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React Interview Questions and Answers in 2023

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Bhoomi Parmar
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5 min read

25 React Interview Questions and Answers in 2023

React is one of the most popular JavaScript libraries for building user interfaces. As React's popularity grows, so does the demand for React developers. This means that being prepared for React interview questions is more important than ever.

In this blog, we will cover the most common and latest React interview questions asked by companies in 2023. We will cover questions about:

  • React Basics

  • Components

  • Props and State

  • Lifecycle Methods

  • Functional Components vs. Class Components

  • Hooks

  • Redux

And more...

So without further ado, here are the top 40 React interview questions and answers:

  1. What is React? React is a JavaScript library for building user interfaces. It is maintained by Facebook and a community of individual developers and companies. React allows you to create reusable UI components.

  2. What are the main features of React?

    • Uses the virtual DOM instead of the real DOM

    • Uses JSX syntax that extends JavaScript

    • Uses single-way reactive data flow

    • Provides reusability using composable UI components

  3. What are components in React? Components are the building blocks of any React application. They accept input and return React elements that describe how UI should look. Components allow you to split the UI into independent and reusable pieces.

  4. What are props in React? Props are inputs that are passed to components. They are read-only - components receiving props cannot update them.

  5. What is state in a React component? State allows a component to change its output over time in response to user actions, network responses, etc. State can only be changed inside the component.

  6. What are the lifecycle methods in a class component?

    • componentDidMount

    • componentWillUnmount

    • shouldComponentUpdate

    • componentDidUpdate

    • etc.

  7. What is the purpose of render()? The render() method returns what needs to be displayed on the screen. If the state or props of the component change, render() will be called to update the UI.

  8. What is the difference between state and props?

    • Props are read-only; state changes can be asynchronous

    • Props are passed from the parent, state is managed within the component

    • State specifies the component's local data; props specify how it is used

  9. What is the purpose of keys in arrays of elements? Keys help React identify which items have changed, are added, or are removed. They should be given to the elements inside the array to give the elements a stable identity.

  10. How to bind methods or event handlers in JSX callbacks? There are 3 ways to bind methods or event handlers in JSX callbacks:

    1. Bind in Constructor:
    constructor() {
      super();
      this.handleClick = this.handleClick.bind(this); 
    }
  1. Arrow functions:
    <button onClick={() => this.handleClick()}>
    </button>
  1. Bind in render:
    <button onClick={this.handleClick.bind(this)}>
    </button>

  1. What is the purpose of constructor in React? The constructor for a React component is used to initialize local state by assigning an object to this.state, bind event handler methods to the component instance, and perform other initialization tasks.

  2. What is the difference between state and context?

    • State is component-specific, but context propagates through the component tree

    • State is mutable, but context is immutable

    • State is used to maintain data particular to a component, whereas context is used to share data between components

  3. What is a controlled component? A component that controls the input elements within the forms on subsequent user input is called a Controlled Component. Every state mutation will have an associated handler function.

  4. What is an uncontrolled component? A component that renders a form and controls its input elements without taking input from the user is called an Uncontrolled Component. It stores all its state internally, and we query the DOM using a ref to find its current state.

  5. What is the purpose of refs? Refs provide a way to access DOM nodes or React elements created in the render method. They allow you to manipulate the DOM imperatively.

  6. What are Higher Order Components? Higher Order Components (HOCs) are functions that take a component and return a new component. HOCs are used to reuse component logic and render hierarchies.

  7. What is the difference between functional and class components?

    • Class components have state and lifecycle methods, but functional components do not have state or lifecycle methods.

    • Functional components are easier to write, test, and think about.

    • Class components allow the use of lifecycle methods and local state, while functional components rely on props and state passed from parent components.

  8. What are hooks in React? Hooks are functions that let you “hook into” React state and lifecycle features from functional components. Hooks allow you to use state and other React features without writing a class.

  9. What are the useState and useEffect Hooks for?

    • useState is used to add state to functional components

    • useEffect is used to perform side effects in functional components. It serves the same purpose as componentDidMount, componentDidUpdate, and componentWillUnmount in class components.

  10. What is the purpose of the useEffect Hook? The useEffect Hook allows you to perform side effects in your components. Some examples of side effects are:

    • Data fetching from an API

    • Setting up a subscription

    • Manually changing the DOM

    • Logging

  11. What is the purpose of the useCallback Hook? The useCallback Hook returns a memoized callback function. It only changes if one of the dependencies has changed. This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders.

  12. What is Redux? Redux is a predictable state container for JavaScript applications. It helps you write applications that behave consistently, run in different environments (client, server, and native), and are easy to test.

  13. What are the three principles of Redux?

    • Single source of truth

    • State is read-only

    • Changes are made with pure functions

  14. What is the difference between Redux Thunk and Redux Saga?

    • Redux Thunk allows writing action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action or to dispatch only if a certain condition is met

    • Redux Saga achieves the same functionality by using generator functions instead of thunks. Sagas are easier to test than thunks.

  15. What is React Router? React Router is a routing library for React that enables navigation among views in a React application. It keeps the UI in sync with the URL.

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