ReactJS - Interview Questions and Answers

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Prepare for your next ReactJS interview with our comprehensive guide to the most frequently asked questions. From component lifecycle to state management, master key concepts and boost your confidence. Ace your React developer interview today!

ReactJS Interview Questions & Answers

Most asked ReactJS Interview Questions

These are the most asked question in any React interview, no matter if your are fresher or an experienced in React. These 10 questions are the base to judge your knowledge on ReactJS.

What is ReactJS?

ReactJS is an open-source JavaScript library for building dynamic and interactive user interfaces(UIs). React is a simple, feature rich and component based UI library. When we say component based, we mean that React develops applications by creating various reusable and independent codes. React is developed and released by Facebook.

What is the use of ReactJS?

ReactJS primary goal is to create user interface to enhance the speed of programs. React enhance the app's efficiency. It allows using HTML-like syntax within JavaScript code, enabling developers to create reusable components and write less code for UI implementation.

What are the differences between library and framework?

As we got to know that React is a JavaScript library so there must be question that arise how a library is difference from a framework!

• Framework: A framework is a set of pre-written code that provides a structure for developing software applications.
• Library: A library is a collection of pre-written code that can be used to perform specific tasks.

How does ReactJS work?

React is totally rely on Virtual DOM. Virtual DOM is a copy of actual DOM so any chanages in state were done was on Virtual DOM after that ReactJS compare that Virtual DOM with the actual DOM and what are the changes have been made on Virtual DOM and update only those components of actul DOM. This procedure takes less time compare to the classic procedure of updating the whole actual DOM.

What is JSX?

React JSX is an extension to JavaScript. It allows writing a JavaScript code that looks like an HTML code. For instance, consider the following code.

const element = <h1>Hello React!</h1>

Know more about React - JSX.

What are the features of ReactJS

There are so many important features of ReactJS as it is the most demanding library for front-end development.

• Virtual DOM: Virtual DOM is a special kind of DOM that is used in ReactJS. Virtual DOM represents the real DOM of the current HTML document. Whenever there is a change in the HTML document, React checks the updated virtual DOM with the previous state of the Virtual DOM and updates only the difference in th actual/real DOM.
• Reusable Components: Components need to be written a single time and can be used multiple times by just calling that component where we require that component.
• One-Way Data Binding: One-way data binding prevents the data in a component from flowing backward. A component can pass the data to its child component only. This will simplify the data handling and reduce the complexity.

What is the main difference between DOM and Virtual DOM?

The main difference is that DOM represents the actual HTML structure of a web page, while the Virtual DOM is a copy of the DOM used by React to enhance performance while updating the actual DOM.

As you can see in this picture that there is Actaul DOM and beside that there is Vistual DOM, so any changes will be done on the virtual DOM wich is represented by New Virtual DOM. Now this New Virtual DOM will be compared with the Actual DOM and updated the new Nodes into the Actual DOM which will be New Actual DOM.

What are the Components in ReactJS?

React component is the building block of a React application. A React component represents a small chunk of user interface in a webpage. The primary job of a React component is to render its user interface and update it whenever its internal state is changed. In addition to rendering the UI, it manages the events belongs to its user interface.

What are the advantages of ReactJS?

React abstracts away the DOM from you, offering a simpler programming model and better performance. React can also render on the server using Node, and it can power native apps using React Native. React implements one-way reactive data flow, which reduces the boilerplate and is easier to reason about than traditional data binding.

Know more about ReactJS - Advantages and Disadvantages.

How ReactJS is difference from React Native?

The main difference between React Native and ReactJS is that ReactJS is used to create UI on web application and on other hand React Native is used to create mobile application that can work on both android and iOS platform.

• ReactJS: ReactJS can be used to develop small applications as well as big, complex applications. ReactJS provides minimal and solid feature set to kick-start a web application.
• React Native: React Native lets you build mobile apps using only JavaScript. It uses the same design as React, letting you compose a rich mobile UI from declarative components.

React Component Interview Questions

These questions focus on the building blocks of React applications. They cover how to create, use, and manage components, which are like reusable pieces of your user interface.

What is a React component?

