50 Most Common React Interview Questions And Answers

This tutorial provides frequently asked React interview questions and answers with explanations to help you prepare for the interview.

This React interview questions tutorial is divided into four parts. First, we will look at the general React.js questions that are asked nowadays concerning React.js. We will then proceed to React components.

Next, we will analyze React Redux interview questions related to React and Redux. Last, we will look at React route interview questions. If you are aiming to ace your React interview, then you have come to the right place.

React JS Interview Questions Quiz

Ace your upcoming React JS Interview with our comprehensive quiz. Boost your React knowledge tremendously and showcase your expertise to impress the recruiter in your upcoming interview.

Top React Interview Questions And Answers

Q #1) What is React, and why is it required?

Answer: Earlier, we utilized applications like jQuery and vanilla JavaScript in our application’s frontend, but nowadays we use a JavaScript library known as React. It is a Facebook innovation developed in 2011, though it started as an open-source project in 2009. React.js follows a component-based approach.

The other thing is that it allows creating reusable UI components, which is why it is widely used. It can be used for both web UI, user-facing, and mobile-based and mobile applications. It was open-sourced in 2015. More so, it has a strong foundation and a very large community base – GitHub and other communities.

Q #2) What are the main characteristics of React?

Answer: These are:

• It uses the concept of a virtual DOM, which is separate from the real DOM, as compared to other client-side libraries, which use the general or real DOM.
• Server-side rendering.
• It follows unidirectional data flow (one-way data binding) – which says all the data binding or the data flow happens in one direction only.

Q #3) What are the various advantages of React as compared to other libraries?

Answer: Advantages include:

• Application performance is highly optimized or highly increased while using React in your application.
• We can use it both on the server and client sides.
• Readability is improved.
• It is very simple to integrate.
• Easy to write your test cases.
• React.js is easy to integrate with various other frameworks and libraries.

Q #4) List the main limitations of React?

Answer: Below are the main limitations:

• React.js is not a full-scale framework; it is a modern view controller. It only handles the view part of MVC.
• The React library is quite extensive.
• It’s very difficult to understand for novice programmers.

Q #5) What do you understand by JSX?

Answer: JSX is a combination of HTML and JS, which stands for JavaScript XML or JavaScript extensions. It also makes HTML very easy to understand. It is robust and boasts most of the performance of your application. JSX expressions must have only one outermost element.

Q #6) What is a Virtual DOM?

Answer: Virtual DOM is separate from the real Dom as compared to other client-side libraries, which use the general or the real DOM. ReactJS uses a virtual DOM and only the changes that are made from the application standpoint get reflected, and then the virtual domain takes care of each and everything.

Q #7) What distinguishes virtual DOM and DOM in regards to React.js?

Answer: Here are the differences:

Q #8) Why can’t you read JSX?

Answer: JSX is not a regular JavaScript – JSX is a combination of HTML and JS file. The browser can read JavaScript objects only and cannot read JSX kinds. Through JSX transformers like Babel, the JSX file gets converted to a JS object before reaching the browser.

Q #9) How did React syntax change from ES5 to ES6?

Answer: ES5 and ES6 are the standards for writing code for JavaScript.

In ES5, we can create a component with the var name and use React.createClass. This is something very important in ES5. We write this type of code or this type of syntax, making use of React.createClass by using a render function.

While in ES6, we have a class keyword that extends React.component.

Q #10) How does React differ from Angular?

Answer: These two are the most famous libraries or frameworks we can say out there in the market nowadays.

The differences can be seen in the following points.

• Architecture: The architecture of React is a Viewpoint and that of Angular is complete.
• Rendering: If we talk about the rendering part, it uses SSR kind of rendering, and AngularJS uses CSR kind of rendering.
• DOM: If we talk about DOM, virtual DOM is the only concept that is introduced with React.js. Angular JS still uses the real DOM.
• Binding: React.js follows the data flow that is from parent to child. This is a one-way binding, but AngularJS follows two-way data binding, which is again sometimes very expensive.
• Debugging: Debugging in React.js is done at compile-time, and in Angular is done at Runtime.
• Authors: React was introduced by Facebook, and Angular is from Google.

These are the basic differences between Angular and React.

