Top 40 ReactJS Interview Questions and Answers to Hire ReactJS Developers

reactjs interview questions and answers
reactjs interview questions and answers

“Respond” is the market’s keyword these days. ReactJS is quickly becoming the most popular front-end technology, with more and more businesses adopting it. If you are looking for ReactJS developers for hire, these ReactJS interview questions are exactly what you need. For front-end developers all across the world, ReactJS is a clear winner. This post on Top 40 ReactJS interview questions is for you if you are an ambitious front-end developer who is preparing for interviews or a business hunting ReactJS developers for hire.

Top 40 ReactJS Interview Questions and Answers

What is ReactJS?

Facebook released the ReactJS front-end JavaScript framework in 2011. It creates reusable user interface components using a component-based methodology. It is used to create sophisticated, dynamic web and mobile user interfaces. Even though it was only open-sourced in 2015, it has one of the largest communities behind it.

What distinguishes ReactJS from other frameworks?

ReactJS is quickly becoming the greatest framework for web developers. The following are the major characteristics of ReactJS,

  • JSX
  • Components
  • One-way Data Binding
  • Virtual DOM
  • Simplicity
  • Performance

What is JSX?

JSX is a syntactic extension for JavaScript. It’s a phrase used in ReactJS to describe how the user interface should seem. Using JSX, we may write HTML structures in the same file as JavaScript code.

Is it feasible for web browsers to directly read JSX?

Web browsers cannot read JSX natively. This is because they were only intended to read regular JS objects, and JSX is not one of them. Before a web browser can read a JSX file, it must be transformed into a regular JavaScript object. For this, we utilize Babel.

List some of the most significant benefits of ReactJS.

The following are some of the ReactJS key advantages,

  • It improves the performance of the application.
  • It is possible to execute it on both the client and the server.
  • Code readability improves as a result of JSX.
  • Other frameworks, such as Meteor, Angular, and others, interact easily with ReactJS.
  • Writing UI test cases is a breeze using ReactJS.

What are the most important downsides of ReactJS?

The following is a list of the significant downsides of ReactJS,

  • ReactJS is mostly a library. It is not an all-inclusive framework.
  • It has a large library that takes some time to learn.
  • Understanding and coding may be tough for inexperienced programmers.
  • ReactJS makes use of inline templating and JSX, which can be challenging and serve as a barrier. It also increases the code’s complexity.

What exactly is a virtual Document Object Model (DOM)?

Document Object Model (DOM) is an abbreviation for Document Object Model. It is a logical tree structure that specifies an HTML page. Each branch of the tree ends in a node, which is where objects are stored. The virtual DOM is a lightweight representation of the real DOM that ReactJS keeps in memory. When the state of an object changes, the virtual DOM updates only that object in the real DOM, not all of them.

How to make browsers read JSX?

Browsers can only read JavaScript objects, but JSX is not one of them. To allow a browser to read JSX, first convert the JSX file into a JavaScript object using JSX transformers such as Babel, and then send it to the browser.

In ReactJS, what are synthetic events?

Synthetic events integrate the responses to several native browser events into a single API, ensuring cross-browser compatibility. It is a constant independent of the browser used to run the program. In this case, preventDefault is a made-up event.

Describe how the lists in ReactJS work.

Lists in ReactJS are formed in the same manner that they are in traditional JavaScript. Lists present data in a logical arrangement. To traverse lists, use the map() technique.

“In ReactJS, everything is a component.” Explain.

A ReactJS application’s user interface is made up of components. These components divide the whole user interface into reusable portions. It then makes each of these components independent of the others without influencing the rest of the user interface.

What is the function render() in ReactJS for?

Every ReactJS component necessitates render(). It returns a single ReactJS element corresponding to the native DOM component. If more than one HTML element is to be rendered, all of them must be included within a single enclosing tag, such as <form>, <group>, <div>, and so on. This function must remain pure, which means it must always provide the same result.

What precisely is a ReactJS state, and how do they work?

ReactJS components are built on top of states. States serve as data sources and should be made as straightforward as feasible. In a nutshell, state objects define how components are presented and behave. In contrast to props, they are changeable and provide dynamic and interactive characteristics. This is how you go about approaching them.state().

How do you change a component’s status?

This might be used to change a component’s status. The setState() function is used to change the state of the system. The State is not usually totally replaced by this strategy. It just modifies the initial state. It is the principal technique for changing the user interface in response to event handlers.

What are the different stages of a ReactJS component’s lifecycle?

The lifespan of a ReactJS component is divided into three stages:

  • The component will begin its life journey and find its way to the DOM during the initial rendering stage.
  • After being added to the DOM, the component may only update and re-render when a prop or state changes. This is peculiar to this period.
  • The component is deleted and wiped from the DOM at the unmounting step of its life cycle.

Describe the lifecycle functions of the ReactJS component in detail.

Some of the most important lifecycle techniques are as follows,

  1. componentWillMount() — On both the client and server sides, this function is called just before rendering.
  2. componentDidMount() – Only invoked after the initial client-side render.
  3. componentWillReceiveProps() – Called as soon as the parent class’s props are received and before the next render.
  4. should
  5. ComponentUpdate() – Returns a true or false answer based on the supplied criteria. Return true if you want your component to update or otherwise return false. By default, it returns false.
  6. componentWillUpdate() — Called just before the DOM is displayed.
  7. componentDidUpdate() – Called after the rendering is finished.
  8. componentWillUnmount() – Called when the component has been unmounted from the DOM.