A React component is a reusable piece of code that represents part of a user interface. It can be a function or a class that return JSX element, describing what should appear on the screen. Components can accept inputs called props and can maintain their own state. They're the building blocks of React application, allowing you to build complex UIs from smaller reusable parts.

What are the differences between class components and functional components in React?

How do you create a class component in React?

To create a class component in React, start by importing React and extend React.Component with your class.Inside this class, include a render() method that returns the JSX to be displayed.

import React, { Component } from 'react';

class Welcome extends Component {
  render() {
    return <div>Welcome to TutorialsPoint</div>
  }
}

export default Welcome;

This creates a class component named Welcome that displays "Welcome to TutorialsPoint".

How do you create a functional component in React?

To create a functional component in React, start by importing React. Then, define a function that returns the JSX you want to display. If you need to use props, pass props as an argument to the function.

import React from 'react';

function MyComponent(props){
    return <div>Welcome to {props.siteName}</div>
}
export default MyComponent;

Here, MyComponent is a functional component that displays a message using the props.siteName value.

What are props in React, and how do you use them?

Props in React are short for "properties" and are used to pass data from a parent component to a child component. You define them in the parent component and pass them as attributes. The child component receives these props as an argument and can access them to render dynamic content.

//Parent Component
function Parent(){
    return <Child message="I am Parent Component."/>;
}
//Child component
function Child(props){
    return <div>{props.message}</div>
}

Here, Parent passes the message prop to Child, which displays it.

What is state in React, and how does it differ from props?

State in React is used to manage data that can change over time within a component. Unlike props, which are passed from a parent component and are read-only, State is managed within the component itself and can be updated using this.section() in class components or the useState in functional components. While Props are for passing data to child components, while state is for handling internal data that affects how the component render.

Can you explain the lifecycle methods of a React component?

Lifecycle methods in a React class component are special methods that allow you to execute code at specific points in the component's lifecycle. Here are the lifecycle methods:

• componentDidMount(): Called once after the component is first added to the page. It is used for initialization tasks, such as fetching data.
• componentDidUpdate(prevProps, prevState): Called after the component updates due to changes in props or state. It is useful for responding to those changes.
• componentWillUnmount(): Called just before the component is removed from the page. It is ideal for cleanup tasks, such as cancelling network requests or removing event listeners.
• componentDidCatch(error, info): Used to handle errors that occur in the component tree.

What are hooks in React, and how do they enhance functional components?

Hooks are special functions in react that enable functional components to use state and other features that were previously only available in class components. Here are a few important hooks:

• useState: Lets functional components manage state, such as tracking user inputs or counters.
• useEffect: Manages side effects like fetching data or updating the DOM after the component has rendered.
• useContext: Allows you to access and use context values directly, without the need to pass props through multiple layers.

How do you handle events in React components?

To handle events in React components:

• Define event handler functions: Create methods within the component to handle specific events.

function ExampleComponent() {
    const handleEvent = () => {
        // Event handling logic
    };
}

Attach handlers to elements: Use camelCase event attributes in JSX to bind handlers.

function ExampleComponent() {
    const handleEvent = () => {
        // Event handling logic
    };          
    return <button onClick={handleEvent}>Activate</button>;
}          

Pass handlers to child components: When necessary, provide event handlers as props to child components.

function ParentComponent() {
    const handleEvent = () => {
        // Event handling logic
    };          
    return <ChildComponent onEvent={handleEvent} />;
}

What is the purpose of the key prop in React, and when should you use it?

The key prop in React helps track which items in a list have changed, been added, or removed.

When to use it?

• Users can use it when rendering lists to allow React handle updates more efficiently.
• Avoid using array indexes as keys because they can lead to performance issues and bugs when items change.

Example:

{items.map(item => <li key={item.id}>{item.name}</li>)}

React Refs Interview Questions

These questions cover how to use refs to access DOM elements or components. when they're necessary, and common mistakes to avoid.

What are React Refs and why are they used?

React refs allow direct access to a DOM element or a React component instance, bypassing the typical data flow in React. They enable direct interaction with elements or components.

Why use them?