Q #11) What is the meaning of the saying ‘Everything is a component in React?

Answer: Components fragment the UI into reusable modules and render each module independently. These container components work as add-ons to the presentation components and JavaScript functions, which take in arbitrary inputs and return XML representations.

Components are JavaScript functions that take in arbitrary inputs, which are most of the time their props, and then return the HTML representation.

Image depicting React.js efficient server rendering:

Q #12) What is the render() function in React?

Answer: Render function, you cannot run your reaction as an application. It will be given with an error that you cannot render the particular view part of the UI that you are going to use. Every Rendered function always returns a React element. Enclosing HTML elements inside render within the closing tag is required.

HTML elements inside renders must be enclosed inside an enclosing tag. For example: <group>, <div>, <form>, etc. There needs to be a pure function.

Q #13) How can we render or embed two components into one?

Answer: This can be done when having two components, for instance, a class myComponent extends React component and class header extends React component, which gets embedded into one component named myComponent.

Q #14) What are Props in React.js?

Answer: This is a short form for the properties. In React, instead of properties, we say Props to point to the properties for a particular HTML element. They are read-only; they are pure, which means they are immutable. 

We cannot change them and always pass them down from parent to child components. React follows a unidirectional data flow or a unidirectional data binding.

So that’s why these Props are always parent-to-child relationship components, and they are used to render dynamic data.

Q #15) What is a state in React?

Answer: When we talk about state in React, we imply that it is the heart of the React components. All components interact with each other using the state. The state should be simple. It’s paramount to understand that the state needs to be immutable in React. It creates dynamic and interactive components.

The state is accessed via dot State. This is a distorted state having two properties: name and ID. In the name we are passing Max and ID is 102, so this is the state and this state is getting rendered over hello this state name, so it will say Hello Max, your ID is 102. We can update the state also using the set state.

Intermediate Level Questions for React JS Interview

Q #16) What is the difference between state and Props?

Answer: We will distinguish in regards to following conditions:

• The first one is receiving the initial value from the parent component; state and Props both receive initial values from the parent component because of unidirectional data flow or unidirectional data binding.
• In React, both state and prop receive initial values from the parent components, so the parent component can change the value concerning the state. We cannot change the component value.
• For Props, we can set default values inside the component, and we can also set the initial values or the default values for the Props.
• Changes inside a component for the state are allowed. Changes are possible within components, but not for Props.
• It is possible to set the child’s initial component value for both state and Props.

Q #17) Briefly describe how you can update a component’s state.

Answer: Since all components are dealing with a state, there is a method called set state. By utilizing the set state function, you can change the component’s state. Change the state or the initial state using this set state function.

Q #18) What is meant by an arrow function?

Answer: Arrow function is represented by the equal to and greater than sign, i.e., =>. They allow proper binding of the context components. Since auto-binding isn’t present in JSX, it needs to be kept in mind that this allows binding the context of the components properly.

Arrow functions are used since auto-binding is not available in the ES6 system. It makes it easier to work with higher-order functions.

Q #19) What are the differences between stateful and stateless components?

Answer: Stateful components are those that store component state changes in memory. Can calculate the internal state components. Stateful components can change the state. Stateless components can’t change state. The stateful component keeps records of past, present, and future forecast changes of state.

Stateless components don’t keep records of past, present, and future forecast changes of state. Stateless components notify in cases of state change requirements. They can then proceed to send out the Props to them.

Q #20) Describe React component lifecycle?

Answer: There are four phases. The three phases that are mostly used are the initial phase, updating phase, and unmounting phase.

Every phase has its methods, which work in a particular order or with particular priorities.

The initial phase is having the function as:

• getDefaultProps()
• getInitialState()
• ComponentWillMount()
• Render()
• componentedMount()

There are 5 methods or functions available in the updating phase. They are:

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

Unmounting phase has one function called componentWillUnmount().

These are the lifecycle methods in React, which we need to have so that whatever Props or components we are using are mounted and unmounted accordingly.

Q #21) List the React component methods.

Answer: These are the various React component methods:

• componentWillMount()
• componentWillReceiveProps()
• componentDidMount()
• shouldComponentUpdate()
• omponentWillUnmount()
• componentWillUpdate()

Q #22) In React, what are events?