What precisely are ReactJS forms?

Forms are used by ReactJS to allow users to interact with web apps. Users can take part in the initiative by filling out forms and sending information as needed. Text fields, buttons, checkboxes, radio buttons, and other elements may be included in a form. Forms are used for a variety of purposes, including user authentication, searching, filtering, indexing, and so on.

What precisely do you mean when you say “refs” in ReactJS?

Refs are the abbreviation for references in ReactJS. It’s an attribute that allows you to save a reference to a certain ReactJS element or component, which will be returned by the render configuration function of the component. It’s used to get references to specific elements or components that the render returns(). They are useful when performing DOM measurements or adding methods to components.

List some of the instances in which references should be used.

Here are some examples of when you should utilize references:

  • Choose text or media playback when you need to modify the focus.
  • to start the required animations
  • Third-party DOM libraries can be included.

What are the ramifications of keys in ReactJS?

A key is a one-of-a-kind identifier. It is used in ReactJS to determine if items in Lists have been changed, updated, or removed. It’s useful when we’ve dynamically added components or when users update the lists. It also aids in determining which parts of a collection should be re-rendered rather than rendering the complete collection every time. It boosts the application’s performance.

What are Higher-order components (HOCs)?

Higher-Order Component is a more sophisticated method of reusing component code. It’s merely a pattern formed from ReactJS’s compositional nature. HOCs are unique components that include another. They will accept any dynamically created child component but will not accept any static child component.

What is the difference between fragments and container divs?

Because they do not construct an extra DOM node, fragments are quicker and use less memory. CSS Grid and Flexbox, for example, have a clear parent-child connection and inject <div> tags in the middle, making it more difficult to keep the intended layout. The DOM Inspector has been improved to make it more user-friendly.

What exactly do you mean by “pure components”?

Natural components are the simplest and quickest to construct. They may be used to replace any component with only a render(). These components enhance both the code’s simplicity and the program’s performance.

What precisely is Redux?

Redux is a JavaScript library that updates the state of an application. Redux is used to build the user interface in ReactJS. It is a state container for JavaScript applications that are used to store the program’s overall state.

What are the components of Redux?

Store: Maintains a record of the application’s current state. The information source for the shop. 

Reducer: Defines how the application’s state changes as a result of actions communicated to the store.

What are HOC purposes?

HOC is capable of a wide range of tasks, some of which are given below:

  • Code Reusability
  • Using props to manipulate
  • meddling with the state
  • Make a highjacking attempt.

Why must component names begin with a capital letter?

In ReactJS, component names must start with a capital letter. If we start the component name in lower case, an error similar to an unknown tag will be issued. This is because lowercase tag names are regarded as HTML tags in JSX.

What does “one source of truth” mean?

The Store method of Redux is used to save the application’s entire state in a single location. As a result, the component’s whole state is kept in the Store, and they receive changes straight from the Store. The single state tree simplifies debugging and analyzing software, as well as tracking changes over time.

In Redux, how are actions defined?

In ReactJS, actions must have a type property that defines the type of action being performed. They must be expressed in the form of a String constant that may be expanded with extra characteristics.

What is the point of utilizing ReactJS Router?

It is used to create single-page web apps since it has a uniform structure and behavior.

Various views may be presented in a single application by configuring distinct routes in the ReactJS application.

Explain Flux.

Flux is an architectural idea that restricts data movement in one direction. It regulates derived data and permits communication across several components by utilizing a central Store with all data authority. Any data updates that occur throughout the program must occur just here. Flux improves application stability while decreasing run-time errors.

What are the benefits of using Refs?

In ReactJS, the Ref is used in the following situations:

  • It is used to acquire the element’s reference.
  • When we require DOM measurements for things like controlling focus, text selection, or media playback, we utilize it.
  • It is used to carry out time-sensitive animations.
  • It’s used to interact with third-party DOM libraries.
  • It is also useful for callbacks.

Why does ReactJS Router v4 utilize the switch keyword?

Among the numerous declared Routes, the switch keyword is used to render just a single Route. Only when the route is matched is the Switch component used to render components. Otherwise, it returns to the component that was not found.

How many different ways can the ReactJS Component be styled?

ReactJS Component may be styled in four different ways, as seen below:

  • Styling on the fly
  • CSS 
  • Module for CSS
  • Components with a Design

What does the term Store signify in Redux?

A store is a JavaScript object that may retain the state of an application and provides a few helpful methods for accessing the state, dispatching actions, and registering listeners. An application’s whole state is kept in a single store. As a result, Redux is highly straightforward and predictable. We may send middleware to the store to process data and maintain track of different actions that affect the storage’s condition. 

What do stateless components mean?

Stateless components are just functions that render the DOM based on the properties provided to them.

What is Jest?

Jest, a Jasmine-based JavaScript unit testing framework, was designed by Facebook. It includes a jsdom environment as well as automatic mock generation. It is also used for testing.

What does a dispatcher do?

A dispatcher serves as the app’s primary hub, accepting actions and distributing payloads to registered callbacks.

What is Babel in ReactJS JS?

Babel is a JavaScript compiler that turns the most recent JavaScript, such as ES6, and ES7, into plain old ES5 JavaScript, which is supported by the vast majority of browsers.

Define reconciliation.

When a component’s state or properties change, Rest will compare the shown element to previously rendered DOM and, if necessary, update the real DOM. This is referred to as reconciliation.


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