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 at acing your React interview, then you have come to the right place.

Top React Interview Questions And Answers

Q #1) What is React and why it is 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 and the reason 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 virtual DOM, which is separate from the real DOM as compared to other client-side libraries which use the general or the 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 listed 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 expression 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. The ReactJS uses a virtual Dom and only the changes which are done 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 browsers can’t 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 the 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 below points can show the differences:

• 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 Angular JS 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 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 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 representation.

Components are the JavaScript functions that take in arbitrary inputs, which are most of the time its 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 part which you are going to use. Every Rendered function always returns a React element. Html elements inside render must be enclosed within the closing tag.

HTML element 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 you can render or embed two components into one?

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

Recommended Reading => Explore the Most Popular Bulma Interview Questions

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 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 component. 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 rendering 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 state. The state should be simple. It’s very 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.

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 value 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 is allowed. We can do the changes inside the component, but for Props, we cannot.
• 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 equal to and greater than sign, i.e. =>. They allow proper binding of the context components. Since auto-binding isn’t present in JSX, this needs to be kept in mind that 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 the 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 which are mostly used are the initial phase, updating phase, and unmounting phase.

Every phase is having its methods which are working in a particular order or with these 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 click, keypress, 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 native event system of browsers. Synthetic events combine the browser behavior into one API. They ensure events ensure properties 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 regards 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 update 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.

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 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 the pure functions and are the custom components that are developed to add functionality to some other components.

Q #31) What can you do with HoC?

Answer: HoC means high 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 there is the simple component or pure components which 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 always in every programming language.

React keys are important because:

• They map the virtual DOM elements into respective data driving 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 for 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 DOM and not on 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 follows the below 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 functions 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 receive 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.

Q #40) How does data flow through Redux?

Answer:

The flow of data in Redux:

[image source]

The container components which is 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. On the code below sample, the function takes text as an input parameter. What it is returning is 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. JavaScript object holds the application state, it maintains the application, or we can say it maintains the state tree of the particular application. That is, 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, disconnected stores in flux.
• React component subscribe to the store in flux state 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 reduced, 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 of the advantages of Redux, which makes React application 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, then 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 the 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 the 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, it appears that 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.

Also Read ->> Create & test a React App using Jest

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.