Top 60 React Interview Questions and Answers in 2021

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What is React? React is a JavaScript library. It is an open-source, front-end library designed for building user interfaces or UI components. React is used to develop single-page or mobile applications. It is maintained by Facebook and a group of individual developers.

Table of Contents

Top React Interview Questions and Answers

1. Explain a few features of React?

Some of the features of React are listed below:

  1. React makes use of the virtual DOM instead of the real DOM.
  2. React uses server-side rendering.
  3. React follows data binding or unidirectional flow of data.

2. Can you explain the limitations of React?

The limitations of React are listed below:

  1. It is just a library. React is not a full-blown framework.
  2. As react uses inline templating and JSX, the coding becomes complex.
  3. The React library is vast. Hence it takes more time to understand.
  4. It might be difficult for the native programmers to understand the concepts.

3. Can you differentiate between Real DOM and Virtual DOM?

Real DOMVirtual DOM
It can directly update the HTML.It cannot update the HTML directly.
The update here is a slow process.The update here is faster compared to the Real DOM.
There is a lot of memory wastage in DOM.There is no Memory wastage.
The manipulation of DOM is costly.DOM manipulation is easy compared to Real DOM

4. Can you list some of the significant advantages of Limit?

Few significant advantages of React are:

  1. There is an increase in readability because of the JSX’s code.
  2. It increases the overall application performance.
  3. It is easy to write the UI test cases using React.
  4. React provides easy integration with other frameworks like Angular, Meteor, etc.
  5. It can be used conveniently both on the client and server-side.
  6. React is SEO-friendly.
  7. It has a gentle learning curve.

5. Explain JSX?

JSX i.e JavaScript XML, allows us to write HTML inside the JavaScript and it places them inside the DOM without making use of the functions like createElement() or appendChild().

6. Can you explain Virtual DOM and tell us how React uses virtual DOM to render the UI?

Virtual DOM in React is a concept where the virtual representation of the Virtual DOM is placed inside the memory, and it is synced with Real DOM by using libraries like ReactDOM.

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DOM manipulation is considered an integral part of any web application, but it is pretty slow compared to other JavaScript operations. It affects the efficiency of other applications when several DOM manipulations are done. Here the javascript framework updates the entire DOM even if the simple part of the DOM is changed.

React mainly uses two virtual DOMs for rendering UI. If one is used for storing the current state of the object, the other DOM is used to store the object’s previous state.

When any of the virtual DOM is updated, the React compares the two virtual DOMs to know which virtual DOM got updated. Based on the update, React renders only the updated objects inside the Real DOM instead of rendering the whole Real DOM.

7. Can you differentiate between Functional and Class components?

Functional ComponentsClass Components
It is a simple javascript function, which accepts props as the argument, and returns the React element.A Class component enforces to extend from React component, and it creates a render function to return a react element.
The functional components do not allow React lifecycle methods.We can use React lifecycle methods inside a class component.
It is also known as stateless components.It is also known as stateful components.

8. Explain the differences between controlled and uncontrolled components?

Controlled componentsUncontrolled components
Here, the Form data is handled by a react component.DOM handles the Form data.
It is used to create Form validations when you always want to know the input’s value and check if it is a valid character.It is used to handle the Form validations without updating the input value. 

9. When are Refs primarily used in React?

Refs are defined as a function provided by React to retrieve the DOM element and the React element that the user has created. They are used in situations like when you want to change the child component’s value without using the props.

10. Can you explain the relation between React and its components?

Reacts and their components are interrelated. Here, the components of React can be defined as the building blocks of the React application for the UI(User Interface). The process of splitting the whole User Interface into small, independent, reusable pieces helps create Reacts application User Interface.

React Interview Questions and Answers

11. Can you explain some of the advantages of using Redux?

Some of the advantages of using Redux are:

  1. It has better code organization.
  2. It has efficient developer tools that allow the developers to track everything, ranging from actions to state changes.
  3. Redux code is composed of isolated, pure, small functions, thus making the testing easy and simple.
  4. The Redux code is easy to maintain.
  5. Redux supports a large-scale community.
  6. It supports server-side rendering.

