Top 50 React Interview Questions You Must Prepare In 2020 – Starweaver
Top 50 React Interview Questions You Must Prepare In 2020

Top 50 React Interview Questions You Must Prepare In 2020

There are many React interview questions that you should know to make sure that you can impress any new and potential employers and really show off some of the skills that you already have. Some of the most common questions you will encounter on any interview for React include:

Differentiate Between Real DOM and Virtual DOM

Both of these are important when you get to work on web design. Real DOM is slower at updating but can directly update the HTML and will be more expensive. Most companies like to work with the Virtual DOM because it updates faster, can update the JSX elements and is easy to manipulate without wasting memory.

What is React?

It is important to know a bit about React. React is a library for JavaScript that was developed by Facebook and can now be used by other companies. It is going to follow more of a component-based approach that will build UI components that are reusable. Many developers will use this to help make interactive and complex designs.

Why should I use React?

React makes it easier to do some of the work that you need with JavaScript and website development.

What are the features of React?

There are many features of React including that it uses the virtual DOM, it will work with server-side rendering, and it can follow what is known as data binding, or unidirectional data flow.

What are the advantages of React?

There are a lot of benefits that come with using React and it really depends on what is important to you and how you plan to use this. Some of the benefits of using React include:

  1. It will increase the performance of the application.
  2. It is easy to use on the server side and the client side.
  3. The code’s readability is good thanks to JSX.
  4. React can work well with other frameworks.
  5. When you use React, writing your test cases is easier than before.

What limitations does React Have?

While there are a lot of great benefits to using React, there are some limitations as well. Some include:

  1. React is only a library, without the full framework to make things easier.
  2. The library is large, which can make it take longer to understand.
  3. It is difficult for beginners to start with.
  4. Coding will get more complex in some cases.

What is JSX

JSX is simply a shorthand way to write JavaScript XML. This is a file that React will use to help get the programs done.

What is Virtual DOM?

This is basically a lightweight object in JavaScript that works just like a copy of the real DOM. It will have the node tree that lists out the elements along with their attributes and content. It works similar to a regular DOM but allows the user to have a bit more speed and less waste of space on the computer.

Will my browser read JSX?

Browsers are only designed to read JavaScript objects, but JSX is not one of these. This means that you need to transform the JSX file into an object in JavaScript and then pass it onto the browser.

How different is the ES6 to ES5 syntax?

There are a few differences that show up in the syntax of both of these syntax. These changes are found in the require vs. import, export vs. exports, component and function, props, and state.

Is React different from Angular?

While these two programs may seem like they are the same, there are a lot of big differences. React will work with server-side rendering while Angular works with client-side. React does a virtual DOM and Angular does a real DOM. React uses one-way data binding and Angular uses two-way data binding. And React will use compile time debugging and Angular focuses on runtime debugging.

What does “in React, everything is a component?

When using React for UI, components are simply the building blocks. These will split up the whole design into some smaller pieces that can be reused. Then it will render these independent of the other without affecting the rest.

How does render() work in React?

This coding will return the only React element which is supposed to be the native DOM component. If there is more than one element that needs rendered, then they will be grouped together.

What is Props?

Props simply stand for Properties in React. These will include read-only components.

What is a state in React?

States are an important part of React. They will become the source of data and you must keep them as simple as possible and can determine the components will render and their behavior.

How are states and props different?

A state will receive the initial value from the parent component, can set the default values inside that component, will do all changes inside a component, and can set the initial value for the child components. The Props will also receive that initial value from the parent component but this parent component can change the value and will set the default values inside. It can also set the initial value for child components and make changes inside of them.

How easy is it to learn how to use React?

If you already know how to use JavaScript, then React is simple to use as well. For those who don’t know how to code in JavaScript, this is a simple language to learn as well.

What is the arrow function?

This is a simple syntax to help us write out the function expression. It can also be known as fat arrow functions and they will bind the context of the components properly.

What is a stateful component?

A stateful component will store info about the components state change while also having authority to change a state. It will contain all the knowledge of the past, current, and future changes in the state and all stateless components will notify them and send the props down.

What is a stateless component?

This component will calculate the internal state of the components and doesn’t have authority to make changes. It will not contain knowledge about any state changes and can receive the props from the stateful components.

What are the phases of the React component’s lifecycle?

There are three phases for this including the initial rendering phase, the updating phase, and the unmounting phase.

