ReactJS Interview Questions for experienced & Freshers

ReactJS won the popularity battle in 2017 & 2018. So if you are looking for ReactJS Interview Questions and Answers for Experienced & Freshers, Here is right place to learn the ReactJS interview questions for HCL, TCS, Cognizant, IBM etc. There are lot of opportunities from many reputed companies in the world like HCL, TCS, Cognizant, IBM, Infosys etc. According to research, ReactJS has a market share of about 7.3%. So, Currently available opportunities in market in ReactJS Developer. Webjustify is offering advanced ReactJS Interview Questions for experienced 2018 that helps you in cracking your interview & acquire your dream career as ReactJS Developer.

ReactJS interview questions for experienced

ReactJS Interview Questions And Answers

1. What is React?

React is a open-source front end JavaScript library developed by Facebook in 2011. It facilitate the creation of interactive, stateful & reusable UI components. React is using for create large interactive web applications and mobile application. It change over time without reloading the page.It is covering for ‘View’ in MVC (Model-View-Controller).

2. What are the ReactJS Features?

Some important features of React.

  • JSX (JavaScript XML)
  • React Native (It uses in mobile application)
  • Virtual DOM (Document Object Model)
  • Single-Way data flow or uni-directional data flow
  • Server-side rendering

3. What is Virtual DOM ?

Virtual DOM is in-memory representation of Real DOM. It is lightweight JavaScript object which is copy of Real DOM that is know as Virtual DOM.

4. Different between Real DOM and Virtual DOM.

Real DOM Virtual  DOM
1. It updates slow. 1. It updates faster.
2. It Can directly update HTML. 2. It Can’t directly update HTML.
3. Creates a new DOM if element updates. 3. Updates the JSX if element updates.
4. DOM manipulation is very expensive. 4. DOM manipulation is very easy.
5. Too much of memory wastage. 5. No memory wastage.

5. Major Advantages of using React.

  • Testability :- it is easy to test, and integrate some tools like jest.
  • Maintainability :- it ensures readability and makes maintainability easier.
  • Performance :-  it is really good performance because it’s use virtual-dom.
  • Rendering :- it’s render server-side and client side as well.
  • Code Reusability :- React all about components. So reactJS provides developers with the opportunity to have more time to use and create common abstractions, setting up the creation, distribution and consumption of isolated reusable parts.
  • Better Combination Technologies :- React makes best use of HTML and JavaScript mixing them ideally and also incorporates CSS to provide your business with the best.
  • Integrate With Others Framework :- it’s easy to integrate with other frameworks like Backbone.js, Meteor, Angular, etc.
  • JSX:- JSX makes it easy to read the code of your components. It is also really easy to see the layout, or how components are plugged/combined with each other.
  • Data Binding :- React uses one-way data binding or uni-directional data flow .

6. Major Disadvantages of using React.

  • It is only covering for ‘View’  layer in MVC (Model-View-Controller).
  • React is just a JavaScript library, Not a framework.
  • Its library is very large and takes time to understand.
  • it uses inline templating and JSX.

7. What is state in react js ?

it holds the information about the component and state is using only inside the component. State is mutable. By default component has no state.

8. What is props in react js ?

Props means properties, It’s a way of passing data from parent to child and we can say that props are just a  communication channel between components. Always moving from parent to child component. It’s a immutable and read-only.

9. What is different b/w super() and super(props)?

Only one reason for using props parameter in super keyword. When you want to access this.props inside the constructor then you need to pass the props parameter in super keyword and no difference outside constructor.
10. What is super keyword in react ?
The super keyword is used to access & call functions on an object’s parent.
11. What is JSX ?
JSX means JavaScript XML and it is a templating syntax. JSX makes React a more elegant. JSX adds XML syntax to JavaScript. This file makes applications robust and boosts its performance. Read more about JSX As like below example of JSX.

12. Why can’t browsers read JSX?

Browsers can only read JavaScript objects but JSX is not a regular JavaScript object. That’s why browsers can’t read JSX. Read JSX on browsers for first of all we need to transform JSX file into a JavaScript object using JSX transformers like Babel and then pass it to the browser.

 13. What is different b/w React and Angular?

1. ARCHITECHTURE Only the View of MVC Complete MVC
2. RENDERING Server side rendering Client side rendering
3. DOM Uses virtual DOM Uses real DOM
4. DATA BINDING One-way data binding Two-way data binding
5. DEBUGGING Compile time debugging Run time debugging
6. Developed By Facebook Google
 14. What is different b/w React ES6 and ES5 ?
1.) require vs import

2.) export vs exports

3.) component and function

4.) props

5.) state

15. What is Props ?

Props are Properties of React. It is immutable Properties of React, In other word you can’t change value of props.They are always passed down from the parent to the child components through out the application. A child component can never send a prop back to the parent component. we can also say, Props are communication channel of two different component. It is using for render the dynamically generated data and helping to maintain the unidirectional data flow.

16. What is State and why use State ?

State is mutable, In other word you can change the state inside component. Component state can be changed over time in response to user actions, API responses, and any other. We have a class based components by extending React. Component class to get state features. Define the state initial inside the constructor . State is accessed via this.state(). It will use this.setState() to schedule updates to the component local state. It is only using inside the component.