12. Can you explain the benefits of HOC?

Few benefits of HOC are listed below:

  1. Code reusability.
  2. HOC offers pop manipulation.
  3. It supports manipulation and state abstraction.
  4. HOC offers a high hacking facility.
  5. It supports the application of logic and bootstrap abstraction.

13. Explain states in React?

State in React is a javascript object used to store components’ dynamic data. It also enables a component to keep track of changes between renders. As the state is defined as dynamic, it is reserved only for interactivity. Hence it is not used with static React objects.

14. What are pure components in React?

Pure components in React are defined as the components that do not re-render whenever the value of a state and props has been updated with the same values. The component is not rendered if the value of the given previous state or pops and the new prop state is the same.

15. Explain props in React?

Props in React is a unique keyword that stands for properties, and it is used to pass data or information from one component to another component, and the data that is passed here is in a unidirectional flow.

16. Why do we use render() in React? Explain?

Rendering in React is one of the most useful procedures that a programmer has to manage in front-end development. The render() method in React is an essential method of the class component, which is responsible for displaying the specific view to be rendered to the browser window.

17. Can you explain What is a store in Redux?

A store in Redux is defined as an immutable object tree. In simple words, a store is a state container that holds the application’s state. Once a store is created in Redux, you have to specify the reducer. A Redux can only have a single store in an application.

18. What is a Stateful component in React?

A stateful component in React is dependent on its state object and has the ability to change its own state. The component re-renders depending on the changes to its states and may pass down its state’s properties to the child components as the properties on a props object.

19. Why do we need a Router to React? Explain?

The router in React is a dynamic, client-side routing that allows you to build a single page web application with navigation without refreshing the page as the user navigates. React router makes use of component structures to call components that displays the appropriate information.

20. Are you familiar with Flux in React?

React Flux is a programming concept where the data flow is unidirectional. The data here enter the app and flows in one direction until it is rendered on the screen.

React Interview Questions and Answers

21. Can you differentiate between React and Angular?

ReactAngular
React can be defined as a Javascript library.Angular can be defined as a complete framework.
It can be packaged with other libraries.It is a complete solution itself.
It doesn’t offer much community support.It offers community support.
It is easy to learn compared to Angular.A lot of training is required to learn Angular.
React gives us the freedom to select the architecture, tools, and libraries used to develop an app.Angular offers limited freedom and flexibility.
It uses one-way data binding.It uses two-way data binding.
React is based on Virtual DOM.Angular is based on the MVC model.
It is written in Javascript.It is written in typescript..
It allows adding a javascript library to the source code.It does not allow adding javascript libraries to the source code.

22. Can you differentiate between Flux and Redux?

FluxRedux
It provides single debugging with the dispatcher.Redux single store makes debugging easy.
Flux includes multiple stores.Redux includes single stores.
Here, the store handles all the logic.Here, the reducer handles all the logic.
It supports front-end frameworks like React, Angular, and Polymer.It supports front-end frameworks like Backbone js, Meteor, Ember, and Polymer.

23. Explain Reducers in React?

Reducers in React is defined as a function that determines the changes to an application’s state. The reducer uses the action that it receives to determine the differences. Tools like Redux help us to manage application state changes in a single store so that their behavior is consistent.

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24. Can you define an event in React?

An event in React is an action that is triggered as a result of the users’ action or a system-generated event. Events in React are named as camelCase instead of the lowerCase. With the help of JSX, a function can be passed as an event handler instead of a string.

25. Can you define HOC in React?

HOC stands for High order component, is a Reacts advanced technique for reusing the component’s logic. HOC is a function that can take the component as a parameter and returns a new component. HOC can be defined as a pattern that has emerged from Reacts compositional nature.

