Top 50 ReactJS Interview Questions and Answers

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REACTJS INTERVIEW QUESTIONS & ANSWERS

It is necessary to get guidance to learn React JS. There are many options available for online and offline training in React JS.

Following are the 50 interview questions and answers for ReactJS

Q 1- What is ReactJS?

React is also known as ReactJS or React.js. React is an open-source library for Javascript. Like Angular and the current Vue, ReactJS is one of the essential tools having more credit on the way we build a web application.

Facebook maintains · ReactJS.

  • It is used for building UI (User Interface) or UI components. 
  • ReactJS or React can be used as a groundwork for developing a single page application (SPA) or mobile applications.
  • ReactJS is mainly concerned with rendering data to the data object model (DOM).
  • ReactJS provides us with a simple programming model with better performance
  • ReactJS presents data which changes over time. It is updated according to the user action.
  • To create ReactJS applications, we require additional libraries, state management and routing. Redux and React router are examples of the libraries we need.

Q 2– How does ReactJS work?

React follows the apparent pattern and tunes in state changes. ReactJS upgrades the virtual DOM tree. Once the virtual DOM has been updated, the comparison is made between the latest version of the virtual DOM with the old version of the virtual DOM in ReactJS. This procedure is called “diffing”.

Q 3- What are the advantages of ReactJS?

React JS is JavaScript library that is very well known because of its astonishing advantages in web development and transforming the workflow. Most of the leading companies, such as PayPal, Apple, Netflix, and other various websites are using React JS framework to improve the performance of their frameworks.

  • Ease up the process of writing components
  • Increases productivity and makes maintenance smoother
  • Delivers an ideal solution to high-load application
  • Guarantees stable code
  • Improves SEO (Search Engine Optimization) performance
  • Approaches with useful developer tools
  • Ideal for mobile app development
  • Easy to grasp and learn   

Q 4- What are the features of ReactJS?

Features of ReactJS are as follows

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

Q 5- What are the limitations of ReactJS?

           Following are the limitations of ReactJS

  • ReactJS is not a qualified framework; it is just a library.
  • Specifies some additional configuration Merging React.js into a conventional MVC framework
  • Increases code complication with inline templating and JSX.
  • Steering to over-engineering or boilerplate due to excessive smaller components

Q 6- What is DOM?

DOM is a bona fide API (Application Programming Interface) for HTML and corresponding XML documents. It explains the logical structure of documents and the way a record is retrieved and coerced. The term “document” is used widely in DOM specification, XML is employed as a way of illustrating many different kinds of information that may be stored in various systems. However, XML exhibits this data as documents and the DOM may be used to handle this data.

DOM helps the programmers to create the documents, navigate their structure, and add, modify, or delete elements and content.

Q 7- What is a virtual DOM?

Virtual DOM object is a compact version of a DOM object. Virtual DOM objects have similar properties as a real DOM object, but it is unable to directly change what’s on the screen as compared to real DOM object. As nothing gets drawn, on-screen virtual DOM is faster than DOM as it gets manipulated. Virtual DOM is like making changes in software code, as opposed to changes in actual hardware

Q 8- What is JSX?

  • JSX means JavaScript XML.
  • With JSX, we can write HTML in ReactJS.
  • With JSX, it is convenient to write and add HTML in ReactJS.
  • With JSX we are able to write HTML elements in JavaScript and locate them in the DOM without any createElement() and/or appendChild() methods.
  • With JSX, we can convert HTML tags into react elements.
  • Using JSX is not necessary, but JSX makes it convenient to write React applications.

Q 9- Why can’t browsers read JSX?

JSX does not have any derived implementation to read and understand it, so browsers are unable to read it. There is a need to convert JSX into native Javascript and HTML so that the browser can understand JSX

Q 10- What is ReactDOM?

ReactDOM connects React and the DOM. Often used for mounting with ReactDOM.render (). Another beneficial feature of ReactDOM is ReactDOM.findDOMNode () which you can use to earn direct access to a DOM element. (Should be used moderately in React apps, but it can be necessary.)