• Direct DOM Manipulation: Useful for actions like focusing on an input field or measuring its size.
• Third-Party Library Integration: Useful for libraries that need direct access to DOM elements.
• Manage Component Instances: Allows you to call methods on child components or directly manage their state.

How do you create a ref in a class component?

To create a ref in a class component in React, follow these steps:

1. Initialize the Ref: Create a ref using React.createRef() in the constructor.
2. Access the Ref: Use the ref attribute to link the ref to an element in render().
3. Use the Ref: Access the element or component using this.myRef.current when needed.

Example:

class MyComponent extends React.Component {
    constructor(props) {
      super(props);
      this.myRef = React.createRef(); // initialization
    }
  
    componentDidMount() {
      this.myRef.current.focus(); // Using
    }
  
    render() {
      return <input ref={this.myRef} />; // Accessing
    }
}

How do you create a ref in a functional component using the useRef hook?

To create a ref in a functional component using the useRef hook, follow these steps:

• Import the Hook: Import useRef from React.
• Initialize the Ref: Call useRef() inside your functional component to create a ref.
• Attach the Ref: Use the ref attribute to link the ref to a DOM element in the JSX.
• Use the Ref: Access the DOM element or component using the ref.current when needed

Example

import React, { useRef, useEffect } from 'react';

function MyComponent() {
  //Initialize the Ref
  const myRef = useRef(null);

  useEffect(() => {
    // Use the Ref
    myRef.current.focus();
  }, []);

  return (
    //Attach the Ref
    <input ref={myRef} />
  );
}

What are some common use cases for React Refs?

React refs are commonly used for accessing DOM elements directly, managing focus on inputs, triggering animations, integrating with third-party libraries, and controlling child component methods or state.

How do you access a DOM element using a ref?

To access a DOM element using a ref, follow these steps:

• Create the Ref: Initialize a ref using React.createRef() in a class component or useRef() in a functional component.
• Attach the Ref: Connect the ref to the target DOM element, by using the ref attribute in your JSX.
• Access the Element: Use ref.current to interact with the DOM element, such as calling methods or getting properties.

Example

import React, { useRef, useEffect } from 'react';

function MyComponent() {
  const myRef = useRef(null); // Creating

  useEffect(() => {
    myRef.current.focus(); // Attaching
  }, []);

  return <input ref={myRef} />;//Accessing
}

What is the difference between createRef and useRef?

The createRef is used for class components, while useRef is used for function components. The createRef creates a new ref each time the component re-renders, but useRef is used in function components and returns the same ref object on every render, making it more efficient.

Can you explain how to use forwardRef in React?

React forwardRef lets you pass a ref from a parent component to a child component. Wrap your component with forwardRef, then accept the ref as the second parameter after props.

How do you handle focus management using refs in React?

Use refs to get direct access to DOM elements. Create a ref with useRef, attach it to an element with the ref attribute, then call focus() on the ref's current property. This works well for managing focus in forms or accessibility features.

What are callback refs and how do they differ from object refs?

The callback refs are functions, called when a component mounts or unmounts. Object refs use the ref attribute with a ref object. The callback refs give you more control over when refs are set and unset, useful for complex ref logic or when you need to know exactly when a ref is attached.

What are some potential pitfalls or common mistakes when using refs in React?

Common ref mistakes include:

• Overusing refs instead of state
• Modifying DOM directly with refs, bypassing React's lifecycle
• Using deprecated string refs.
• Not handling null refs on initial render
• Not cleaning up refs in useEffect's return function

React Router Interview Questions

These questions cover how to set up routes, handle navigation, and manage views in single-page applications without page reloads.

What is React Router and why is it important in React applications?

React Router is a library for handling routing in React apps. It's crucial for creating single-page applications, allowing navigation between views without page reloads. This improves user experience and helps organize complex applications.

How do you create a simple route that renders a component when the URL matches a specific path?

Use the Route component from React Router:

<Route path="/about" element={<About />} />;

This renders the About component when the URL path is /about.

What is the difference between BrowserRouter and HashRouter?

BrowserRouter uses clean URLs with HTML5 history API while HashRouter uses hash portion(#) of the URL. BrowserRouter is preferred for modern browsers, while HashRouter is useful for older browsers or static file serving.