26. Can you explain Synthetic Events in React?

A Synthetic Events in React is defined as a cross-browser wrapper around the browser’s native event. A Synthetic Event has the same interface as the Browsers’ native event, including the stopPropagation() method and the preventDefault() method. Here the events work identically across all the browsers.

27. Can you explain Strict Mode in React?

Strict mode in React is a tool for highlighting the potential problems in any application. It does not render any visual user interfaces. It is used to activate additional checks and warnings for its descendants.

28. What are the different lifecycle methods in React? Explain?

Each and every component in React has to go through a lifecycle of events they are:

  1. Mounting: This can be defined as giving birth to your component.
  2. Update: It can be defined as the growth of your component.
  3. Unmount: It can be defined as the death of the component.

The different lifecycle methods in React are as follows:

  1. render(): React uses render() to return the same output whenever the same inputs are passed.
  2. componentDidMount(): This method is called as soon as the component is ready and mounted, where you can initiate API calls, and you can load data from a remote end-point. You can also modify the components state within this method.
  3. componentDidUpdate(): This method is called as soon as the update takes place. This method updates the DOM with the response to the state or props changes.
  4. componentWillUnmount(): This method is invoked just before the component is unmounted and destroyed. You can do any clean-up actions at this stage.

29. Can you differentiate between states and props?

PropsState
They are immutable.It is mutable.
They are read-only.The state changes can be asynchronous.
It allows you to pass data or information from one component to another component as an argument.Here, the state holds the data or information about the components.
They are used to communicate between components.They are used to render dynamic changes with the component.
The props make the components reusable.Here, the state cannot make components reusable.
Their child components cannot access them.They cannot be accessed by their child components.

30. Explain the arrow function in React and its uses?

The arrow function in React is more of syntax for writing function expressions. The function allows the user to bind the context of the components’ property. They are helpful when we are working with high-order functions.

31. Can you list out some of the cases when you should use refs?

The following are some of the cases where you should use the refs:

  1. They are used when you need to select the text or media playback.
  2. Refs are used for triggering imperative animations.
  3. Refs easily integrate with the third-party DOM libraries.

32. Can you explain how to create Forms in React?

Forms in React are similar to HTML Forms. Here the state is contained in the components state property, and it is only updated through setState(). Thus the elements here cannot update their state directly, and their submission is handled by the JSX(Javascript XML) function. This function has full access or control to the data that the user enters through the Form.

33. Explain how to create an event in React with code?

The event in React is created by the given below code.

class Display extends React.Component({
show(evt) {
// code
},
render() {
// Render the div with an onClick prop (value is a function)
return (
<div onClick={this.show}>Click Me!</div>
);
}
});

34. Can you explain how to modularize code in React?

The code in React is modularized using the import and export properties, and they also help in writing the components separately in various files.

//ChildComponent.jsx
export default class ChildComponent extends React.Component {
render() {
return(
<div>
<h1>This is a child component</h1>
</div>
);
}
}
//ParentComponent.jsx
import ChildComponent from './childcomponent.js';
class ParentComponent extends React.Component {
render() {
return(
<div>
<App />
</div>
);
}
}

35. Can you explain the significant problems with the MVC framework?

The below mentioned are some of the major problems of the MVC framework.

  1. The DOM manipulations are very expensive.
  2. There was a lot of memory wastage.
  3. Because of the circular independencies, a highly complex model was created around the views and models.
  4. The applications that used the MVC framework were slow and inefficient.

36. Can you explain the importance of keys in React?

Keys in React are used for identifying unique virtual DOM elements with their related data driving the user interface. They play a significant role in React to optimize the rendering by recycling all the DOM elements (Document Object Model). The keys have to be a unique number or string; based on these, React just reorders the elements instead of re-rendering the elements, and keys increase the application’s performance.

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37. Can you describe the three principles that Redux follows?