Q 11- Difference between ReactDOM and React?

React, and ReactDOM are recently divided into two different libraries. All ReactDOM functionality was part of React in preceding v0.14. This confuses since recent documentation won’t mention the React / ReactDOM distinction. ReactDOM connects React and the DOM. Often used for mounting with ReactDOM.render (). Another beneficial feature of ReactDOM is ReactDOM.findDOMNode () which you can use to earn direct access to a DOM element. (Should be used moderately in React apps, but it can be necessary.) You have to use ReactDOM.renderToString() in your back-end code if your app is “isomorphic”. There’s React for all the other things. React is used to define and create your elements, for lifecycle hooks, etc. i.e. the guts of a React application. The arrival of React Native React and ReactDOM were divided into two libraries. React contains functionality which is used in web and mobile apps. ReactDOM functionality is only used in web apps. 

Q 12- Differences between the class component and functional component?

A functional component is just a standard JavaScript function. 

Class component specifies you to extend from React. This sets more code but will also give you some benefits which you will see later on. Component, create a render function which returns a React element.

Q 13- What are controlled and uncontrolled components in React?

In controlled components via callbacks like onChange takes its current value through props and notifies change. New costs as props are passed to the controlled element by parent components “controls” it by handling the callback and managing its state. It is known as this a “dumb component”.

In uncontrolled components, self-state is stored internally, and you request the DOM using a ref to trace its current value when needed. It is somewhat similar to conventional HTML. Most native React supports both controlled and uncontrolled components.

// Controlled:

// Uncontrolled:

// Use `inputRef.current.value` to read the current value of

Q 14- What are higher order components?

In React for reusing components, a technique is developed, which is a higher-order component (HOC). React API has nothing to do with HOC. HOC is a pattern that results from React’s compositional nature. A higher-order component is a function that escorts a component and returns a new component.

Q 15- What is PureComponent?

PureComponent is identical to Pure Functions in JavaScript. A React component is justified PureComponent if it procures the same output for the same state and props value. React contributes to the PureComponent base class for these class components. React.PureComponent extends class components are treated as pure components.

Q 16- What are the different phases of the React component’s lifecycle?

    A React Component can go through four stages as follows.

  • Initialization 
  • Mounting
  • Updating
  • Unmounting 

Q 17- Explain the lifecycle method of React components in detail?

 A React Component can go through four stages of its life as follows.

Initialization: With the given Props and default state, a component is constructed. This is concluded in the constructor of a Component Class.

Mounting: Rendering of the JSX returned by the render method itself in this stage.

Updating: The State of a component is updated, and the application is renovated in this stage.

Unmounting: The last step of the component lifecycle where the component is removed from the page  

Q 18- What is Redux?

It is a predictable state container for JS Apps. Redux is a small open-source JavaScript library for managing application state. It is a limited and straightforward API designed to be a predictable container for application state. It is executed similarly as the rendering function. The functional programming language Elm has a significant impact on Redux. It is commonly used with libraries such as React or angular for building UI. It is similar to Flux

Q 19- What is Redux Thunx used for?

Redux Thunx middleware that is used to manage the asynchronous actions in Redux.

Q 20- Explain Flux

While working with React, Facebook uses Flux architecture internally. Flux cannot be considered as a library or framework. It is the new architecture that supplements React and the concept of Unidirectional Data Flow. Facebook provides dispatcher library repo. 

Q 21- What principles that Redux follows?

  • State is read-only
  • Single source of truth
  • Changes are made with pure functions

Q 22- Show how the data flows through Redux?

Redux architecture rotates around a stern unidirectional data flow. This indicates the data in an application follows the same life cycle pattern, making the logic of your app more foreseeable and straightforward to understand. To avoid resulting in multiple, independent copies of the same data that are not aware of each other, Redux instigate for data normalization.

Q 23- How is an action defined in Redux?

Actions are a payload of information that works in the transfer of data from the application to your store. It is done with the help of a store using store.dispatch().

Q 24- Explain the role of reducer?

