React JS interview questions: Top ones on the list

Introduction

The fact that React JS is now gaining popularity cannot be disputed. Consequently, there is currently a growing need for React JS developers. You should be prepared with React JS interview questions and answers if you plan to attend a job interview.

This blog provides a thorough list of all the typical React JS interview questions, which will help you better understand the key React JS ideas.

React is a library created by Facebook to make user interfaces. It has quickly become one of the most popular libraries in use today, and it even plays a role in some of the company's larger projects like Instagram, WhatsApp, and React Native.

Why React JS?

React, developed by Facebook, is widely used in projects like Instagram, WhatsApp, and React Native. Its popularity stems from several key advantages:

• Speed and Performance: React’s virtual DOM enables fast rendering, updating only the necessary components, making applications highly responsive.

• Simple and Intuitive API: React’s idiomatic API allows developers to build high-quality UI components without complex JavaScript syntax.

• Composability: Components can be combined to create complex interfaces, simplifying code management.

• Declarative Syntax: Developers describe how UI elements should look and function, reducing errors and improving maintainability.

• Ease of Learning: React’s straightforward syntax makes it accessible to developers familiar with HTML/CSS.

• Versatility: React integrates well with modern frameworks like Angular and Vue, as well as legacy platforms like Ruby on Rails and PHP.

These features make React an excellent choice for building sophisticated web applications, from responsive designs to large-scale projects.

Common React JS Interview Questions and Answers

Interviewing techniques also use React js interview questions to assess how the applicant thinks on their feet and responds to challenges. By doing this, you can determine whether the candidate would be a good fit for your team or not.

What is useState() in React?

UseState() is a function in React that lets you store the current state of a component. This can be useful for maintaining track of modal dialogs, scroll bars, and other UI elements.

What are the keys in React?

Keys are objects used to persist data between occasions when the same fragment or page is loaded. They're just like normal JavaScript properties, but they live in your app's global namespace (rather than within any individual react element).

What is JSX?

JSX is an HTML extension that allows you to embed JavaScript inside your HTML tags. This makes it easy to include code from other files into your React components without having to use special directive syntax or import statements. 

What is the virtual DOM? How does React render the UI using it?

The virtual DOM is a representation of the actual DOM tree on the server side. It's used to react to render user interfaces interactively and determine which parts of the tree need to be updated during rendering. The virtual DOM updates are handled using reactive programming techniques so that as changes happen on the client side, those changes are immediately reflected on the server side.

What are controlled and uncontrolled components?

Controlled components are those that have been specifically designed, manufactured, and tested to perform a specific function. Uncontrolled components can be either internal or external to the system they're being used in.

An example of an uncontrolled component would be a light switch. A controlled component might be a lamp that is specifically designed to reduce energy consumption. Uncontrolled components can also include sensors or actuators that aren't properly configured or maintained, which can lead to inaccurate readings or unintentional actions.

In most cases, it's important to always use controlled components when possible because they're more likely to produce accurate results and have less potential for problems down the road. By following these simple guidelines, you can ensure optimal performance from all of your electronic devices!

What are props in React?

Props are the simplest way of managing state in React. They allow you to bind customized values to any DOM element, making it easy to maintain a consistent look and feel across your application. For example, you could use props to bind the title of a blog post to the heading tag on its page. You can also use them to style elements like buttons or labels.

When you create a prop, you need to provide an identifier as well as the data that will be bound to it. The value for this data can come from inside your code or from another source, such as JSON or XML files located outside of your project directory structure. Finally, when necessary, you can set up callbacks so that React will handle UI updates triggered by changes made in props automatically.

What are React Hooks?

React Hooks are a feature of React that lets you add custom behavior to your components without writing any code.

- They work by intercepting state changes to your component and running specific actions.

• This way, you can add complex logic to your components without writing any code.

Why were Hooks introduced in React?

