Top React Interview Questions & Answers You Need to Know

Hey there! Are you preparing for a React interview and feeling a little overwhelmed? Not so fast! We’ve covered you with top-notch interview questions and code examples to help you understand the concepts better. In other words, we’ll give you the means to ace your interview and impress your potential employer.

React has over 200,000 stars and over 14.1 million users on GitHub and is used by companies like Netflix, Airbnb, and Dropbox. It has gained wide popularity among web developers due to its simplicity, flexibility, and efficiency. React has a vast ecosystem, which includes tools, libraries, and frameworks that make it easy to build scalable and robust web applications.

By now, you may be wondering what kinds of questions to expect. Don’t worry! We’ll cover everything from React components and refs to Redux. So, without further ado, let’s jump in and start with some general React interview questions.

General React Interview Questions

1. What Is React, and Its Key Features?

React is a JS library used for building user interfaces. Its key features include:

Component-based architecture: React uses a component-based architecture where UI elements are broken down into smaller components. This makes it easy to reuse code and maintain a consistent look and feel across the application.

2. What is a Virtual DOM, and How Does It Help in React?

The virtual DOM is a lightweight representation of the actual DOM (Document Object Model) in React. It is a JavaScript object that mirrors the real DOM tree and allows React to update only the parts of the actual DOM that need to be changed rather than the entire DOM tree.

Here’s how the virtual DOM works in React

• Whenever there is a modification in a React component, a new virtual DOM tree is generated that reflects the updated state of the component.
• React then compares the new virtual DOM tree to the old one to identify the differences.
• React then generates minimal updates to apply to the actual DOM to make it reflect the updated virtual DOM tree.
• React applies these updates to the actual DOM.

React can avoid updating the entire DOM tree using the virtual DOM whenever component changes occur. This can lead to significant performance improvements in applications that require frequent updates. Additionally, because React handles the updates to the actual DOM, it helps ensure that the application remains consistent and bug-free.

3. What are the Limitations of React?

However, like any technology, React also has its limitations. Here are some of the limitations of React

Steep Learning Curve

React has a relatively steep learning curve, especially for developers who are new to JavaScript and web development. It requires a solid understanding of JavaScript, HTML, and CSS, as well as the React component architecture, JSX syntax, and other concepts unique to React.

Boilerplate Code

The process of initializing a new React project often involves writing a substantial amount of repetitive code, which can be a tedious and time-consuming task.

Performance Issues

React’s virtual DOM can lead to performance issues when rendering large or complex UIs. While React is generally fast and efficient, it may not be the best choice for complex or real-time applications requiring instant updates.

Poor SEO Support

React was originally designed for building single-page applications (SPAs), which can have poor SEO performance due to the lack of server-side rendering (SSR). While there are ways to add SSR to React applications, it can be complex and requires additional setup and configuration.

Tooling

React’s ecosystem is vast and constantly evolving, making it difficult to choose the right tools and libraries for a project. Additionally, setting up and configuring tooling for a React project can be time-consuming and require significant knowledge.

4. What Is Create React App?

Create React App is a tool developed by Facebook that provides a streamlined setup for creating React applications. It sets up the development environment, including setting up the build process, configuring Webpack and Babel, and providing a development server for live reloading.

With Create React App, developers can get started with React development quickly and without dealing with the complexities of configuring a new project from scratch. Additionally, it provides a command-line interface for managing the development process, making creating, testing, and deployment easy.

5. What is JSX, and How Does It Work in React?

Before JSX, creating React elements involved manually creating JavaScript objects with properties that described the element’s type, attributes, and children. Here’s an example of what that would look like to create a simple h1 element:

const element = React.createElement(
  'h1',
  { className: 'greeting' },
  'Hello, world!'
);

In the above code, React.createElement() takes three arguments: the element type (‘h1’), an object of attributes ({ className: ‘greeting’ }), and any children elements (‘Hello, world!’)

JSX provides a more intuitive syntax for creating React elements that looks like HTML markup. Here’s what the same h1 element looks like using JSX:

“greeting”>Hello, world!
;

In this code, the h1 element is created using angle brackets (< and >) and is similar to HTML syntax. The className attribute uses the same syntax as HTML attributes, and the element’s text content is specified between the opening and closing tags.

Under the hood, JSX gets transpiled into the React.createElement() calls we saw earlier. Here’s what the JSX above looks like after being transpiled:

const element = React.createElement(
  'h1',
  { className: 'greeting' },
  'Hello, world!'
);

This transpiled code is what actually gets executed by the browser or Node.js, and results in the creation of a h1 element with a className attribute of ‘greeting‘ and text content of ‘Hello, world!‘.

JSX allows you to write more concise and readable code than using createElement() directly. It also provides some additional syntax features, such as

1. Embedding JavaScript expressions within JSX using curly braces

const name = 'John';
const element = 
Hello, {name}!
;

2. Using custom components

function Greeting(props) {
  return 

Hello, {props.name}!

;
}

const element = "John" />;

3. Specifying attributes using JSX

const element = "https://example.com/image.png" alt="An example image" />;

6. Can You Explain the Difference Between Props and State in React?

Props and state are both used to manage data in React. The main difference between them is that props are passed down from a parent component to a child component, while state is managed within a component.

Here’s an example:

class ParentComponent extends React.Component {
  render() {
    return (
      <ChildComponent name="John" />
    );
  }
}

class ChildComponent extends React.Component {
  render() {
    return (
      <div>Hello {this.props.name}!</div>
    );
  }
}

In this example, the name prop is passed down from the ParentComponent to the ChildComponent.

