Top 10 React JS Interview Questions and Answers

5 min readSep 7, 2023

React, also known as React.js or ReactJS, is an open source front-end JavaScript library used to build user interfaces using user interface components. If you are a Java programmer, you can expect advanced questions about ReactJS during your front-end engineer interview.

To share your experience with different tools and understand Java framework , FAANG+ companies often challenge your knowledge of Java application tools and microservices, including ReactJS. Knowing the answers to frequently asked ReactJS advanced interview questions is critical to your technical interview success.

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Popular Advanced ReactJS Interview Questions and Answers

Q1. What are some key features of the ReactJS framework?

This again This is a common question in technical interviews about ReactJS.

The most important features of the ReactJS framework include:

ReactJs enables data binding, allowing developers to choose which elements users can access and which elements are hidden. Security for the applications they create is also simple with this feature.
ReactJS has a one-way data feed.
ReactJs uses Virtual DOM instead of RealDOM.
React primarily uses server-side rendering.

Q2. What Are The Different Lifecycle Methods In The Mounting Lifecycle Stage?

Render
ComponentWillMount
ComponentDidMount

Q3. What is an Event in ReactJS?

Events are actions performed by the user or by the computer. Actions like clicks and keypresses are examples of events in ReactJS. Unlike HTML where events are written in lowercase, events in React are written in CamelCase.

Q4. What are Higher Order Components (HOC) in React?

A higher order component (HOC) is a function that takes a component and returns a new component. It is essentially a model derived from the compositional nature of React. We call them “pure” components because they can accept any dynamically provided child components, but they cannot modify or copy the behavior of their input components.

const EnhancedComponent = higherOrderComponent(WrappedComponent);

HOC can be used for many use cases as below,

Code reuse, logic and bootstrap abstraction
Render High jacking
State abstraction and manipulation
Props manipulation

Q5. In which situation would you use useMemo() in React?

Your candidates know what developers can useMemo() to avoid unnecessary re-rendering? You can also say: useMemo() can be useful in situations where large amounts of processing are involved.

Q6. Why should you use superconstructors with prop arguments?

Applicants can explain what these are which the candidates explain. to go to this.props in the constructor. You can mention it if you have a constructor() function in React component, use super() to call the main constructor .

Q7. What is the meaning of the component-based architecture of React?

In React, components are the building blocks used to create user interfaces for applications. Thanks to the component-based system, all individual units become completely reusable and independent of each other. This means that application rendering is simple and does not depend on other UI components.

Q8. What is the use of an arrow function in React?

Use an arrow function to write an expression in React. Allows users to manually connect components with ease. The arrow functions functionality can be very useful, especially when working with core components. command functions.

render() {
return(
<MyInput onChange={this.handleChange.bind(this) } />
);
}
//Making use of the arrow function
render() {
return(
<MyInput onChange={ (e) => this.handleOnChange(e) } />
);
}

Q9. Why API calls should be made from ComponentDidMount and not the constructor/ComponentWillMount?

It is advisable to avoid side effects. The Render method is called immediately after the WillMount component, and we cannot make the Render method wait for the API to return.

The constructor is a place in which we are setting the variables and do not make any API calls. API calls can have side effects and should not be used inside the constructor.

React expects state to be available, since the render function is the next one after ComponentWillMount and is called by code may fail if any of the mentioned state variables are missing, which can happen with Ajax API calls.

Another reason is when we need server-side rendering for React components, the WillMount component is called on the server side and again on the client side, resulting in the extract being called twice. So this is definitely not where we should integrate our APIs.

NOTE: A side effect is any change to the application state observable outside of the invocation function as the return value. Example: Changing a global variable

Q10. Explain the new lazy loading and code splitting feature in React v16.6?

Not surprisingly, this appears frequently in React questions.

Lazy Load is the new feature introduced in React v16.6 that allows some components to be loaded later than normal. This allows us to load fast components like text first and components that load images a bit later.

Consider the following code for App.js. It contains two components, ContentComponent and myComp.
One contains a paragraph containing lorem ipsum and the other contains an image containing unsplash which can be loaded .

Now we are loading myComp was delayed because there is an image to download. Note the special import type and we also need to include the component in Suspense. Suspense now includes the alternate component displayed when loading myComp.

The other ContentComponent will be loaded immediately.

//App.js
import React, { Component, lazy, Suspense } from "react";
import "./App.css";
import ContentComponent from './components/ContentComponent';
const MyComp = lazy(() => import("./components/myComp"));
class App extends Component {
 render() {
   return (
     <div className="App">
       <header className="App-header">  
         <h1>Lazy Loading Demo</h1>        
          <Suspense fallback={<div>Loading.....</div>}>
            <MyComp />
          </Suspense>
         <ContentComponent />
       </header>
     </div>
   );
 }
}
export default App;
//ContentComponent.js
import React from 'react'
export default function ContentComponent() {
 return (
   <div>
     <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry...</p>
  </div>
 )
}
//myComp.js
import React from "react";
export default () => {
 return <img src="" width="960" height="480" alt="acceronix.com" />;
};