React Hooks are a way of modularizing your React codebase into reusable components. This makes it easier to read, maintain, and scale your codebase as you grow. They work by allowing you to define dependencies between props and states in an easily comprehensible manner, which greatly reduces the amount of boilerplate that is necessary when using React.

Additionally, hooks allow for declarative state management which eliminates the need for unnecessary calculations or brute force updates while maintaining performance.

Describe the strict mode in React JS!

The main purpose of the React Developer Tool StrictMode is to draw attention to potential flaws in a web-based application. For its child components, it turns on additional gradation checks and warnings. Its ability to display visible feedback (warning/error messages) when the React rules and suggested practices are not adhered to is one of the factors contributing to its popularity. The React StrictMode Component does not generate any visible UI, just like the React Fragment.

List a few methods for enhancing the performance of a React App!

The efficiency of a React app may be optimized in a variety of ways; let us just look at some of them:

By using the React hook useMemo(), CPU-intensive functions can be cached.

React apps occasionally run CPU-expensive functions frequently as a result of component re-renders, which can slow down rendering.

Such routines can be cached using the useMemo() hook. The CPU-Expensive method is only called when necessary thanks to useMemo().

utilising React. PureComponent is a base element class that determines whether a component needs to be changed by looking at its state and properties.

To eliminate the needless re-rendering of a component, we can utilize React.PureComponent in place of the straightforward React. Component.

Bringing the state as near to where you require it as possible is the procedure of maintaining state colocation.

Sometimes in React apps, the parent component has a lot of extraneous states, which makes the code difficult to read and maintain. Not to mention that having many states within a single component causes the component to be rendered again needlessly.

Moving states that are less essential to the parent element to a different component is preferable.

Lazy loading is a method for speeding up the loading of React applications. Slow loading minimizes the likelihood of poor web app performance.

What does a React event mean?

An event is an action that is triggered by a user action or a system-generated event, such as a mouse click, web page loading, key press, window resizing, etc. The event-handling approach in React is quite similar to how DOM elements handle events. Synthetic Event, a cross-browser encapsulation of the native event of the browser, is the name of the React programming mechanism.

There are some syntactic variations when handling events using React, including:

React events have camelCase names rather than lowercase.

In JSX, a function rather than a string is supplied as the event handler.

When should we utilize a class component instead of a function component?

You should consider using a class component in React if you're working on a legacy codebase or require features that are easier to implement in classes.

However, with the introduction of React Hooks in React 16.8, function components can now manage state, side effects (like lifecycle methods), context, and more — which were previously exclusive to class components.

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      inputValue: '',
    };
    this.handleChange = this.handleChange.bind(this);
  }

  handleChange(event) {
    const value = event.target.value;
    this.setState({ inputValue: value });
    console.log("Input changed:", value);
  }

  render() {
    return (
      <div>
        <label htmlFor="shopify-input">Enter Product Note:</label>
        <input
          id="shopify-input"
          type="text"
          value={this.state.inputValue}
          onChange={this.handleChange}
        />
        <p>Value: {this.state.inputValue}</p>
      </div>
    );
  }
}
  

So, prefer function components with hooks in modern React development unless you're maintaining older code that still uses class components.

Overall, React is a practical and modern technology for creating web applications in 2022. This package allows for quick development, high performance, and simple maintenance, enabling the creation of great web masterpieces. ReactJS can be used to build complex interface designs, Responsive Web Applications (Progressive web apps), and generally anything that needs frequent page updates (even big-scale projects).

Moreover, as mobile-first strategies dominate the digital world, hiring expert react native developers has become essential for businesses aiming for cross-platform success.

A leading developer of e-commerce solutions, Lucent Innovation is a full-service software creation and development firm. We provide custom software for our customers, which boosts productivity, business expansion, and profitability. Additionally, we create e-commerce platforms that are prepared to expand, scale, and sell. As a Reactjs development company, Lucent Innovation has the skills to develop strong front-end applications that follow the most recent trends.