On the other hand, state manages data within a component. Here’s an example:

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

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

In this example, the count state is managed within the Counter component.

7. What is the Difference Between a Functional Component and a Class Component?

Functional components are simpler and easier to read and write than class components. They are JavaScript functions that take in props and return JSX. They don’t have their own internal state and are often used to present data.

Here’s an example of a functional component:

function Greeting(props) {
  return <div>Hello {props.name}!</div>;
}

On the other hand, class components are more powerful and offer more features than functional components. They have their own internal state, can use lifecycle methods, and can be used to manage more complex UI elements.

Here’s an example of a class component:

class Greeting extends React.Component {
  constructor(props) {
    super(props);
    this.state = { name: 'John' };
  }

  render() {
    return <div>Hello {this.state.name}!</div>;
  }
}

Let’s look at the table below to summarize:

8. What is the Purpose of the Key Prop in React?

The key prop is used to give each item in a list a unique identifier. This helps React to identify which items have changed, been added, or been removed from the list. The key prop should be unique within the list and should be stable across re-renders. If the key prop is not provided, React will generate a warning.

Here’s an example of using the key prop in a list:

const items = [
  { id: 1, text: 'Item 1' },
  { id: 2, text: 'Item 2' },
  { id: 3, text: 'Item 3' }
];

function ItemList() {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.text}</li>
      ))}
    </ul>
  );
}

In this example, the key prop is set to the id property of each item.

9. Explain What React Router is and Its Purpose in a React Application?

React Router is a library that enables client-side routing in React applications. It allows you to define routes for different URLs in your application and map them to specific components that should be rendered when those URLs are accessed.

This allows for a more seamless user experience by allowing users to navigate between different views of your application without requiring a full page refresh. React Router also provides features like nested routing, programmatic navigation, and URL parameters, which can be useful for building complex applications.

10. What is Redux?

Redux is a state management library for JavaScript applications, including those built with React. It provides a predictable, centralized way to manage the state of an application, making it easier to reason about and debug.

Redux follows the principles of a unidirectional data flow, where state is kept in a single store and can only be modified through actions. These actions describe what happened in the application and are dispatched to a reducer, which calculates the new state of the application based on the current state and the action. The new state is then returned to the store and any subscribed components are notified of the change.

In a Redux application, the state is kept separate from the components and is accessed through a series of functions called selectors. This helps to decouple the state from the UI, making it easier to test and refactor the code. Redux also provides a set of middleware, which can be used to intercept and modify actions or perform asynchronous operations such as network requests.

11. What is the Role of setState in React?

setState is a method provided by React that is used to update the state of a component. When setState is called, React re-renders the component with the updated state. setState can take either an object or a function as an argument.

Here’s an example of using setState with an object:

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

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

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.handleClick()}>
          Click me
        </button>
      </div>
    );
  }
}

In this example, the handleClick method calls setState with an object that increments the count state.

12. How Will You Differentiate Between componentWillMount and componentDidMount Lifecycle Methods?

componentWillMount is called right before a component is rendered for the first time, while componentDidMount is called after the component is rendered for the first time. componentWillMount is rarely used, while componentDidMount is commonly used for data fetching and other side effects.

Here’s an example of using componentDidMount for data fetching:

class DataFetcher extends React.Component {
  constructor(props) {
    super(props);
    this.state = { data: null };
  }

  componentDidMount() {
    fetch('/api/data')
      .then(response => response.json())
      .then(data => this.setState({ data }));
  }

  render() {
    return (
      <div>
        {this.state.data ? (
          <div>{this.state.data}</div>
        ) : (
          <div>Loading...</div>
        )}
      </div>
    );
  }
}

In this example, the componentDidMount method fetches data from an API and updates the data state when the response is received.

13. What are Hooks in React?

React hooks allow developers to use React features such as state in function components without the need to write a class component. Essentially, hooks are functions that allow you to “hook into” the React state and lifecycle methods.

Here’s an example of using the useState hook to manage state in a function component:

import React, { useState } from 'react';

function Counter() {
  // Declare a state variable called 'count', initialized to 0
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked the button {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

In this code, the useState hook is used to declare a state variable called count and its corresponding update function, setCount. The initial value of count is passed as an argument to useState and is set to 0.

The Counter component renders the current count value in a p element, along with a button that, when clicked, updates the count value using setCount. Since updating the state causes the component to re-render, the new count value is reflected in the UI.

Hooks can also be used for other purposes, such as managing side effects with the useEffect hook or creating reusable logic with custom hooks.

Conclusion

In conclusion, the world of React is vast and ever-evolving, with new updates, features, and best practices always emerging. The demand for React development services has increased rapidly in recent years, with React being one of the most popular and widely used JS library for building user interfaces. Companies that want to build complex and dynamic web applications are looking to hire React developers who understand the library’s core concepts and can create efficient and effective code.

With these interview questions and answers, you should be well-prepared to tackle the most common and important topics in React interviews and demonstrate your expertise and experience to potential employers. Good luck on your journey as a React developer!

If you enjoyed this, be sure to check out our other React articles.

• The Power of Hooks in React: Streamlined Dynamic Applications
• Unit Testing in React: In-Depth Guide
• In Which Projects Can Your Company Use React?
• What You Need to Know About React Before Using It for Your Business
• 6 Reasons Why React Is So Popular