The three principles that Redux follows are listed below:

  1. Single Source of Truth: Here, the state of the whole application in React is stored in an object or state tree inside a single store. This single state tree makes it easy to keep track of changes over a period of time and debug or inspect the application.
  2. The state is read-only: The only way to change the state in React is to trigger an action. Again an action in React is a simple JS object that specifies the changes. Just like, how the state is defined as the minimal representation of data, the action is also the minimal representation of the change to that specified data.
  3. The changes are made with pure functions: Pure functions are needed to specify how actions transform the state tree. In the case of pure functions, the return value depends on the argument’s value.

38. Can you list the components of Redux?

The components of Redux are listed below:

  1. Action: It is an object in Redux that describes what had happened.
  2. Reducer: It is a place that determines how the state can be changed.
  3. Store: The state or object tree of the whole application is saved in the store.
  4. View: It simply displays the data or information provided by the store.

39. Explain Actions in Redux?

The Actions in React should have the type property to indicate the type of action to be performed. They have to be defined as a string constant, and it also lets you add more properties. Actions are created using the function Action creators.

unction addTodo(text) {
return {
type: ADD_TODO,
text
}
}

The above code defined is an example of Action and Action Creator.

40. Differentiate between React Routing and Conventional Routing?

React RoutingConventional Routing
It involves single HTML pages.Here, each view corresponds to a new file.
The user here is tricked, thinking that he is navigating across various pages.Here, the user actually navigates across various pages for each view.
Here, the history attribute is only changed.Here, it sends an HTTP request to the server and receives a corresponding HTML page.

41. Can you tell us why the switch keyword is used in React router v4?

We use <div> to encapsulate various routes inside the router. We use a switch keyword whenever we want to display a single route to re-rendered among the several defined routes. When we use the <switch> tag, it matches the typed URL with the specified routes in sequential order. When it finds the first match, it renders the specified route and thereby passing all the remaining routes.

42. Can you list the advantages of React router?  

The advantages of React router are:

  1. It lets us understand what our app views are because of the viewing declarations in a standardized structure.
  2. It supports dynamic route matching.
  3. It supports lazy code loading.
  4. One can easily handle nested views and progressive resolutions of the views using React router.
  5. With the help of the browsing history feature in React router, one can easily navigate back and forth by restoring the state of views.
  6. It is beneficial in working with teams because of the standardized app structure and behavior.

43. Can you explain why browsers can’t read JSX?

Browsers are designed to read only javascript objects, but JSX is not a regular javascript object. In order to allow a browser to read JSX, first, we have to convert the JSX file into a javascript object using the JSX transformers like Babel, and then you have to pass it to the browser.

44. Can you differentiate between createElement and cloneElement?

React.createElement() function is used to create elements that are going to be used for the object representation of the UI(User Interface).

CloneElement in React is used to clone an element and pass it to the props.

45. Can you explain with code to create props proxy for HOC component?

One can add or edit props that are passed to the component using the props proxy pattern as shown below.

function HOC(WrappedComponent) {
  return class Test extends Component {
    render() {
      const newProps = {
        title: 'New Header',
        footer: false,
        showFeatureX: false,
        showFeatureY: true
      }
      return <WrappedComponent {...this.props} {...newProps} />
    }
  }
}

46. Explain JEST?

JEST is defined as a JavaScript unit testing framework developed by Facebook based on jasmine, and it provides an automated mock creation and a jsdom environment. It is mainly used to test React components.

47. Can you explain how to write comments in React?

The comments in React are similar to that of javascript multiline comments, but they are wrapped in curly braces.

We have two types of comments in React, namely,

  1. Single line comments.
  2. Multiline comments.

Single line comments example:

<div>
  {/* Single-line comments(In vanilla JavaScript, the single-line comments are presented by double slash(//)) */}
  {`Hello ${user}, let's play React`}
</div>
Multiline comment example:
<div>
  {/* Multi-line comments for more than
   one line */}
  {`Hello ${user}, let's play React`}
</div>

48. Explain what is the context in React?

The context in React is a mechanism to pass data or information through the component tree without passing the props manually at the entry-level.

