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31+ React Interview Questions And Answers

React is a widely used JavaScript library for building user interfaces, especially single-page applications where you need a fast, interactive user experience. React enables developers to create large web applications that use data which can change over time, without reloading the page. Its key feature is the ability to build components, which are custom, reusable HTML elements, that can be nested, managed, and handled independently. This summary encompasses the essence of React and serves as a prelude to various React interview questions, tailored for both beginners and advanced developers.


Most asked React interview questions



What is React and who created it?

React is a JavaScript library for building user interfaces. It was created by a team of engineers at Facebook.


Can you explain what JSX is?

JSX stands for JavaScript XML. It allows us to write HTML inside JavaScript and place them in the DOM without using functions like createElement() or appendChild().


What are components in React?

Components are the building blocks of any React application. They describe a part of the user interface. They are reusable and can be nested inside other components.


What is the virtual DOM?

The virtual DOM is a programming concept where an ideal, or 'virtual', representation of a UI is kept in memory and synced with the 'real' DOM by a library such as React DOM. This process is known as reconciliation.


What is state in React?

State of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components.


What are props in React?

Props (short for properties) are a way of passing data from parent to child components. They are read-only and cannot be modified by the child components.


What is a key in React lists and why is it important?

Keys are attributes that help React identify which items have changed, been added, or been removed in a list. They should be given to the elements in a list to give the elements a stable identity.


How do you handle events in React?

Events in React are handled using camelCase syntax and by passing functions rather than strings. The event handling function is passed as a prop to the component element.


How can you update the state of a component?

You can update the state of a component using the setState() method.


What is a higher-order component?

A higher-order component (HOC) is a function that takes a component and returns a new component. It's a pattern used in React for reusing component logic.


What is the difference between state and props?

State is managed within the component and can be changed, while props are passed to the component by the parent and are immutable.


Can you show an example of using state in a component?

Certainly, consider a button that tracks how many times it has been clicked:

class ClickCounter extends React.Component {
  constructor(props) {
    this.state = { count: 0 };

  handleClick = () => {
    this.setState({ count: this.state.count + 1 });

  render() {
    return (
      Clicked {this.state.count} times


What are life cycle methods in React?

Life cycle methods are special methods on React components that run automatically at specific points during a component's life cycle. Examples include componentDidMount(), componentDidUpdate(), and componentWillUnmount().


Can you explain what a functional component is?

A functional component is a JavaScript function that returns a React element. They often result in simpler and less code compared to class components.


What is a controlled component?

A controlled component is a component that derives its input values from state. For example, a form input whose value is controlled by React in this way is considered a controlled component.

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Explain the virtual DOM diffing algorithm.

React's diffing algorithm is used for updating the DOM. It compares the previous and next virtual DOMs, identifies changes, and updates the real DOM efficiently. This allows for minimal DOM manipulation, improving performance.


What are Pure Components?

PureComponent is similar to Component, but it handles the shouldComponentUpdate method for you. It does a shallow comparison on the state and props data to determine if re-rendering is necessary.


How does React handle forms?

React relies on controlled components for form handling. The form's data control can be centralized in the state of the component. Thus, inputs receive their current values from the state, mutating the state to update inputs.


Can you explain context?

Context provides a way to pass data through the component tree without having to pass props down manually at every level. It's used for data that can be considered 'global' for a tree of React components.


How can you optimize the performance of a React application?

To optimize performance, make sure to use the production version of React, utilize PureComponent and memo() for preventing unnecessary renders, implement lazy loading with React.lazy() and Suspense, and use shouldComponentUpdate wisely.


Explain React Fiber.

React Fiber is the new reconciliation engine or core algorithm in React 16. Its main goal is to enable incremental rendering of the virtual DOM.


What is a Fragment in React?

React Fragments allow components to return multiple elements without wrapping them in an extra DOM element, such as a div.


Explain the concept of higher-order components (HOCs) with an example.