Answer: Events are actions, e.g., mouse clicks, key press, etc. For example, switching on a bulb. This is an event that is getting triggered, i.e., reacts to specific actions.

Q #23) How can you create an event?

Answer: An event in React can be created using a render function, which will return a tip, and the on click is the event. That is how we can create events in React.js.

Q #24) Describe what synthetic events are in React?

Answer: Synthetic events are the cross-browser wrappers of the native event system of browsers. Synthetic events combine the browser behavior into one API.

They ensure that events ensure property consistency across different browsers. If you want to have a React sharing application running on different browsers, you need to use synthetic events in ReactJS.

Q #25) What are Refs in regard to React?

Answer: Refs stand for references. They are used to add methods to components.

Q #26) When should you use Refs?

Answer: Refs are used in managing media playback or focused text selection for triggering imperative animations. They are also used if you want to display animation – for imperative animation, we will use Refs in React.

Q #27) How can you achieve code modularization in React?

Answer: You can modulate code by using the import and export standard or by using the import and export properties. We can write the component separately in different files. So for one file, we will use the export property, and for another file, we can use the import property. That is how you can modulate the code.

Q #28) How forms are created in React?

Answer: HTML form elements maintain their own state, and regular domain updates themselves based on user input. However, a state in React.js is contained in the component’s state property, and it gets only updated via text state. The JavaScript function is used for handling the form solution.

React JS Scenario-Based Interview Questions

Q #29) What kinds of components do we have in React?

Answer: There are two types of components:

• Control components that do not maintain their state.
• Data is controlled by the DOM, and Refs are used to get their current value, thus maintaining their state.

If you want to use Refs, you need to have uncontrolled components.

Q #30) What are higher-order components?

Answer: These are the custom components that are developed while coding in React.js. They wrap other components. They accept child components that are provided dynamically, and the input component isn’t modified.

They do not copy any behavior from the input component. They are pure functions and are custom components that are developed to add functionality to other components.

Q #31) What can you do with HoC?

Answer: HoC means higher other components.

Uses:

• Reuse the code logic and bootstrap abstraction using HoC.
• Render hijacking state abstraction manipulation
• Do state and Props manipulations

Q #32) Explain Pure components in React?

Answer: Pure components are simple and easy to write. They are very fast. They can replace only the component having render(). Pure components enhance the simplicity and performance of the application.

Pure component syntax:

ReactDOM.render(<h1>Hello</h1>, document.getElementById(‘content’));

Render <h1>hello and content, these are the two parameters that are passed to the render function, and they are simple components or pure components that we can write in React.js, and hence the fastest.

Q #33) Why are React keys important?

Answer: There is a key-value pair combination in every programming language.

React keys are important because:

• They map the virtual DOM elements into the respective data-driven UI.
• They must be a unique number or string.
• By recycling DOM elements, they assist React js in optimizing the rendering process.
• Instead of re-rendering with keys, React just reorders the element.
• The performance of the application is optimized and highly increased.

Q #34) What are the major problems associated with the MVC framework?

Answer:

[image source]

The major problems with Model View Controller are:

• DOM manipulation is very expensive because the MVC framework is completely based on the DOM and not on the virtual DOM.
• They are inefficient and slow.
• MVC framework wastes memory.
• Circular dependency leads to a complicated model.

Q #35) Explain flux.

Answer: Before Redux, there was a technology called Flux, which was introduced by Facebook to develop client-side web-based applications. Using unidirectional data flow compliments Reacts view components. It is less of a formal framework and more of a pattern, and you can get started with less code.

Q #36) What is Redux?

Answer: Redux is a state container that is predictable and can develop JavaScript applications that are consistent in behavior across native, server, and client environments. Redux is easy for testing. It is commonly utilized with React.js as a state management tool; it can be used with various JavaScript libraries and frameworks.

Q #37) Redux follows which principles?

Answer: Redux operates according to these three principles:

• A Single Source of Truth – there is only one Store in a Redux or React-Redux application.
• Its state is read-only in Redux.
• Pure function libraries make the changes.

Q #38) Single source of Truth refers to?

Answer: When we say a Single source of Truth, it is the Store that we are talking about in the Redux application. Components state receives updates directly from the store. It makes it easier to record application changes.