49. Explain reconciliation?

Reconciliation in React is a process. When a component’s state or prop changes, React has to decide whether an actual DOM update is necessary or not by comparing the newly returned element and with the previously rendered one. When the React finds that it is not equal, then the updating of the DOM takes place.

50. What is a children’s prop in React?

Children are a prop (this.props.children) in React when a component’s state or prop changes, allowing it to pass the components as data to the other components. The component tree that is put between components opening and closing tag will be passed to that component as a child prop.

A few of the methods that are available in the React API to work with the prop are listed below:

  1. React.Children.map()
  2. React.Children.forEach()
  3. React.Children.count()
  4. React.Children.Only()
  5. React.Children.toArray()

Example of children prop

const MyDiv = React.createClass({
  render: function() {
    return <div>{this.props.children}</div>
  }
})
ReactDOM.render(
  <MyDiv>
    <span>{'Hello'}</span>
    <span>{'React'}</span>
  </MyDiv>,
  node
)

51. What are fragments in React?

Fragments in React are common patterns that are used for a component to return multiple values. Fragments help us to group a list of children without adding the extra nodes to the DOMs. 

52. Can you tell us about portals in React?

Portals in React are recommended to render the children into a DOM node outside of the parent component’s DOM hierarchy.

Syntax:

ReactDOM.createPortal(child, container)

53. Can you explain error boundaries in React v16?

Error boundaries in React are components that are used to catch JavaScript errors in the child component tree, log those errors and display a fallback user interface instead of the crashed component tree.

54. Can you explain the usage of the React-DOM package?

The package provides DOM-specific methods which are used at the top level of an App. Some of the methods of the package are listed below:

  1. render()
  2. hydrate()
  3. unmountComponentAtNode()
  4. findDOMNode()
  5. createPortal()

55. Explain ReactDOMServer?

The ReactDOMServer object allows us to render components to static markup. The ReactDOMServer is mainly used for server-side rendering. 

The below-mentioned methods can be used for both the server and browser environments.

  1. renderToString()
  2. renderToStaticMarkup()

56. Can you explain how to enable production mode in React?

We have to make use of webpacks’ DefinePlugin method to set NODE-ENV to production, where it strips out things like extra warnings and pop validations.

57. Explain the lifecycle method orders in mounting?

The lifecycle methods are invoked in the below-mentioned order:

  1. constructor()
  2. Static getDerivedStateFromProps()
  3. render()
  4. componentDidMount()

58. Can you explain the recommended ordering of methods in a component class?

The below mentioned are the recommended ordering of methods from mounting to render stage:

  1. static methods
  2. constructor()
  3. getChildContext()
  4. componentWillMount()
  5. componentDidMount()
  6. componentWillReceiveProps()
  7. shouldComponentUpdate()
  8. componentWillUpdate()
  9. componentDidUpdate()
  10. componentWillUnmount()
  11. click handlers or event handlers like onClickSubmit() or onChangeDescription()
  12. getter methods for render like getSelectReason() or getFooterContent()
  13. optional render methods like renderNavigation() or renderProfilePicture()
  14. render()

59. Explain React Mixins?

Mixins in React are used to totally separate components to have common functionality. PureRenderMixin is one of the most commonly used mixins. We use this in some components to prevent the unnecessary re-rendering when the states and props are equal to the previous state and prop.

60. Name the pointer-events supported in React?

Pointer events in React provide a unified way to handle all the input events.

The following events are supported in React:

  1. onPointerDown
  2. onPointerMove
  3. onPointerUp
  4. onPointerCancel
  5. onGotPointerCapture
  6. onLostPointerCapture
  7. onPointerEnter
  8. onPointerLeave
  9. onPointerOver
  10. onPointerOut

Good luck with your React interview, and we hope our React Interview Questions and Answers were of some help to you. You can also check out our SVN Interview Questions and Answers, which might help you.

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