ReactJS Interview Questions

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ReactJS Interview Questions

Q1. What is React JS?

Ans: React is a front-end JavaScript library developed by Facebook in 2011. It follows the component-based approach which helps in building reusable UI components. It is used for developing complex and interactive web and mobile UI. Even though it was open-sourced only in 2015, it has one of the largest communities supporting it.

Q2. What are the features of React?

Ans: Here are some of the main features of React:

  • JSX: JSX is a JavaScript syntax extension.
  • Components: React is all about components.
  • One-direction flow: React implements a one-way data flow which makes it easy to reason about your app

All of these features make ReactJS a very in-demand technology with many companies looking to hire ReactJS developers.

Q3. What are the advantages of React JS?

Ans: Some of the major advantages of React are:

  • It increases the application’s performance
  • It can be used on client and server-side
  • Code’s readability increases, because of JSX.
  • It is easy to integrate with other frameworks such as  Angular, Meteor etc
  • Using React, writing UI test cases becomes extremely easy
  • React uses virtual DOM which is a JavaScript object.
  • This will improve apps performance
  • Component and Data patterns improve readability.

Q4. What are the limitations of React?

Ans: Limitations of React are listed below:

  • React is just a library, not a full-blown framework
  • Its library is very large and takes time to understand
  • It can be a little difficult for novice programmers to understand
  • Coding gets complex as it uses inline templating and JSX

Q5. What is JSX?

Ans: JSX is a shorthand for JavaScript XML. This is a type of file used by React that utilizes the expressiveness of JavaScript along with HTML-like template syntax. This makes the HTML file really easy to understand. This file makes applications robust and boosts their performance.

Q6. Differentiate between Real DOM and Virtual DOM.


Real DOM Virtual DOM
It updates slowly. It updates faster.
Can directly update HTML. Can’t directly update HTML.
Creates a new DOM if the element updates. Updates the JSX if the element updates.
DOM manipulation is very expensive. DOM manipulation is very easy.
Too much memory wastage. No memory wastage.

Q7. Real DOM vs Virtual DOM

Ans: Browsers can only read JavaScript objects but JSX is not a regular JavaScript object. Thus to enable a browser to read JSX, first, we need to transform the JSX file into a JavaScript object using JSX transformers like Babel and then pass it to the browser.

Q8. How is React different from Angular?

Ans: React vs Angular

ARCHITECTURE Only the View of MVC Complete MVC
RENDERING Server-side rendering Client-side rendering
DOM Uses virtual DOM Uses real DOM
DATA BINDING One-way data binding Two-way data binding
DEBUGGING Compile time debugging Run time debugging
AUTHOR Facebook Google

Q9. Why can’t browsers read JSX?

Ans: Components are the building blocks of a React application’s UI.

These components split up the entire UI into small independent and reusable pieces.

Then it renders each of these components independent of each other without affecting the rest of the UI.

Q10. Explain the purpose of render() in React.

Ans: In React, the render() method is a fundamental part of a component's lifecycle and plays a crucial role in determining what should be displayed on the screen. The purpose of the render() method is to define the structure and appearance of the component's UI based on its current state and props.
The render() method must return a single React element, but it can encompass multiple HTML elements by wrapping them in a single enclosing tag, such as a <div> or a <React.Fragment>. This allows you to group multiple elements together and return them as a single unit.

Reactjs Interview Questions For Freshers

Q11. What are the Props?

Ans: Props are shorthand for Properties in React. They are read-only components that must be kept pure i.e. immutable. They are always passed down from the parent to the child components throughout the application. A child component can never send a prop back to the parent component. This help in maintaining the unidirectional data flow and is generally used to render the dynamically generated data.

Q12. What is a state in React and how is it used?

Ans: States are the heart of React components. States are the source of data and must be kept as simple as possible. Basically, states are the objects which determine components rendering and behaviour. They are mutable unlike the props and create dynamic and interactive components. They are accessed via this. state().

Q13. Differentiate states and props.

Ans: States vs Props

Conditions State Props
1. Receive the initial value from the parent component Yes Yes
2. Parent component can change the value No Yes
3. Set default values inside the component Yes Yes
4. Changes inside the component Yes No
5. Set the initial value for child components Yes Yes
6. Changes inside child components No Yes

Q14. What is the arrow function in React? How is it used?

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

//General way
render() {
//With Arrow Function
render() {
this.handleOnChange(e) } />

Q15. Differentiate between stateful and stateless components.

Ans: Stateful vs Stateless Components

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

Q16. What are the different phases of React component’s lifecycle?

