React top 100 interview questions and answer

1.) What is React and how does it work?

React is a JavaScript library for building user interfaces. It allows developers to create reusable UI components and efficiently update them when the underlying data changes. React uses a virtual DOM (Document Object Model) to optimize rendering performance by minimizing direct manipulation of the actual DOM.

2.) What are the key features of React?

The key features of React include:

• Component-based architecture
• Virtual DOM for efficient updates
• JSX syntax for writing components
• Unidirectional data flow
• Reusable and composable UI components
• React Native for mobile app development

3.) What is JSX in React?

JSX is an XML-like syntax extension used in React that allows you to write HTML-like code within JavaScript. It helps to define the structure of UI components in a more declarative way.

4.) What is the difference between a functional component and a class component?

A functional component is a JavaScript function that receives props as input and returns JSX elements. It is simpler and easier to understand. A class component, on the other hand, is an ES6 class that extends the React.Component class. It has additional features like state management and lifecycle methods.

5.) What is the significance of keys in React lists?

Keys are used in React lists to help identify each item uniquely. They improve the performance of list rendering and reconciliation, especially when items are added, removed, or re-ordered. Keys should be stable, unique within their siblings, and not dependent on the item’s index.

6.) What is the state in React?

State is an internal data storage mechanism provided by React. It represents the mutable data that affects a component’s rendering and behavior. State can be updated using the setState method, and when the state changes, React automatically re-renders the component and its children.

7.) What are React hooks?

React hooks are functions introduced in React 16.8 that allow functional components to use state and other React features without writing a class. Some commonly used hooks include useState, useEffect, useContext, and useReducer.

8.) What is the purpose of setState in React?

The setState method is used to update the state of a component. It accepts a new state object or a function that returns a new state based on the previous state. When setStateis called, React re-renders the component and its children with the updated state.

9.) Explain the component lifecycle methods in React.

In class components, the lifecycle methods are:

componentDidMount: Called after the component is mounted to the DOM.
componentDidUpdate: Called after the component is updated.
componentWillUnmount: Called before the component is removed from the DOM.
With the introduction of hooks, functional components can use the useEffect hook to achieve similar functionality.

10.) What is the purpose of React Router?

React Router is a popular library used for routing in React applications. It allows developers to create single-page applications with multiple views by mapping different URLs to different components. It provides declarative routing and navigation capabilities.

11.) What is the purpose of the “useState” hook in React?

The useState hook is used in functional components to add state management capabilities. It allows you to declare state variables and provides a way to update them. By using the useState hook, you can add state to functional components without using class components.

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

12.) What are controlled components in React?

Controlled components are React components where the value of form elements, such as input or textarea, is controlled by React’s state. This means that the state of the component is the single source of truth for the input’s value. To update the value, an onChange event handler is used to update the state, which in turn updates the input.

13.) How can you optimize the performance of React applications?

To optimize React applications, you can:

• Use the production build of React for deployment.
• Implement code splitting and lazy loading to load only necessary components and resources.
• Use shouldComponentUpdate or React.memo to prevent unnecessary re-renders.
• Implement virtualized lists for long lists to improve rendering performance.
• Use React’s profiling tools to identify and optimize performance bottlenecks.

14.) What is the difference between controlled components and uncontrolled components?

In controlled components, form data is managed by React’s component state. The value of the form elements is controlled by React and is updated via event handlers. In uncontrolled components, the form data is handled by the DOM itself, and you can retrieve the values using refs.

15.) What is the purpose of the useEffect hook?

The useEffect hook is used to perform side effects in functional components. It allows you to perform actions such as data fetching, subscriptions, or manually manipulating the DOM after the component has rendered or when certain dependencies change.

16.) What is the purpose of PropTypes in React?

PropTypes is a built-in feature of React that allows you to specify the types of props expected by a component. It helps in validating that the correct data types are passed to a component and provides warnings in the browser console if there are any mismatches.

17.) What are props in React?

Props (short for properties) are a mechanism for passing data from a parent component to its child components. Props are read-only, meaning that child components cannot modify the props received from their parent. Props help in creating dynamic and reusable components by allowing different values to be passed to the same component.

18.) What is the difference between state and props?

Props are used to pass data from a parent component to its child components and are read-only. State, on the other hand, is managed within a component and can be changed using setState(). While props are used for component configuration, the state is used for component-specific data that can change over time.

19.) What is the purpose of the shouldComponentUpdate method?

shouldComponentUpdate is a lifecycle method in class components that allows you to optimize rendering performance. By default, React re-renders a component whenever its state or props change. By implementing shouldComponentUpdate, you can control when a component should re-render by comparing the previous and next state or props and returning a boolean value indicating whether an update is necessary.

20.) What are React hooks? How do they differ from class components?

React hooks are functions that allow you to use state and other React features in functional components. They were introduced in React 16.8 as a way to use stateful logic without writing class components. Hooks like useState, useEffect, and useContext provide a way to manage state, handle side effects, and access context in functional components. Unlike class components, hooks don’t require you to use the this keyword and can be used more easily with JavaScript’s functional programming paradigm.