How do you handle nested routes in React Router?

To handle nested routes in React Router, you nest Route components inside a parent route. The child routes inherit the path of the parent and can extend it.

<Route path="/users" element={<Users />}>
    <Route path=":id" element={<UserProfile />} />
</Route>

What is the purpose of the Switch component in React Router?

Switch(now Routes in React Router v6) component renders only the first matching route, ensuring that only one route is shown at a time.

How can you programmatically navigate to a different route using React Router?

We can programmatically navigate to a different route using the useNavigate hook.

const navigate = useNavigate();
navigate('/new-route'); 

This programmatically changes the route without user interaction.

What are route parameters and how do you use them in React Router?

Route parameters are dynamic segments in URLs. Define them in the route path with a colon, like this:

<Route path="/users/:id" element={<UserProfile />} />

Access them using the useParams hook:

const { id } = useParams();

How do you protect routes in a React application using React Router?

Create a PrivateRoute component that checks authentication status. Wrap protected routes with this component:

<Route
path="/dashboard"
element={
    <PrivateRoute>
        <Dashboard />
    </PrivateRoute>
}
/>

What is the Link component and how does it differ from a regular HTML anchor tag?

Link is a React Router component for navigation. Unlike <a> tags, Link prevents full page reloads, maintaining the single-page app experience. It updates the URL without refreshing the page.

How do you handle 404 pages (not found routes) in React Router?

Use a catch-all route at the end of your route definitions:

<Routes>
    {/* Other routes */}
    <Route path="*" element={<NotFound />} />
</Routes>

This renders the NotFound component for nay unmatched routes.

React Styling Interview Questions

These questions cover various methods for styling React components, from traditional CSS to React-specific techniques.

What are the different ways to style React components?

• Inline styles
• CSS stylesheets
• CSS Modules
• CSS-in-JS libraries(e.g., styled-components)
• Utility-first CSS frameworks (e.g., Tailwind CSS)
• SAAS/LESS with appropriate loaders
• CSS custom properties(variables)

How do you apply inline styles in React, and what are the pros and cons of this approach?

Apply inline styles using the style prop with a JavaScript object:

<div style={{ color: 'blue', fontSize: '14px' }}>Hello</div>

• Pros: Easy to use and supports dynamic styling.
• Cons: No CSS reuse, no media queries and can clutter components.

What are CSS Modules, and how do they help in styling React components?

CSS Modules are CSS files with locally scoped class names by default, which helps avoid naming conflicts and improve modularity.

Usage:

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

function Button() {
    return <button className={styles.button}>Click me</button>;
}    

How do you use the styled-components library to style React components?

The styled-components library allows you to write CSS directly within you javascript.

import styled from 'styled-components';

const Button = styled.button`
    background-color: blue;
    color: white;
`;

function MyComponent() {
    return <Button>Click me</Button>;
}

What is the difference between CSS-in-JS and traditional CSS?

How can you implement theming in a React application using styled-components or another CSS-in-JS library?

Use ThemeProvider component and pass a theme object:

import { ThemeProvider } from 'styled-components';

const theme = {
  colors: {
    primary: 'blue',
    secondary: 'green'
  }
};

function App() {
  return (
    <ThemeProvider theme={theme}>
      {/* Your app components */}
    </ThemeProvider>
  );
}

What are the benefits of using a utility-first CSS framework like Tailwind CSS in React?

Using a utility-first CSS framework like Tailwind CSS offers these benefits:

• Rapid development
• Consistent design
• Small bundle size
• Highly customizable
• No need to create custom class names
• Easy responsive design

How do you handle global styles in a React application?

To handle global styles, follow these steps:

• Create a global.css files and import it into your main App component.
• Use CSS custom properties for global variables.
• Utilize createGlobalStyle from style-components for CSS-in-JS.
• Apply styles to high-level components that wrap the entire app.

What are some best practices for maintaining responsive design in React?

For a responsive design in React, follow these:

• Use CSS media queries or responsive utilities (e.g., Tailwind's responsive classes).
• Implement a mobile-first approach
• Use relative units (em, rem, %) instead of fixed pixels
• Utilize CSS Flexbox and Grid layout.
• Test on various devices and screen sizes.
• Consider using React's useMediaQuery hook for responsive logic.