Ans: There are three different phases of React component’s lifecycle:

Initial Rendering Phase: This is the phase when the component is about to start its life journey and make its way to the DOM.

Updating Phase: Once the component gets added to the DOM, it can potentially update and re-render only when a prop or state change occurs. That happens only in this phase.

Unmounting Phase: This is the final phase of a component’s life cycle in which the component is destroyed and removed from the DOM.

Q17. Explain the lifecycle methods of React components in detail. 

Ans: The most important lifecycle methods are:

1. componentWillMount() – Executed just before rendering takes place both on the client as well as server-side.

2. componentDidMount() – Executed on the client side only after the first render.

3. componentWillReceiveProps() – Invoked as soon as the props are received from the parent class and before another render is called.

4. shouldComponentUpdate() – Returns true or false value based on certain conditions. If you want your component to update, return true else return false. By default, it returns false.

5. componentWillUpdate() – Called just before rendering takes place in the DOM.

6. componentDidUpdate() – Called immediately after rendering takes place.

7. componentWillUnmount() – Called after the component is unmounted from the DOM. It is used to clear up the memory spaces.

Q18. What is an event in React?

Ans: In React, events are the triggered reactions to specific actions like mouse hover, mouse click, keypress, etc. Handling these events are similar to handling events on DOM elements. But there are some syntactical differences like:

Events are named using camel case instead of just using lowercase. Events are passed as functions instead of strings.

The event argument contains a set of properties, which are specific to an event. Each event type contains its own properties and behaviour which can be accessed via its event handler only.

Q19. What are synthetic events in React?

Ans: Synthetic events are the objects which act as a cross-browser wrapper around the browser’s native event. They combine the behaviour of different browsers into one API. This is done to make sure that the events show consistent properties across different browsers.

Q20. List some of the cases when you should use Refs.

Ans: Following are the cases when refs should be used: When you need to manage focus, select text or media playback To trigger imperative animations Integrate with third-party DOM libraries

Q21. What do you know about controlled and uncontrolled components?

Ans: Controlled vs Uncontrolled Components

Controlled Components Uncontrolled Components
They do not maintain their own state They maintain their own state
Data is controlled by the parent component Data is controlled by the DOM
They take in the current values through props and then notify the changes via callbacks. Refs are used to get their current values

Q22. What are Higher-Order Components(HOC)?

Ans: Higher-Order Component is an advanced way of reusing the component logic. Basically, it’s a pattern that is derived from React’s compositional nature. HOC are custom components that wrap another component within it. They can accept any dynamically provided child component but they won’t modify or copy any behavior from their input components. You can say that HOC is a ‘pure’ component.

Q23. What can you do with HOC?

Ans: HOC can be used for many tasks:

  1. Code reuse, logic and bootstrap abstraction
  2. Render High jacking
  3. State abstraction and manipulation
  4. Props manipulation

Q24. What are Pure Components?

Ans: Pure components are the simplest and fastest components that can be written. They can replace any component which only has a render(). These components enhance the simplicity of the code and the performance of the application.

Q25. What is the significance of keys in React?

Ans: Keys are used for identifying unique Virtual DOM Elements with their corresponding data driving the UI. They help React optimize the rendering by recycling all the existing elements in the DOM. These keys must be a unique number or string, using which React JS just reorders the elements instead of re-rendering them. This leads to an increase in the application’s performance.

Reactjs Interview Questions For Experienced

Q26. What were the major problems with the MVC framework?

Ans: Following are some of the major problems with the MVC framework:

  • DOM manipulation was very expensive
  • Applications were slow and inefficient
  • There was huge memory wastage
  • Because of circular dependencies, the complicated model was created around models and views

Q27. What is Redux?

Ans: Redux is one of the hottest libraries for front-end development in today’s marketplace. It is a predictable state container for JavaScript applications and is used for the entire application's state management. Applications developed with Redux are easy to test and can run in different environments showing consistent behaviour.

Q28. What do you understand by “Single source of truth”?

Ans: Redux uses ‘Store’ for storing the application’s entire state in one place. So all the component states are stored in the Store and they receive updates from the Store itself. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.

Q29. List down the components of Redux.

Ans: Redux is composed of the following components:

Action – It’s an object that describes what happened.

Reducer –  It is a place to determine how the state will change.

Store – The state/ Object tree of the entire application is saved in the Store.

View – Simply displays the data provided by the Store.

Q30. How are Actions defined in Redux?

Ans: Actions in React must have a type property that indicates the type of ACTION being performed. They must be defined as a String constant and you can add more properties to it as well. In Redux, actions are created using the functions called Action Creators.

