspense>
);
}
export default App;

62. What are common performance bottlenecks in React applications, and how can they be mitigated?

1. Unnecessary Re-renders

• Cause: Props/state changes triggering full component re-renders
• Fix: Use React.memo, useMemo, and useCallback to memoize values and functions

2. Large Bundle Size

• Cause: Loading all code at once
• Fix: Use code splitting with React.lazy() and dynamic imports

3. Inefficient Lists Rendering

• Cause: Rendering long lists without optimization
• Fix: Use windowing/virtualization (react-window, react-virtualized)

4. Expensive Calculations in Render

• Cause: Heavy logic inside render methods
• Fix: Move logic outside render or memoize with useMemo

5. Blocking the Main Thread

• Cause: Synchronous heavy tasks (e.g., loops, JSON parsing)
• Fix: Use Web Workers or async APIs

6. Inefficient State Management

• Cause: Global state updates re-rendering all children
• Fix: Localize state, split contexts, or use libraries like Redux Toolkit or Zustand efficiently

7. Missing Keys in Lists

• Cause: Using an array index as a key causes re-renders
• Fix: Use unique, stable keys for list items

8. Unoptimized Images/Assets

• Cause: Loading large media files
• Fix: Use compressed, responsive images and lazy-load assets

63. How to implement optimistic UI updates in React, and what are the trade‑offs involved?

Optimistic UI updates show the “expected” result immediately, then reconcile with the server response.

Implementation Process:

import { useMutation, useQueryClient } from "@tanstack/react-query";
import { likePost } from "./api";
function useOptimisticLike(postId: string) {
const qc = useQueryClient();
return useMutation({
mutationFn: () => likePost(postId),
onMutate: async () => {
await qc.cancelQueries({ queryKey: ["post", postId] });
const prev = qc.getQueryData<{ id: string; likes: number }>(["post", postId]);
qc.setQueryData(["post", postId], (d: any) => ({ ...d, likes: d.likes + 1 }));
// optimistic
return { prev };
},
onError: (_err, _vars, ctx) => {
if (ctx?.prev) qc.setQueryData(["post", postId], ctx.prev); // rollback
},
onSettled: () => qc.invalidateQueries({ queryKey: ["post", postId] }), // refetch
});
}

Trade-offs:

• Pros: Snappy UX, perceived speed, fewer loading states
• Cons: Possible state/server mismatch, rollbacks on failure, handling race conditions & conflicts (e.g., double-clicks), more complex error handling, and cache invalidation

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64. What are React Server Components, and how do they differ from traditional client components?

About React Server Components

RSCs are a modern React feature that allows parts of a React application to be rendered on the server rather than in the browser. They let the server handle data fetching, computation, and rendering of non-interactive UI elements, which are then streamed to the client.

This approach reduces the client JavaScript bundle size, speeds up rendering, and improves performance, especially on data-heavy pages.

About Traditional Client Components

Traditional client components run entirely in the browser. They handle both logic and rendering, requiring all necessary JavaScript to be downloaded, parsed, and executed by the client.

While this enables full interactivity, it can lead to slower load times for large applications.

65. Explain concurrent rendering in React.

Concurrent Rendering in React (introduced in React 18) enables React to prepare multiple UI updates simultaneously without blocking the main thread. It makes the UI more responsive and interruptible.

React can pause, resume, or discard rendering work based on priority (e.g., a user typing vs. background data loading).

Example:

import { useTransition } from "react";
function SearchBox() {
const [query, setQuery] = useState("");
const [isPending, startTransition] = useTransition();
function handleChange(e) {
const value = e.target.value;
setQuery(value);
// Defer expensive update (like filtering)
startTransition(() => {
filterLargeList(value);
});
}
return (
<div>
<input value={query} onChange={handleChange} />
{isPending && <span>Loading...</span>}
</div>
);
}

66. What is useDeferredValue, and in what scenarios is it used?

useDeferredValue is a React Hook that helps delay re-rendering of non-urgent updates to keep the UI responsive.

It displays the current value immediately while deferring expensive updates (like filtering, searching, or rendering large lists) to a lower priority.

Example:

import { useState, useDeferredValue } from "react";
function SearchList({ items }) {
const [query, setQuery] = useState("");
const deferredQuery = useDeferredValue(query); // lower-priority value
const filtered = items.filter(item =>
item.toLowerCase().includes(deferredQuery.toLowerCase())
);
return (
<>
<input value={query} onChange={(e) => setQuery(e.target.value)} />
<ul>
{filtered.map((item) => <li key={item}>{item}</li>)}
</ul>
</>
);
}

When to Use it:

• When typing or having quick interactions, they should stay smooth while heavy computations run in the background
• Ideal for search bars, large tables/lists, or data filtering, where real-time rendering may cause lag

67. Describe how Suspense works for data fetching and code splitting.

Suspense in React is a mechanism that lets components “wait” for something (like data or code) before rendering, showing a fallback UI (e.g., a loader) in the meantime.

1. Code Splitting

Suspense works with React.lazy() to load components on demand.

const Profile = React.lazy(() => import('./Profile'));
<Suspense fallback={<div>Loading...</div>}>
<Profile />
</Suspense>

2. Data Fetching

Suspense lets you defer rendering until async data is ready.

const data = use(fetchUserData()); // imaginary Suspense-enabled hook
<Suspense fallback={<div>Fetching user...</div>}>
<UserProfile data={data} />
</Suspense>

68. What is the difference between useMemo and useCallback?

• useMemo → caches values
• useCallback → caches functions

69. How does React’s automatic batching improve performance?

Automatic batching in React groups multiple state updates that occur within the same event loop into a single re-render, rather than re-rendering after each update.

Example:

setCount(c => c + 1);
setFlag(f => !f);
// React batches these → only one re-render

Result:

Fewer renders → less DOM work, better performance, and smoother UI.

70. What are React Portals, and what problems do they solve?

React Portals render a component’s output outside its parent DOM hierarchy into a different part of the DOM tree.

Example:

import { createPortal } from "react-dom";
function Modal({ children }) {
return createPortal(
<div className="modal">{children}</div>,
document.getElementById("modal-root")
);
}

They solve the following problems:

• Rendering modals, tooltips, or dropdowns outside parent containers
• Avoiding CSS overflow or z-index issues (e.g., when parent has overflow: hidden)
• Maintaining a logical React hierarchy while adjusting visual placement in the DOM

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