21.) What is context in React? How is it used?

Context provides a way to pass data through the component tree without explicitly passing props down the hierarchy. It is useful for sharing data that is considered “global” for a tree of components, such as user authentication status or theme preferences. Context consists of two parts: the context provider, which sets up the data to be shared, and the context consumer, which accesses that data. In React, you can use the createContext function to create a context, and the useContext hook or the Context.Consumer component to consume it.

22.) What is the purpose of React’s reconciliation process?

React’s reconciliation process is responsible for efficiently updating the user interface when the application’s state or props change. When a component’s state or props change, React compares the previous and next states/props, determines the differences, and updates only the necessary parts of the DOM. This process, also known as diffing, helps to minimize the number of actual DOM manipulations, resulting in better performance.

23.) What is React Router? How does it work?

React Router is a library that enables declarative routing in React applications. It allows you to define different routes and associate them with corresponding components, enabling navigation between different views within a single-page application. React Router uses a <BrowserRouter> or <HashRouter> component to listen to changes in the URL and render the appropriate component based on the current route. It provides components like <Route>, <Link>, and <Switch> for defining routes and creating links.

24.) What is server-side rendering (SSR) in React? What are its benefits?

Server-side rendering is a technique where the initial rendering of a web page is done on the server and sent as HTML to the client, rather than rendering it solely in the browser. React supports server-side rendering using libraries like Next.js. The benefits of SSR include improved SEO, faster initial page loading, and better performance on low-powered devices. SSR also provides a better user experience by displaying content while JavaScript is still downloading and executing.

25.) What are React portals? When and why would you use them?

React portals allow you to render a component’s children into a different DOM node outside of its parent hierarchy. They provide a way to render components outside the normal DOM hierarchy, such as modals or tooltips, which need to appear above the other components. By using portals, you can ensure that the content appears in the

26.) What are Higher-Order Components (HOCs)?

Higher-Order Components (HOCs) are a design pattern in React where a function takes a component and returns a new enhanced component. HOCs enable code reuse, logic abstraction, and cross-cutting concerns like authentication, logging, or data fetching. By wrapping a component with an HOC, you can add additional props or behavior to the component.

27.) What are React Fragments?

React Fragments are a way to group multiple child elements without adding extra nodes to the DOM. Fragments allow you to return multiple elements from a component’s render method without wrapping them in a container element. This can be useful when you don’t want to introduce unnecessary markup or when a parent component expects a single child element.

28.) What are React fragments and why are they useful?

React fragments (short syntax: <>…</>) allow you to group a list of children without adding extra elements to the DOM. They are useful when you need to return multiple elements from a component’s render method without introducing a wrapping parent element.

29.) How can you handle routing in a React application?

• Use a routing library like React Router to handle client-side routing.
• Set up route configurations with corresponding components for each route.
• Utilize dynamic routing by passing URL parameters to components.
• Implement nested routes and handle route transitions as needed.

30.) How can you handle asynchronous operations in React?

• Use promises or async/await syntax within lifecycle methods or hooks to handle asynchronous operations.
• Utilize the useEffect hook with an empty dependency array to perform one-time initialization or fetch data when the component mounts.
• Show loading spinners or placeholders while waiting for asynchronous operations to complete.
• Handle error scenarios and display error messages or fallback UI appropriately.

31.) How do you handle forms in React?

• Maintain form state in React component state and bind form inputs to the state.
• Use controlled components, where the value of an input element is controlled by React state and updated via onChange event handlers.
• Implement form validation by validating the form state before submitting or processing the data.
• Utilize form libraries like Formik or React Hook Form to simplify form handling and validation.

32.) How can you handle state management in a large React application?

• Use state management libraries like Redux or MobX to manage complex state across multiple components.
• Utilize React’s built-in Context API for managing state that needs to be shared across components without the need for a full-fledged state management library.
• Follow the principle of lifting state up, where state is moved to the nearest common ancestor of components that need access to it.
• Utilize component composition and pass data and functions as props to child components.

33.) What are some best practices for organizing a React codebase?

• Use a modular component structure and organize components into directories based on their functionality.
• Separate presentational components (focused on UI) from container components (focused on data and logic).
• Follow a consistent naming convention for files, components, and variables.
• Use a state management library like Redux or React Context API for managing global state.
• Extract reusable components and create a library of shared components.
• Use a build tool like Webpack or Parcel to bundle and optimize the code.

34.) How can you optimize the performance of a React application?

• Use React’s shouldComponentUpdate or PureComponent to prevent unnecessary re-renders.
• Utilize the React.memo Higher Order Component or React’s memoization hook (useMemo) to memoize expensive calculations.
• Implement code splitting and lazy loading to load components only when they are needed.
• Use production builds and minimize JavaScript bundle sizes.
• Optimize images and other assets for faster loading.
• Utilize server-side rendering (SSR) to improve initial page load time.
• Perform performance profiling and use tools like React Developer Tools to identify and address bottlenecks.