An HOC is a function that takes a component and returns a new component. It's a pattern used for reusing component logic. For example, if you had an HOC that provided data fetching capabilities, it could be used with different components to handle data requests.

function withDataFetching(WrappedComponent) {
  return class extends React.Component {
    // Logic to fetch data on mount
    // Pass data as props to the WrappedComponent


Give an example where you would use the useRef hook.

useRef hook can be used to directly access a DOM element. It's used, for instance, to focus an input element on component mount.

function TextInputWithFocus() {
  const inputEl = useRef(null);

  useEffect(() => {
  }, []);

  return ;


What are render props?

Render props are a technique for sharing code between React components using a prop whose value is a function. They allow you to tell a component what it should render.


How do you ensure that a component is using the latest state or props?

You can use the useEffect hook to ensure that a component is working with updated state or props. This hook can be configured to rerun whenever certain values change.


What does this code snippet do?

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

This code sets up an effect with React's useEffect hook that logs 'Component mounted' to the console when the component mounts.


Can you explain the use of useMemo and useCallback?

useMemo is a hook that memoizes a computed value so that it does not need to be recomputed on every render if the dependencies have not changed. useCallback is a hook that memoizes a callback function to prevent unnecessary renders.


How does the shouldComponentUpdate lifecycle method work?

shouldComponentUpdate is a lifecycle method in class components that allows you to decide whether the next component's update should trigger a re-render. Returning false from it prevents the component's update.


Give an example of a side effect caused by a React component and how to handle it.

A side effect could be a data fetch on component mount. It is handled using the useEffect hook, making sure to properly cleanup on component unmount if necessary.

useEffect(() => {
  const fetchData = async () => {
    const result = await myApi.getData();


  return () => {
    // cleanup logic here
}, []);


What is the children prop?

The children prop is used to pass elements directly into components that represent where those children should be placed in the component tree. It is the content between the opening and closing tags of a component.


React Interview Tips

Stay Calm and Composed

  • Maintaining a calm demeanor during an interview is key. Despite the pressure, staying composed ensures that you can think clearly and communicate effectively. Practice breathing exercises, positive affirmations, or mindfulness techniques before the interview to reduce anxiety and manage stress. In the moment, feel free to take a short pause to collect your thoughts before answering tough questions. This shows that you are thoughtful and deliberate rather than reactive. Remember, interviewers are looking for confidence and the ability to handle challenging situations, which is often reflected in your composure.

Understand the Question Before Answering

  • One common mistake during interviews is rushing to answer without fully understanding the question. Take a moment to clarify the question if needed. It's perfectly acceptable to ask the interviewer to repeat or to elaborate on the question. This shows that you are attentive and committed to providing a thoughtful response. It’s much better to give a well-considered answer than to hurriedly offer one that misses the point. If a question is complex, breaking it down into parts can help you to address each component fully. If you're unsure about your interpretation, repeat the question back in your own words to confirm your understanding.

Use the STAR Method

  • For behavioral interview questions, which often start with 'Tell me about a time when...', use the STAR method: Situation, Task, Action, Result. This technique allows you to structure your responses in a clear, concise manner. Begin by describing the Situation and your assigned Task. Then, discuss the Action you took to address the task or problem, and end with the Result of your action. It's a powerful approach to illustrate your skills, show how you handle real-world problems, and provide concrete examples that demonstrate your experience and value to the employer.

Be Honest About What You Don't Know

  • It's better to admit when you don't know an answer than to pretend you do. If you're asked a technical question that you can't answer, be honest about your current knowledge, and express a willingness to learn. You can say something like, 'I'm not familiar with that specific technology or technique, but I'm eager to learn more about it. Can you tell me more?' or 'That's an area I haven't had much experience in, but I'm interested in expanding my skills in that direction.' Employers appreciate honesty and a growth mindset over a know-it-all attitude.

Provide Evidence and Examples

  • Whether answering technical or behavioral questions, using specific examples to back up your statements makes them more compelling. When discussing your skills or experiences, reference projects you've worked on, challenges you've overcome, or achievements you've accomplished. This not only demonstrates your expertise but also your hands-on experience. For instance, talk about a difficult bug you solved or a feature you developed, and explain the impact it had. These real-world examples give credibility to your claims and help you stand out from other candidates.


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