React JS Interview Questions

By Albert Howard

The right set of tools and technologies greatly impacts the success of an application development project. It begins with knowing different tools and technologies and how they work, which can be challenging with technologies being invented every other day. However, after training in different tools, working on some projects, and gaining years of experience, individuals have a shorter learning curve for newer technologies within the same domain.  

React JS is an open-source JavaScript framework used for front-end development. React JS is specifically used for developing user interfaces. As a full stack or front-end developer, you will be required to be familiar with certain programming languages for front-end development like CSS, HTML, and JavaScript, along with their libraries like ReactJS for JavaScript and Bootstrap and Semantic UI for CSS. 

React JS is among the popular and powerful frameworks adopted widely for building user interfaces. As it were, it is hard to miss React JS interview questions when interviewing for front-end or full-stack development roles. 

Why do developers prefer React JS for front-end development? 

React JS is easy to learn and use, especially if you have undertaken a JavaScript of React course. This is because React JS features declarative programming in which developers describe user interfaces making the coding process fast and easy. React JS code is easy to read and understand. 

React JS is also component-based, meaning that components are built and encapsulated for easy and independent state management of the components. React JS components are thus reusable, and new UIs can be developed faster without writing new code or reformulating existing ones. 

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Are you an aspiring front-end developer? 

Here are common React JS interview questions you should expect in the interview room and answers to help you prepare for your big day. 

  • What is JSX? 

JSX stands for JavaScript XML. It is a syntax extension to JavaScript that let’s developers write HTML in React. JSX is used together with React to write readable understandable markup. 

  • Can browsers read a JSX file? 

Browsers cannot read a JSX file. 

This is because JSX is not a valid JavaScript code but merely a syntax extension to JavaScript. For this reason, a JSX code should be transformed into a valid JavaScript code using JSX transformers such as Babel, a JavaScript compiler. 

  • What is the difference between ReactJS and AngularJS?

Both ReactJS and AngularJS are JavaScript frameworks used to render user interfaces. The differences between ReactJS and AngularJS are: 

ReactJS  AngularJS
DOM  Uses virtual DOM  Uses real DOM 
Code dependency  Declarative language that uses descriptions for objects hence is easily readable and understandable. ReactJS depends less on code.   AngularJS requires code to be written from scratch.  
Structure  Only supports the view element of MVC   Full MVC support 
Inventors  Facebook  Google 
Language  JavaScript with extended XML syntax  TypeScript, a superset of JavaScript
Data binding  One-way data binding  Two-way data binding 


  • Outline the life cycle of components in ReactJS 

Components in ReactJS go through lifecycle events in four phases in which the states and properties of components are managed. These phases are: 

  • The initialization phase. In the initialization phase, a component’s initial state and default properties are defined and initialized using the getDefaultProps() method for defining the default properties and the getInitialState() method for defining the initial state of a component. 
  • The mounting phase. Here an instance of the component is created and positioned over the DOM. 
  • The state/property update phase. In the updating phase, the state and properties are updated with changes on the component. 
  • The unmounting phase. This is the final phase of the component life cycle where the component is unmounted from the DOM and destroyed. A component cannot be mounted again once it has been unmounted from the DOM. 
  • State the difference between an element and a component

A React Element is a plain object representing a DOM node. An element describes a DOM node and its properties or a component instance. An Element can contain other elements within it. 

A component, on the other hand, is a core building block of React applications. A component can be a function or class that accepts input and returns an HTML element. Components simply represent specific elements describing a UI function.

  • What are Props 

Props refer to properties. These are inputs to React components that pass information from one component to another. Data in the Props is read-only and is passed from parent to child component in a unidirectional flow. 

  • What is a callback function in React and what is its use? 

A callback function is a function that is passed to another function as an argument to allow the function to call another function. 

  • How can you bind methods or event handlers in JSX callbacks? 

This can be done by: 

  • Binding in Constructor 
  • Using arrow functions directly in callbacks
  • Using public class field syntax to bind callbacks 
  • What is a virtual DOM? 

A virtual DOM is a lightweight in-memory representation of the DOM (Document Object Model) elements before any changes are made to the UI. Every DOM object in React JS has a parallel virtual DOM that is manipulated instead of manipulating the real DOM directly. 

Data changes in a React app are re-rendered in the VDOM. Afterward, the new VDOM version is compared to the previous VDOM, and the difference is calculated in a process known as diffing. React then modifies only the objects that have changed in the real DOM rather than rendering the entire DOM. 

  • Explain the difference between Shadow DOM and Virtual DOM 

A virtual DOM is an in-memory copy of the real DOM. A virtual DOM creates the whole copy of the real DOM object. 

Shadow DOM is the creation of separate instances of pieces of the DOM object. A shadow DOM has its own isolated scope for the part of the DOM that it represents. 

  • What are controlled components in React JS? 

A form is a core element in React applications. Forms store information in the document section. This can be the login screen to a checkout page, select, textarea, etc. This information is transmitted to a server to perform an action.  

Form data in components can be controlled using two different methods in React. When it is controlled by the state within the component, this is known as a controlled component.

Alternatively, when form data in a component is controlled by the DOM, it is known as an uncontrolled component. 

  • What are stateless components and how are they different from stateful components? 

A stateless component in React is a component that has no state. In other words, the behavior of the component is not dependent on its state. Stateless components can be created by a class or a function and their output is determined by the props given to them. 

A stateful component, on the other hand, is one that holds some state. This means that the behavior of the component is determined by its state. Stateful components can only be created by the class function and their states are initialized in the constructor. 

  • What is polling in React JS? 

Polling is a technique used to monitor the server for updates by making API requests to the server from time to time. For example where a website is checked every five seconds for new comments. 

  • Explain synthetic events in React

A synthetic event is a cross-border wrapper that is similar to the native browser that React creates to allow the events to adapt consistently to different browsers. This prevents the creation of multiple implementations for the same event for different browsers ultimately improving the performance of a React application. 

  • What is the importance of using keys in React? 

Keys are used to identifying unique virtual DOM elements. Keys are important in React because they help to differentiate different elements to increase performance by recycling elements in the DOM instead of rerendering them. 


As React JS continues to grow in popularity, both large and small enterprises alike are warming up to its benefits in web development. We have covered some frequently asked React JS interview questions that we believe will help you to adequately prepare for your big day. 

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