17. Differentiate between states and props?

State Props
Receive initial value from parent component Receive initial value from parent component
Parent component can not change value Parent component can change value
Set default values inside component Set default values inside component
Changes inside component Changes inside child components
Set initial value for child components Set initial value for child components
Mutable -you can change the value Immutable (Read-only)- you can’t change the value

18. What is Higher-Order Components in react ?

A higher order component is an advanced way to re-using component logic and it is a function that takes a component and returns a new component that is known as Higher-Order Components.

19.Differentiate between stateful and stateless components ?

Stateful Component Stateless Component
1. Stores info about component’s state change in memory 1. Calculates the internal state of the components
2. Have authority to change state 2. Do not have the authority to change state
3. Contains the knowledge of past, current and possible future changes in state 3. Contains no knowledge of past, current and possible future state changes
4. Stateless components notify them about the requirement of the state change, then they send down the props to them. 4. They receive the props from the Stateful components and treat them as callback functions.

20. Differentiate between controlled and uncontrolled components?

Controlled Components Uncontrolled Components
1. It doesn’t  maintain their own state 1. It maintain their own state
2. Data is controlled by the parent component 2. Data is controlled by the DOM
3. It takes in the current values through props and then notify the changes via callbacks 3. Refs are used to get their current values

21. What are Pure Components?

Pure components don’t have functionality of re-render. It is simplest and fastest components only have render() that is called pure components.

22. What is React Router?

Reactjs router most powerful routing library, It helps in adding new screens and flows of application. It provides a sync URL with data that’s be displayed on the web page. It maintains standard structure and behavior and It is used for developing single page web applications.

23. Why do we need a Router in React?

Reactjs router use to define multiple routes and when a user types a specific URL into the browser, if this URL path matches any ‘route’ defined inside the router file, then the user will be redirect to that particular route. So basically, we need to add a Router library to our application that allows creating multiple routes with each leading to us a unique screen.

24. What is the benefits Of React Router?

  1. You don’t need to set the browser history manually.
  2. React Router library is split into 3 separate packages like react-router-dom, react-router-native, react-router-core.
  3. Link uses to navigate the internal links in your application. It’s as like anchor tag.
  4. Uses Switch feature.
  5. The Router have only a Single Child element.
  6. Every Component specified in <Route>.
  7.  IndexRoute is replaced by ‘exact‘.

25. How Does react Virtual DOM Work?

In react Virtual DOM is a node tree that have lists of elements and their attributes, content as object and their properties. React js methods create node tree react components. It updates node tree in the response to mutation in the data model.
Virtual DOM representation the user interface has created.If you are thinking about how many phase in update the browser’s DOM.

26. Why virtual dom is faster ?

Virtual dom is faster because it uses.
1. Efficient diff algorithm
2. Batched update operations
3. Efficient update of sub tree only
4. Uses observable instead of dirty checking to detect change
Note : ReactJS maintains two virtual DOM, one of the updated state Virtual DOM and another with the prior state Virtual DOM.

27. Which feature can we use to cause a component to render only when its ID changes?


28. Does ReactJS use HTML?

No, It uses JSX which is simiar to XML.

29. What are the life Cycle phase of ReactJS?

1. Initialial Phase
2. State/Property Updating Phase
3. Destruction or Unmounting Phase

30. What is Repository URL of ReactJS?

31. How to embed two components in One component?

Simply you can copy and paste below code inside index.js file in root directory.

32. What is difference b/w Class Component or Functional Component?

If you use state or a lifecycle methods of react component that is class component. If you don’t use state or lifecycle methods that is Functional component.

33. What’s the difference b/w an Element and a Component in React?

Basically, a React component describes what you need to see on the screen. Not all that basically, a React element is a protest portrayal of some UI.

A React component is a function or a class which alternatively acknowledges input and returns a React component (ordinarily by means of JSX which gets transpiled to a createElement invocation).

34. What is a three reasons behind the success of ReactJS?

  1. ReactJS is a technology that can be trusted for complex tasks.
  2. Developers don’t worry about the bugs because it always ensures error free outcomes it offers scalable apps.
  3. It is very fast technology and can simply be trusted for quality outcomes.

35. What is the difference between createElement and cloneElement?

createElement is the thing that JSX gets transpiled to and is the thing that React uses to make React Elements (protest representations of some UI). cloneElement is utilized as a part of request to clone a component and pass it new props.

36. What is arrow function in React? How is it used?

Arrow functions are more of brief syntax for writing the function expression. That is also called ‘fat arrow‘ (=>) the functions. These functions allow to bind the context of the components properly since in ES6 auto binding is not available by default. Arrow functions are mostly useful while working with the higher order functions.

37. How can you pass or send data parent to child component in react ?

You can send data parent to child component.

Parent Component

Child Component

38. How can you pass or send data child to parent component in react ?

You can use callback and state for send data child to parent component or receive data parent component from child component.

Parent Component

Child Component

39. What is difference between export and export default in react?

1. If you have requirement for export multiple class go with named export(without default keyword).

Hello.js file

index.js file

2. Otherwise single class export for, you can use simply, export default.

Hello.js file

index.js file

40.) What is yield keyword in JavaScript ?

The yield keyword is used to pause and resume a generator function. That is known as yield keyword.


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Thank you for posting the valuable information.

Anish PR
Anish PR

Very good article 🙂

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