  1.  With the ascend of Redux as State Management Solution, the concept of reducer became famous in JavaScript. You don’t need to master Redux to understand Reducers. Reducers handle state in an application. 

Q 25- Significance of Store in Redux?

In one central location, all of your components are placed with the help of Store in Redux. This makes it available to all components without demanding a parent/child relationship. Store. The store is defined as the central location in which we store the state

Q 26- How is Redux different from Flux?

Flux includes multiple Stores per-app whereas Redux consists of a single Store per-app. This is the main difference between Flux and Redux. Redux places state information in one region of the app, whereas Flux places state information in multiple Stores across the application

Q. 27- What is a React router?

With user action or request, routing helps the user to direct to different pages. To develop Single Page Web Application, ReactJS Router is used. ReactJS Router describes multiple routes in the application. Redirection to a particular route takes place when the user types a specific URL in the browser. On top of the React, a React Router standard library system is built, and by using React Router Package, a routing is created in the React application.

Q 28- Why we need a Router to React?

A key role is played by the React Router to display multiple views in a single page application. In React application, it is impossible to view various views without React Router, react. The router is used for rendering multiple views in most of the social media websites like Facebook, Instagram.

Q 29- Advantages of React Router?

  • Due to standardized structure viewing declarations, you can quickly understand what is viewed by the app
  • Slow code loading
  • Nested views and the progressive resolution of views can be easily managed using React Router
  • Viewing declarations in a standardized structure helps us to understand what are our app views quickly
  • User can navigate backwards/forward and restore the state of the view by using the browsing history feature

Q 30- Advantages of Flux?

  • It is easy to understand the unidirectional data flow.
  • Maintenance is easy.
  • In comparison with the formal framework like MVC architecture, Flux has more design pattern as it is the source

Q 31- List down the components of Redux?

  • Action
  • Reducer
  • Store
  • View

Q 32- Explain the components of Redux

     Redux has the following four components.

  • Action: By issuing an action is the only way to change the state. Actions are the plain JavaScript objects. Actions are the primary source of information used to transfer data from the application to the store. Actions are responsible for providing information to the store. Using store.dispatch() the action sends information to store
  • Reducer: Job of the reducer is to specify how the application’s state changes in response. It receives a callback (reducer) and allows you to get one value out of multiple values, sums of integers, and all this is based on the array reduce method. Take the present state of the application and an action and then return a new state is the function of reducers in Redux. Reducers perform most of the work. 
  • Store: Store provides access to the state, dispatch actions and register listeners. It is a JavaScript that holds the application state. In a single store, the entire state/ object tree of an application is saved. Because of store Redux is more comfortable and straightforward to understand. The store can handle the processing of data and to keep a log of various actions that change the state of stores bypassing the middleware to store. 
  • View: To build the view, smart and dumb components are put together. View main and only purpose is to display the data passed down by the store. The intelligent component takes actions charge. The dumb components which lie underneath the smart components notify them in case they need to trigger the action. The intelligent components, in turn, pass down the props which the dumb components treat as callback actions.

Q 33- What do you mean by “Single source of truth”?

Your application complete state is stored in an object tree within a single store. It makes it easy to create universal apps as the state from the store is sequential, and clients can acquire it without extra coding effect. Debugging of an application becomes more comfortable with a single state tree. It allows you to hold on your app’s state of development, which speeds up the development cycle.

Q 34- What is the difference in state and props?

The dissimilarity between state and props is that props are fixed, whereas the state is changeable. The state can be updated and changed as defined by container components whereas prop is used to pass data from the state by child component.

Q 35- What is the purpose in between ReactNative and React?

The main dissimilarity between React and React Native is that the React is a JavaScript library and based on React ReactNative is a JavaScript framework. In both of the platforms, tags can be used differently. Development of UI and Web Applications React is used, whereas to build cross-platform mobile apps ReactNative is used

Q 36- What is a state in react also how it is used?

In ReactJS component state is like a data store. The state is used to update the component according to user action like clicking the button, typing some text, pressing some key, etc. All class-based ReactJS components have to React—component as a base.