Q #39) What are the various components of Redux?

Answer: Redux components are:

• Action: This is an object that describes what happened.
• Reducer: It determines state change.
• Store: It is a single source of truth; it’s a single store that is used for state management of all the components.
• View: It displays the data provided by the Store.

React JS Interview Questions for Experienced Professionals

Q #40) How does data flow through Redux?

Answer:

The flow of data in Redux:

[Via O’Reilly]

The container components, that are passing data as Props, they are passing the data to actions, action is passing the data to the reducer. The input for the store is the reducer, and the reducer is passing the data to the provider component. That is how the data flows in Redux applications.

Q #41) In Redux, how are Actions defined?

Answer: Always remember that action should have the type of action indicated by the type property to be implemented should have a type property. Type is very important for every action. In the code sample, the function takes text as an input parameter. What it is returning is that it is a time, and what type of action it is Add_Task.

Code showing Action implementation

function addingTask (str) {
   return {
     type: Add_Task,
     text
   }
}

Q #42) What is the work of reducers?

Answer: Reducer is a pure function that, in case there is no work, returns the previous state. If there are no changes in the state or if there are no updates, it will return the previous state, which it takes as an input, and it returns a new state with the changed parameters as output.

Q #43) Why is the store in Redux important?

Answer: Redux is important since the store is the distinct source of truth. A JavaScript object holds the application state; it maintains the application, or we can say it maintains the state tree of the particular application. The store provides helper functions to access the dispatch action, state, and applications that are unregistered with a Store.

Redux synchronizing the data state from the server level to the client layer is easier. It’s a single store, a single source of truth, a single place to maintain the state tree of the application, and it makes the development of large applications very easy and fast.

Q #44) How does Redux differ from Flux?

Answer: These are the basic differences:

• The store contains state and change logic, has multiple stores, and disconnected stores in flux.
• React components subscribe to the store in Flux state, which is mutable, so you can change the state in Flux.
• Redux store and change logic are separate. We have a single store in Redux single store with hierarchical reducers. You can have multiple reducers in Redux, but the store needs to be single in the React-Redux application.
• They use connect container components.
• Their state is immutable.

Q #45) What are the advantages of Redux?

Answer: Some of the advantages of Redux are:

• Predictability
• Maintainability – the application will be quick and optimized.
• It supports many developer tools.
• The testing and debugging part will is easy
• A huge open source community for products is available out there on the internet.
• Accurate organization of the course, because there is only one store which is maintaining the complete state of the application.

These are some advantages of Redux, which make React applications very famous and very popular.

Q #46) What is React Router?

Answer: It is a very powerful routing library. It adds screens and flows to the app. It also keeps the URL synchronized with the data displayed on the web page. In addition, it is a very simple API. More so, the React Router API is very simple to use.

Q #47) In React Router v4, when would you use the switch keyword?

Answer: The switch keyword is used when you want to have multiple defined routes, but only a single route needs to be displayed. We use the switch statement.

Q #48) Why is the router in React important?

Answer: The router helps in defining multiple routes inside the router, with each route leading to a unique view.

That is why we need a router and React.

Q #49) List down the pros of React Router.

Answer: 

Pros:

• The API is very simple.
• API is all about the component.
• Component-based API.
• The router APIs.
• History value isn’t set manually.
• It has separate packages for web and native platforms.
• Very famous.

Q #50) How does React Router differ from conventional routing of data?

Answer: In React Router, only single HTML pages are involved. In conventional routing, we used to have multiple pages involved in the application.

In React Router, the past attribute is the only one that’s updated. This is a very drastic change in the request-response time in logic.

If we talk about the look and feel, the user navigates across various pages for each view in conventional routing. But for React Router, the user is navigating across different pages, but it is not the case.

Conclusion

In this tutorial, we have looked at the first part of the trending React.js interview questions that are being used by the majority of interviewers in both online and onsite interviews.

We have broadly analyzed the questions in a way that seals all the loopholes that are there as far as React Native interview questions are concerned.

For more React-related guides, you can explore our range of tutorials below:

• How To Test React Apps Using Jest Framework
• How to Use Create React App With TypeScript