Top React Js Interview Questions | Answers, Coding Tips & Examples

Preparing for a React interview can feel overwhelming, especially when you're trying to master both the theoretical concepts and hands-on coding challenges. In this guide, we break down the most asked React JS interview questions, offering clear explanations and practical tips to help you navigate your next interview with confidence. Whether you're just starting out or looking to refine your skills, this resource is designed to give you real-world insights into what interviewers are really asking and how you can prepare effectively.

Why are React JS Skills in High Demand?

React JS has rapidly become one of the most famous library in modern web development. Its component-based architecture and virtual DOM enable developers to create highly interactive, efficient, and scalable user interfaces. Companies favor React because it not only streamlines development with reusable components but also enhances performance, making it ideal for building dynamic web applications. With a strong community backing continuous innovation and a vast ecosystem of libraries and tools, professionals skilled in React JS are in high demand, opening up exciting opportunities across the tech industry.

Basic ReactJS Interview Questions for freshers

1. What is React JS?

React is an open-source JavaScript library for building user interfaces (UIs). It focuses on creating reusable UI components and efficiently updating the DOM using a virtual DOM for performance optimization.

2. What are Components in React?

Components are reusable, independent blocks of code that return HTML (via JSX) to define parts of a UI. They can be functional (using functions) or class-based (using ES6 classes).

What is JSX?

JSX (JavaScript XML) is a syntax extension that allows writing HTML-like code in JavaScript. It makes React code more readable and is transpiled to React.createElement() calls by tools like Babel.

Example:

1const element = <h1>Hello, React!</h1>;

4. What is the Virtual DOM? How is it different from the Real DOM?

The Virtual DOM is a lightweight, in-memory representation of the actual DOM (Document Object Model). React uses it to optimize updates by first making changes to the Virtual DOM, then efficiently syncing only the differences with the Real DOM.

Virtual DOM

• Lightweight JavaScript object.
• Updates are fast (no screen repaints).
• Used by React to minimize DOM operations.

Real DOM or Actual DOM

• Heavyweight browser structure.
• Updates are slow (triggers reflows).
• Directly manipulated by vanilla JS.

5. Difference between State vs. Props?

State

• Mutable: Managed internally by a component (e.g., useState hook).
• Purpose: Tracks dynamic data that changes over time (e.g., form input, UI toggles).
• Scope: Local to the component unless passed as props to children.
• Example:
  .jsx

  1const [username, setUsername] = useState("");

Prop

• Immutable: Passed from parent to child components (read-only).
• Purpose: Share data/configurations between components (e.g., <UserProfile name="John" />).
• Scope: Received by child components and cannot be modified by them.
• Example:
  .jsx

  1<Component title="Hello" />

6. What are React Hooks?

Hooks (introduced in React 16.8) let you use state and lifecycle features in functional components. Common hooks:

• useState: Manage state.
• useEffect: Handle side effects (e.g., API calls).
• useContext: Access context values.

7. Why are Keys Important in Lists?

Keys (key={id}) help React identify which items in a list have changed, been added, or removed. They optimize re-rendering and prevent bugs.

8. Controlled vs. Uncontrolled Components

• Controlled: Form data is handled by React state (e.g., <input value={value} onChange={...} />).
• Uncontrolled: Form data is handled by the DOM itself (e.g., using useRef).

Controlled

• Definition: Form elements (like <input>) managed by React state.
• Behavior: Values are stored in state and updated via onChange.
• Use Case: Predictable forms with validation or dynamic UI updates.
• Example:
  .jsx

  1function Form() {  
  2const [value, setValue] = useState("");  
  3  return (  
  4      <input  
  5      value={value}  
  6      onChange={(e) => setValue(e.target.value)}  
  7      />  
  8  );  
  9}  
  10 

Uncontrolled

• Definition: Form elements managed directly by the DOM.
• Behavior: Values accessed via ref (e.g., useRef hook).
• Use Case: Integrating with non-React code or handling large forms.
• Example:
  .jsx

  1function Form() {  
  2    const inputRef = useRef(null);  
  3    return <input defaultValue="React" ref={inputRef} />;  
  4 }

Learn More about useRef Hook

9. What is the Purpose of render() in Class Components?

The render() method returns the JSX that defines the UI of a class component. It’s called whenever the component’s state or props change.

10. How Does React Handle Conditional Rendering?

Use JavaScript operators like &&, ternary (? :), or variables to conditionally render JSX.

Example:

1{isLoggedIn ? <Dashboard /> : <Login />} 
2// or using && operator
3{isLoggedIn && <Dashboard /> />

Advanced React JS Interview Questions for Experienced

1. How do React.memo, useMemo, and useCallback optimize performance?

• React.memo: Memoizes functional components to prevent re-renders if props don’t change.
• useMemo: Caches expensive calculations (e.g., filtered lists).
• useCallback: Caches function references to avoid unnecessary child re-renders.

2. Explain React Server Components (RSC) and their benefits.

RSCs run on the server and:

• Reduce client-side bundle size by sending only HTML/JSON.
• Directly access backend resources (DBs, APIs).
• Integrate with client components via "use client" directives.

  Example :

  .jsx

  1// ServerComponent.server.js  
  2export default function ServerComponent() {  
  3const data = fetchDataFromDB(); // Runs on server  
  4return <ClientComponent data={data} />;  
  5}  

3. How does React Suspense work with data fetching?

Suspense lets components "wait" for data by throwing a promise. Use frameworks like React Query or Relay to integrate:

1const data = fetchData(); // Throws promise  
2return <DataView data={data} />;  
3
4// Wrap in Suspense:  
5<Suspense fallback={<Spinner />}>  
6<DataComponent />  
7</Suspense>   

4. Describe "lifting state up" in React.

Lifting state up involves moving shared state from child components to their closest common parent. This allows sibling components to access and update the same state.