Q 37- How are components and containers different in React Redux?

Part of a React UI is described as a component is a class or function. The container is an unofficial term for a React component which is connected to a Redux store. Containers accept the Redux state and update it and dispatch actions, and there is no need to render DOM elements; they substitute rendering to presentational child components.

Q 38- What is the difference between reacting and containers?

React is a combo of two components that are smart component(containers) and dumb (presentation component). Containers are very identical to components; the only difference is that containers are very conscious of the application state. 

Q 39- How is React different from Angular?

React:

  •   The architecture consists only the view of MVC.

  •   Rendering is done at server-side. 

  •   Virtual DOM is used

  •  Data binding is unidirectional.

  •   Debugging is done while compiling.

  •   Maintained by Facebook

Angular:

  • Its architecture consists of complete MVC.

  • Rendering is done at client-side. 

  • Real DOM is used

  • Data binding is bidirectional.

  • Debugging is done during run time.

  • Maintained by Google

Q 40- How is virtual DOM different from DOM?

Real DOM:

  •   Updates are slow

  •   HTML can be directly updated

  •   DOM needs to create a new element for update

  •   DOM Manipulation is very costly.

  •   Memory wastage is extreme.

Virtual DOM:

  • Updates are fast

  • Direct update of HTML is not possible.

  • If element updates JSX is updated

  • DOM manipulation is simple and therefore not costly as Virtual DOM.

  • There is no memory wastage.

Q 41- How is a state different from Props? 

State:

  •  Parent component cannot change the value.
  • Changes inside the component are possible.
  •  No changes can be done inside child components.

Props:

  • Parent component cannot change the value.
  • Changes cannot be done inside the component.

  • Changes can be done inside child components.

Q 42- How is Redux different from Flux? 

Flux:

  • The store holds state and change logic
  • Multiple stores are present.
  • All the stores are uncoordinated
  • Dispatcher entity is present
  • React components contribute to the store
  • State is variable

Redux:

  • Store and change logic are segregated
  • It consists of only one store.
  • The dispatcher is not present.
  • Container components are used to connect
  • The state is fixed.

Q 43- How is React Router difference from conventional routing?

In Conventional Routing new file correlated with each view whereas, in React routing, only one HTML page is implicated. In Conventional Routing, an HTTP send a request to the server, and a correlated HTML page receives it, and in React Routing only the History attribute is changed. In conventional routing navigation by the user goes through different pages for each view and in React Routing user is deceived into thinking as if they are navigating through other pages

Q 44- Significance of keys in React?

The primary function of keys in React is to identify the specific virtual DOM correlated to data driving UI. Keys are helpful as they minimize the rendering by reprocessing all the current elements in the DOM. Key should have a specific identification like a unique number or string, so that react can just change the aspect instead of again rendering them. Because of this reason, application performance increases.

Q 45- What are some of the cases when you should use Refs?

Refs are used in the following scenarios,

  • To handle focus, select text or media playback it is needed.
  • Needed when you have to prompt necessary animations
  • It combines with third-party DOM libraries

Q 46- What is an event in React?

When particular actions like mouse hover, mouse click, keypress occur, events are triggered. How DOM elements are managed in the same way event managing takes place. Some syntactic differences are there such as 

  1. Using camel case events are labelled instead of using the lowercase.
  2. Strings are not used to pass an event as functions instead of strings.

Set of properties are included in the event argument, which and they are unique to an event. Every event type comprises its properties and behaviour that can be accessed via its event handler only.

Q 47- What are synthetic events in React?

They are objects that act as a cross-browser wrapper around the browser’s native event. In one API, they blend the behaviour of different browsers. Due to this, an event shows dependable properties across other browsers.

Q 48- What did you understand by refs in React?

It means References in React. Ref is an attribute which is used to store a reference to a specific React element or component, that is returned by the components render configuration function. By render() it returns references to a particular element or component. When DOM measurements or to add methods to is needed, it is useful.

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