Below is an example of an Action and Action Creator:

function addTodo(text) {
return {
type: ADD_TODO,

Q31. Explain the role of the Reducer.

Ans: Reducers are pure functions that specify how the application’s state changes in response to an ACTION. Reducers work by taking in the previous state and action, and then it returns a new state. It determines what sort of update needs to be done based on the type of action, and then returns new values. It returns the previous state as it is if no work needs to be done.

Q32. What is the significance of Store in Redux?

Ans: A store is a JavaScript object which can hold the application’s state and provide a few helper methods to access the state, dispatch actions and register listeners. The entire state/ object tree of an application is saved in a single store. As a result of this, Redux is very simple and predictable. We can pass middleware to the store to handle the processing of data as well as to keep a log of various actions that change the state of stores. All the actions return a new state via reducers.

Q33. How is Redux different from Flux?

Ans: Flux vs Redux

Flux Redux
The Store contains state and change logic Store and change logic is separate
There are multiple stores There is only one store
All the stores are disconnected and flat Single store with hierarchical reducers
Has singleton dispatcher No concept of a dispatcher
React components subscribe to the store Container components utilize connect
State is mutable State is immutable

Q34. What are the advantages of Redux?

Ans: The advantages of Redux are listed below:

1. Predictability of outcome:

Since there is always one source of truth, i.e. the store, there is no confusion about how to sync the current state with actions and other parts of the application.

2. Maintainability:

It is simple to maintain a strict structure and predictable outcome.

3. Server-side rendering:

To the client side, You just need to pass the store created on the server. This is helpful for the initial render and provides a high-quality user experience as it optimizes the application performance.

4. Developer tools:

From actions to state changes, developers can track everything going on in the application in real-time.

5. Community and ecosystem:

Redux has a huge community behind it which makes it even more captivating to use. A large community of talented individuals contribute to the betterment of the library and develops various applications with it.

6. Ease of testing:

Redux’s code is mostly functions that are small, pure and isolated. This makes the code testable and independent.

7. Organization:

Redux is precise about how code should be organized, this makes the code more consistent and easier when a team works with it.

Q35. What is React Router?

Ans: React Router is a powerful routing library built on top of React, which helps in adding new screens and flows to the application. This keeps the URL in sync with data that’s being displayed on the web page. It maintains a standardized structure and behaviour and is used for developing single-page web applications. React Router has a simple API.

Q36. Why is the switch keyword used in React Router v4?

Ans: The ‘switch’ keyword is used when you want to display only a single route to be rendered amongst the several defined routes. The tag when in use matches the typed URL with the defined routes in sequential order. When the first match is found, it renders the specified route. Thereby bypassing the remaining routes.

Q37. Why do we need a Router to React?

Ans: A Router is used to define multiple routes and when a user types a specific URL, if this URL matches the path of any ‘route’ defined inside the router, then the user is redirected to that particular route. So basically, we need to add a Router library to our app that allows creating multiple routes with each leading to us a unique view.






Q138. List down the advantages of React Router.

Ans: Few advantages are:

Just like how React is based on components, in React Router v4, the API is ‘All About Components’. A Router can be visualized as a single root component () in which we enclose the specific child routes ().

No need to manually set History value: In React Router v4, all we need to do is wrap our routes within the component.

The packages are split: Three packages one each for Web, Native and Core. This supports the compact size of our application. It’s easy to switch over based on a similar coding style.

Q39. How is React Router different from conventional routing?


Topic Conventional Routing React Routing
PAGES INVOLVED Each view corresponds to a new file Only a single HTML page is involved
URL CHANGES An HTTP request is sent to the server and the corresponding HTML page is received Only the History attribute is changed
FEEL The user actually navigates across different pages for each view The user is duped into thinking he is navigating across different pages

Q40. How React JS differs from AngularJs?

Ans: Both React JS and AngularJS are very potent with a diverse approach from front-end web applications. It supports all kinds of enterprise applications from small, and medium to large ones. React JS –is SEO friendly, easily understandable and simple application whereas AngularJs training is easy to develop an application and supports different testing forms.

Q41. What are the primary reasons to use React JS?

Ans: In spite of all front-end frameworks, React JS is gaining massive popularity with SEO-friendly applications and easily understandable methodologies. It was the perfect fit for our needs. The primary reasons for its popularity are as follows:

  • Fast Learning Curve
  • Reusable Components
  • Quick render with Virtual DOM
  • Clean Abstraction
  • Flux and Redux
  • Great Developer Tools
  • React Native

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