How do you manage and apply animations in React components?

There are several ways to manage and apply animations in React components:

• CSS animations and transitions
• React transition Group for entering/exiting animations
• CSS-in-JS libraries like styled-components for dynamic animations
• React, Spring or Framer Motion for more complex animations.
• Use the useSpring hook for declarative animations in functional components

For example, using React Spring:

import { useSpring, animated } from 'react-spring';

function AnimatedComponent(){
    const props = useSpring({opacity:1, from: { opacity: 0 } });
    return <animated.div style={props}> I will fade in</animated.div>
}

React Redux Interview Questions

These questions cover how Redux manages state in react apps, including setup and best practices.

What is Redux and how does it integrate with React?

Redux is a state management library for javascript apps. It integrates with React using the react-redux package, helps manage app state in a single-place. This makes it easier for components to access and update state without passing props through many layers.

What are the core principles of Redux?

The core principles of Redux are:

• Single source of truth: All app state is stored in one object tree.
• State is read-only:You can only change state by dispatching actions.
• Changes are made with pure functions: Reducers are pure functions that take the previous state and an action to return the next state.

How do you set up a Redux store in a React application?

To set up a Redux store in a React application:

• Create a root reducer
• Set up the store with createStore
• Wrap your app with Provider

Here is an example to set up a Redux store in a React application:

import { createStore } from 'redux';
import { Provider } from 'react-redux';
import rootReducer from './reducers';

const store = createStore(rootReducer);

function App() {
  return (
    <Provider store={store}>
      {/* Your app components */}
    </Provider>
  );
}

What are actions and reducers in Redux, and how do they work together?

Actions are plain objects describing what happened in the app. They have a type and optional payload. Reducers are pure functions that take the current state and an action, then return a new state. Actions describe the change, reducers specify how the state changes.

How do you connect a React component to the Redux store?

Use the useSelector and useDispatch hooks in functional components.

import { useSelector, useDispatch } from 'react-redux';

function MyComponent() {
  const data = useSelector(state =< state.someData);
  const dispatch = useDispatch();

  // Use data and dispatch actions
}

What is the purpose of middleware in Redux, and how do you use it?

Middleware is used to handle things like logging and async operations before actions reach the reducers. Apply middleware when setting up the store:

import { createStore, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';

const store = createStore(rootReducer, applyMiddleware(thunk));

How do you handle asynchronous actions in Redux?

Use middleware like redux-thunk or redux-stage. With redux-thunk, create action creators that return functions instead of objects.

const fetchData = () => {
    return async dispatch => {
        dispatch({ type: 'FETCH_START' });
        try {
          const data = await api.getData();
          dispatch({ type: 'FETCH_SUCCESS', payload: data });
        } catch (error) {
          dispatch({ type: 'FETCH_ERROR', error });
        }
    };
};

What are some common patterns for structuring Redux state?

Consider the following approaches:

• Domain-based: Organize by feature or domain.
• Normalized state: Flatten nested data structures.
• Separate data and UI state.
• Use slices: Break state into smaller, manageable pieces.
• Keep minimal and derived state separate.

How do you use the useSelector and useDispatch hooks in a functional component?

The useSelector selects data from the store, while useDispatch dispatches actions. Here's how you use them:

import { useSelector, useDispatch } from 'react-redux';

function Counter() {
  const count = useSelector(state => state.counter);
  const dispatch = useDispatch();

  return (
    <div>
      <span>{count}</span>
      <button onClick={() => dispatch({ type: 'INCREMENT' })}>
        Increment
      </button>
    </div>
  );
}

What are some best practices for optimizing performance in a React-Redux application?

To optimize performance in a React-Redux app, follow these best practices:

• Use memoization (React.memo, useMemo, useCallback)
• Implement reselect for efficient derived data
• Normalize state to avoid deep nesting
• Use shallowEqual for object comparisons in useSelector
• Batch updates with Redux Toolkit's createSlice
• Avoid re-renders by selecting only necessary state.
• Use Redux DevTools for performance profiling
• Implement code splitting and lazy loading for large apps