What is an event in React?

An event is something that is a triggered reaction to specific actions. These can be like a keypress, mouse click, our mouse hover.

How to create an event in React?

class Display extends React.Component({

show(evt) {

// code


render() {

// Render the div with an onClick prop (value is a function)

return (

Click Me!




What are synthetic events?

These are objects that will act as the cross-browser wrapper around the native event of that browser. They can combine the behavior from different browsers into one.

What are refs in React?

Refs are short for References to React. This attribute will help store a reference to a particular element or component in React.

When would I use a ref?

There are several times when you are able to use Refs. These include when you need to manage focus, media playback or select text. They can work when triggering imperative animations and when you need to integrate with third-party DOM libraries.

How to Modularize code in React?

Some code to help get this done includes:


export default class ChildComponent extends React.Component {

render() {


This is a child component


import ChildComponent from './childcomponent.js';

class ParentComponent extends React.Component {

render() {



How are forms created?

If you would like to create forms, you can do so with the following:

handleSubmit(event) {

alert('A name was submitted: ' + this.state.value);



render() {

return (




What are controlled components?

These are components that do not stay in their own state and the data is controlled by the parent component. They will instead take on the current values of props and then can notify all the changes.

What are uncontrolled components?

These components can maintain their own state and data is controlled through the DOM. The REfs are used here to get their current values.

What are Higher Order Components

This is an advanced way to reuse some of the component logic. They are custom components that will wrap another component inside.

What can you do with HOC?

There are different tasks that will use HOC. This include code reuse, logic, and bootstrap abstraction, render hijacking, props manipulation and more.

What are pure components?

This is the simplest and fastest components a coder can write out. They will replace any component that is found with render() in front of it.

Why are keys significant in React?

Keys are important because they will help us identify the unique virtual DOM elements with the corresponding data in the UI.

What is the Flux?

This is simply an architectural pattern that will enforce the idea of uni-directional data flow. It will control the derived data.

What is Redux?

This is another library that works well for front-end development. It is seen as a predictable state container for your applications.

What are three principles for Redux?

The three principles include single source of truth, state is read-only, and changes are made with pure functions.

What is the single source of truth?

Reduce will use “store” for storing the entire state of the application in one place. So all of the component’s state will be in the Store while receiving updates from the Store itself. This makes it easier to keep track of things.

What are the components of Redux?

Reduce has four main components including action, reducer, store, and view.

How are actions defined?

Actions in React need to have a type property that will let us know the action that they perform. You need to do this definition as the string constant and then add in as many properties that you need.

What is the Reducer?

Reducers are functions that are able to specify how the state will change based on the action that you pick. They can take in the previous action and state before returning a brand new state.

What is a store?

This is an object that is meant to hold onto the state of the application and can provide several helper methods to access this while also registering listeners and dispatching actions.

When would I use Flux?

Flux will work when you want to have the store hold the state and change logic, when there are more than one store, when all of the stores are disconnected, and when the state is mutable.

When would I use Redux?

This is a good option when you want the change logic and store to be separate and you only have one Store, the state is also immutable with this.

What are the benefits of using Redux?

There are several benefits to using Redux including:

  1. Predictability of the outcome.
  2. Easy to maintain
  3. Server-side rendering.
  4. Developer tools
  5. Community and a good ecosystem
  6. Ease of testing
  7. Organization

What is React Router

This is a routing library that works with React and can help add in new screens and many flows to your application. It can also keep the URL in sync with data that is shown on your web page.

Why is the switch keyboard in this router v4?

The switch tag will be used here when it finds a match with the typed URL with the defined routes in sequential order. The switch can help when there is just one single route amongst many different and defined routes.

Why is Router in React important?

This router is used to define many routes and when a user types in the exact URL they want, if the URL does match the path, then the user will be sent that way. This allows the app to create more than one route to a unique view.

What are the benefits of React Router?

There are many advantages. React Router can be visualized and used with many different paths to make things work better. You also do not need to manually set the value of the History and the packages can be split based on how you would like to use them.

Is React Router different than traditional routing?

In conventional routing, each view will correspond to a new file and the HTTP request will be sent in to the server before getting the right page. The user will need to go around many pages for each view. With React Routing, there is just one HTML page involved and only the History attribute will change. The user may feel they are going through many pages, though it is just one.

And there you have it! 50 questions to help you with your React interview today!


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