ReactJS interview Questions and Answers

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The Best ReactJS Interview Questions and Answers for all to crack your interview process.

What are synthetic events in React?

  • Synthetic events combine the response of different browser’s native events into one API, ensuring that the events are consistent across different browsers.
  • The application is consistent regardless of the browser it is running in. Here, preventDefault is a synthetic event.

Name some advantages of React.

 One of the basic React.js interview questions. You should be able to name some of the React’s biggest advantages such as: 

  • High efficiency: React is known for its flexibility and efficiency because of its own virtual DOM that it creates. It automatically calculates the changes needed to be made in DOM thus avoiding expensive DOM options and making regular updates whenever needed.
  • Easier JavaScript writing: Thanks to the JSX syntax which allows you to mix HTML with JavaScript, writing is much easier with React. The user can simply drop a bit of HTML in the render function without having to concatenate strings and React then turns those HTML bits into functions with its JSXTransformer.
  • A variety of developer tools: in order to get the most out of React.js, you should install its official browser extension. It comes in quite handy, especially for debugging the app. It gives you a direct look into the virtual DOM as if you were browsing a regular DOM tree in the elements panel.
  • Great for SEO: Most of the JavaScript frameworks aren’t particularly search engine friendly, but that is not the case with React. Thanks to its unique virtual DOM that is rendered to the browser as a regular page, the search engines will not encounter the same issues as with reading other JavaScript-heavy apps.
  • UI Test Cases: It is very easy to write UI test cases thanks to the virtual DOM in JavaScript.

Explain how lists work in React

  • We create lists in React as we do in regular JavaScript. Lists display data in an ordered format
  • The traversal of lists is done using the map() function

Why is there a need for using keys in Lists?

Keys are very important in lists for the following reasons:

  • A key is a unique identifier and it is used to identify which items have changed, been updated or deleted from the lists
  • It also helps to determine which components need to be re-rendered instead of re-rendering all the components every time. Therefore, it increases performance, as only the updated components are re-rendered

What are forms in React?

React employs forms to enable users to interact with web applications.

  • Using forms, users can interact with the application and enter the required information whenever needed. Form contain certain elements, such as text fields, buttons, checkboxes, radio buttons, etc
  • Forms are used for many different tasks such as user authentication, searching, filtering, indexing, etc

What are th disadvantages of React.js?

 A: When talking about React questions, one should be well aware of the biggest disadvantages of this framework. Let’s take a look at some you should name. 


  • Integration with traditional MVC framework requires major configurations
  • The steep learning curve for the beginners

 What that means in practice is that you can write concise HTML/XML-like structures in the same file as you write JavaScript code. These structures will then be automatically transformed into JavaScript code. 

What are refs in ReactJs ?

In React ref is used to store the reference of element or component returned by the component render() configuration function.Refs should be avoided in most cases, however, they can be useful when we need DOM measurements or to add methods to the components.

Refs can be used in the following cases

Managing focus, text selection, or media playback.

Triggering imperative animations.

Integrating with third-party DOM libraries.

What is the difference between createElement and cloneElement?

This is one of the quite logical React interview questions and answers. Basically, as the name suggests, createElement what React uses to create React Elements, cloneElement, on the other hand, is used to clone an element and pass it new props. Might seem quite obvious, but it is still one of the top 50 interview questions and answers. 

What is the second argument that can optionally be passed to setState and what is its purpose?

 A: When preparing for your React js interview, you need to know that a callback function is invoked when setState has finished and the component is re-rendered. However, keep in mind that setState is asynchronous and that is the reason why it requires a second callback function. However, rather than relying solely on the callback function, one should use another lifecycle method. 


Explain life Cycle of React JS Component ?

React JS Component Lifecycle

Each component has several “lifecycle methods” that you can override to run code at particular times in the process. Methods prefixed with will are called right before something happens, and methods prefixed with did are called right after something happens.


These methods are called when an instance of a component is being created and inserted into the DOM:

  • constructor()
  • componentWillMount()
  • render()
  • componentDidMount()


An update can be caused by changes to props or state. These methods are called when a component is being re-


  • componentWillReceiveProps()
  • shouldComponentUpdate()
  • componentWillUpdate()
  • render()
  • componentDidUpdate()


This method is called when a component is being removed from the DOM:

  • componentWillUnmount()

Other APIs

Each component also provides some other APIs:

  • setState()
  • forceUpdate()

Class Properties

  • defaultProps
  • displayName

Instance Properties

  • props
  • state

What is the difference between state and props?

This is one of the ReactJS interview questions and answers for experienced as well as for the beginners.  Long story short, the state is a data structure that starts with a default value when a component mounts. It may be mutated across time, mostly as a result of user events.  Props, on the other hand, are a component’s configuration. Think of them as the way for the components to communicate with each other. They are received from the above component and immutable as far as the component receiving them is concerned. A component isn’t able to change its props, but it has to put together the props of its child components. While the props are mostly data, other things can be passed as them as well, for example, callback functions.  Props and state do similar things but are used in different ways. The main difference is that props are immutable and cannot be changed while the state is used for mutable data or data that will change.

What is the difference between React Native and React?

This is another one of the common React interview questions. It all starts with the fact that React is a JavaScript library and it supports both front-end web and being run on the server and it is commonly used for building user interfaces and web applications.  React Native, however, is a mobile framework that compiles to native app components. It allows us to build native mobile applications for Windows, Android, and iOS in JavaScript while we can use ReactJS to build our components.  With React Native, we can mimic the behavior of the native app in JavaScript and get a platform-specific code as the output. It is also possible to mix the native code with the JavaScript if we need to optimize our application further. 

What do you understand by “Single source of truth”?

We have mentioned the single source of truth many time earlier in this article but would you know how to explain what it actually means? Well, you should because this is one of the React interview questions that are pretty basic.  The single source of truth refers to the store used for storing the app’s entire state at one place. The benefits of the single source of truth include the facts that all the components stored there receive updates from the store itself, it is easier to keep track of their changes, as well as debug and inspect the application.

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