Top React Interview Questions and Answers You Need to Know

Hey! Are you preparing for a React interview and feeling a little overwhelmed? Not so fast! We've got you covered with high-level interview questions and code samples to help you better understand the concepts. In other words, we will provide you with the means to ace your interview and impress your potential employer.

The reaction 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 great 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 might be wondering what kind of questions to expect. Don't worry! We'll cover everything from React components and references to Redux. So, without further ado, let's start with some general React interview questions.

General Reaction Interview Questions

1. What is React and its main features?

React is a JS library used to build user interfaces. Its main features include:

Component-based architecture: React uses a component-based architecture where UI elements are divided into smaller components. This makes code reuse easier and maintains a consistent look and feel throughout the application.

2. What is a virtual DOM and how does it help with react?

Virtual DOM is a lightweight representation of the real 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 real DOM that need to be changed, rather than the entire DOM tree.

See how the virtual DOM works in React

• Whenever there is a modification to 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 with the old one to identify the differences.
• React then generates minimal updates to apply to the real 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 by using virtual DOM whenever component changes occur. This can lead to significant performance improvements in applications that require frequent updates. Additionally, because React handles 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. Requires a solid understanding of JavaScript, HTML, and CSS, as well as React component architecture, JSX syntax, and other concepts unique to React.

Standard code

The process of starting 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. Although React is generally fast and efficient, it may not be the best choice for complex or real-time applications that require instant updates.

Poor SEO support

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

Tools

The React ecosystem is vast and constantly evolving, making it difficult to choose the right tools and libraries for a project. Additionally, installing and configuring tools 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 simplified setup for creating React apps. 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 start developing React quickly and without dealing with the complexities of setting up a new project from scratch. Additionally, it provides a command-line interface to manage the development process, making it easier to build, test, and deploy.

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 it would look like to create a simple h1 element:

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

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

JSX provides a more intuitive syntax for creating React elements that look like HTML markup. This is 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 text content of the element is specified between the opening and closing tags.

Behind the scenes, the JSX is transpiled to the React.createElement calls we saw earlier. This is what the JSX above looks like after being transpiled:

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

This transpiled code is what is actually executed by the browser or Node.js and results in the creation of an 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 like

1. Embedding JavaScript expressions in 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 = " alt="An example image" />;

6. Can you explain the difference between props and state in React?

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

Here is 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 property is passed from ParentComponent to ChildComponent.

On the other hand, state manages the data within a component. Here is 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 counting state is managed in 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 props and return JSX. They have no internal state of their own and are often used to present data.

Here is 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 is 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 core support in React?

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

Here is an example of using the key property 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 property is set to the id property of each item.

9. Explain what is React Router 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 to 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, allowing users to navigate between different views of your app 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 reasoning and debugging easier.

Redux follows the principles of a one-way 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 storage and all subscribed components are notified of the change.

In a Redux application, state is kept separate from components and is accessed through a series of functions called selectors. This helps decouple state from the UI, making code easier to test and refactor. 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 an object or a function as an argument.

Here is 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 first rendered. componentWillMount is rarely used, while componentDidMount is commonly used for data fetching and other side effects.

Here is 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 state of the data 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 needing to write a class component. Essentially, hooks are functions that allow you to “connect” to 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 set to 0.

The Counter component renders the current count value in an ap element, along with a button that, when clicked, updates the count value using setCount. Because 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 constantly 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 libraries for building user interfaces. Companies that want to build complex and dynamic web applications look to hire React developers who understand the library's basic concepts and can create efficient and effective code.

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

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

• The Power of Hooks in React: Dynamic Applications Made Simple
• Unit Testing in React: Detailed Guide
• What projects can your company use React for?
• What you need to know about React before using it in your business
• 6 reasons why React is so popular