Example

1function Parent() {
2const [count, setCount] = useState(0);
3return (
4  <div>
5    <ChildA count={count} />
6    <ChildB setCount={setCount} />
7  </div>
8);
9} 

• Use Case: Synchronizing data between siblings (e.g., form inputs, filters).

5. Functions of Higher-Order Components (HOCs):

• Reuse Logic: Share code (e.g., auth checks, logging) across components.
• Props Manipulation: Inject additional props into wrapped components.
• Conditional Rendering: Control component rendering based on conditions.
  .jsx

  1const withLogger = (Component) => {
  2return (props) => {
  3console.log("Rendered:", Component.name);
  4return <Component {...props} />;
  5};
  6};

6. State the components of the React router.

• BrowserRouter: Wraps the app for client-side routing.
• Routes: Container for defining route groups.
• Route: Maps a path to a component (e.g., <Route path="/" element={<Home />} />).
• Link: Navigates between routes without reloading (e.g., <Link to="/about">About</Link>).
• Navigate: Programmatically redirect users (e.g., <Navigate to="/login" />).

7. What is the strict mode in React?

React Strict Mode is a development tool that helps identify potential problems in an application. It activates additional checks and warnings for its child components

8. What is Babel?

Babel is a popular JavaScript transpiler that converts modern JavaScript code—including ES6+ syntax and JSX—into a version that can run in older browsers and environments. It allows developers to write code using the latest language features without worrying about compatibility issues. Additionally, Babel’s ecosystem of plugins and presets makes it highly customizable to fit various project needs, ensuring that your code remains both modern and accessible across different platforms.

ReactJS Redux Interview Questions

1. What is Redux, and what are its core principles?

Redux is a state management library for JavaScript apps. Its three core principles are:

• Single Source of Truth: The entire app state is stored in one object (the store).
• State is Read-Only: State can only be modified by dispatching actions.
• Changes via Pure Functions: Reducers (pure functions) take the previous state + action to return a new state.

2. What’s the difference between an action and a reducer?

• Action: A plain JavaScript object describing what happened (e.g., { type: 'ADD_TODO', payload: 'Buy milk' }). Created via action creators.
• Reducer: A pure function that takes the current state + action and returns the new state.

Example

1// Reducer  
2const todosReducer = (state = [], action) => {  
3switch (action.type) {  
4  case 'ADD_TODO':  
5    return [...state, action.payload];  
6  default:  
7    return state;  
8}  
9};  

3. What is the purpose of middleware like Redux Thunk?

Middleware like Redux Thunk allows writing async logic (e.g., API calls) by dispatching functions instead of plain action objects.

1// Async action creator  
2const fetchUser = () => {  
3return async (dispatch) => {  
4  const response = await fetch('/api/user');  
5  const data = await response.json();  
6  dispatch({ type: 'FETCH_USER', payload: data });  
7};  
8};  

4. How do selectors optimize Redux performance?

Selectors (e.g., via Reselect) memoize derived data from the Redux store to avoid unnecessary re-renders.

1import { createSelector } from 'reselect';  
2
3// Memoized selector  
4const getCompletedTodos = createSelector(  
5(state) => state.todos,  
6(todos) => todos.filter((todo) => todo.completed)  
7);  

5. What are the benefits of Redux Toolkit (RTK)?

Redux Toolkit simplifies Redux setup by:

• Reducing boilerplate with createSlice (auto-generates actions/reducers).
• Including default middleware (Thunk, DevTools).
• Offering utilities like configureStore and createAsyncThunk.

Example

1// createSlice example  
2const todosSlice = createSlice({  
3name: 'todos',  
4initialState: [],  
5reducers: {  
6  addTodo: (state, action) => {  
7    state.push(action.payload);  
8  },  
9},  
10});  

Most Asked ReactJS Interview Questions and Answers

1. What is React, and why is it used?

React is a JavaScript library used for building user interfaces. It is component-based, declarative, and efficient, making it ideal for developing fast, interactive web applications. React uses a Virtual DOM to optimize updates and re-renders, improving performance.

2. What is the Virtual DOM, and how does React use it?

The Virtual DOM is a lightweight copy of the actual DOM. React updates this Virtual DOM first, compares it with the previous version (diffing process), and only updates the necessary parts in the real DOM. This optimizes performance and speeds up UI updates.

3. What are React Hooks? Name some commonly used hooks.

React Hooks allow function components to use state and lifecycle features without writing class components.

• useState – Manages state in a functional component.
• useEffect – Runs side effects like fetching data.
• useContext – Accesses values from React’s Context API.
• useRef – Maintains a reference without causing re-renders.
• useMemo / useCallback – Optimizes performance.

Learn More about Hooks

4. What is Prop Drilling, and how can it be avoided?

Prop Drilling happens when you pass props through multiple components only to pass them to a deeply nested child.

Ways to avoid Prop Drilling:

• Context API:
  .jsx

  1const ThemeContext = React.createContext();
  2    const Parent = () => (
  3    <ThemeContext.Provider value="dark">
  4        <Child />
  5    </ThemeContext.Provider>
  6    );

• Redux or Zustand: Centralized state management.
• React Query / SWR: Fetching & caching data directly in components.

Conclusion

Preparing for a React JS interview requires a solid understanding of its core concepts, from components and state management to advanced topics like hooks, performance optimization, and server-side rendering. By practicing these commonly asked questions and building real-world projects, you can strengthen your React skills and boost your confidence for the interview. Keep learning, stay updated with the latest React features, and approach each interview with a problem-solving mindset. Good luck!

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