Sunday, April 21, 2024

React Js Developer Interview Questions

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Use A Function Component With Hooks

React.JS Mock Interview | Interview Questions for Senior React.JS Developers

Using the hooks functionality in React it is possible to use state without using this, which simplifies component implementation and unit testing.

For example:

const SubmitButton =  =>  }> Submit< /button>   )} 

Explain the positives and negatives of shallow rendering components in tests.


  • It is faster to shallow render a component than to fully render it. When a React project contains a large number of components, this performance difference can have a significant impact on the total time taken for unit tests to execute.
  • Shallow rendering prevents testing outside the boundaries of the component being testeda best practice of unit testing.


  • Shallow rendering is less similar to real-world usage of a component as part of an application, so it may not catch certain problems. Take the example of a < House /> component that renders a < LivingRoom /> component. Within a real application, if the < LivingRoom /> component is broken and throws an error, then < House /> would fail to render. However, if the unit tests of < House /> only use shallow rendering, then this issue will not be identified unless < LivingRoom /> is also covered with unit tests.

If you wanted a component to perform an action only once when the component initially renderede.g., make a web analytics callhow would you achieve this with a class component? And how would you achieve it with a function component?

The componentDidMount lifecycle hook can be used with class components:

Explain The Useeffect Hook

The useEffect Hook allows you to perform side effects in your components like data fetching, direct DOM updates, timers like setTimeout, and much more.

This hook accepts two arguments: the callback and the dependencies, which allow you to control when the side effect is executed.

Note: The second argument is optional.


import React,  from 'react' const App =  =>  , 2000)   }, )   return}export default App 

Icipate In Every Stage Of The Development Lifecycle

While not necessarily an expert in every area, an experienced React developer should have some input at every stage of application development. Expressing an opinion in all phases, from initial planning to deployment and support, can prevent issues from arising later. Developers with a great deal of experience in personal projects may even have practical knowledge and experience in every phase.

Asking about side projects during the interview can be a great way to find out what experience a developer has alongside their professional credentials.

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What Do You Understand With The Term Polling

The server needs to be monitored for updates with respect to time. The primary aim in most cases is to check whether novel comments are there or not. This process is basically considered pooling. It checks for updates approximately every 5 seconds. It is possible to change this time period easily. Pooling help keep an eye on the users and always make sure that no negative information is present on the servers. Actually, it can create issues related to several things, and thus pooling is considered.

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How To Create Components In React

34 Javascript Debugging Interview Questions

Components form the fundamental building blocks of a React application. The candidate should be able to explain the two approaches to creating a component in React along with their differences and applications.

React provides two ways to create a component function components and class components.

  • Function Components is the simplest way to create a component. It uses pure JavaScript functions that accept props and returns a React element.
  • functionWelcome `}< /h1> }
  • Class Components, on the other hand, uses ES6 class to define a component. The same function component can be rewritten as the following class component:
  • classWelcomeextendsReact.Component `}< /h1>   }}

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    Sample Advanced Reactjs Interview Questions For Practice

    In this section, weâll look at some additional ReactJS interview questions that you can practice for your upcoming interview.

  • Why do we use Keys in ReactJS lists?
  • How can the transversal of lists be achieved in ReactJS?
  • What do you understand about components in React?
  • Explain the two types of Components in ReactJS?
  • What are Synthetic events in React?
  • What are Pure Components in ReactJS?
  • What are some important features of Redux?
  • What do you understand about Flux in React?
  • Why are Routers used in ReactJS?
  • What are some disadvantages associated with the ReactJS framework?
  • Why canât browsers read JSX? How can browsers be made to read JSX?
  • What do you understand about Higher-Order Components in ReactJS?
  • What do you understand by References in ReactJS?
  • What are Props in ReactJS?
  • How do you modularize code in the ReactJS framework?
  • What is the use of Webpack in React?
  • What is Babel in ReactJS?
  • What is Prop-drilling in ReactJS?
  • What are Error boundaries in ReactJS?
  • What do you understand about the Strict mode in React?
  • These above advanced ReactJS interview questions will help you understand the type of questions asked in your Java developer interview, especially if you are a front-end developer.

    React Interview Questions 2022 For Mid

    If you are looking to hire mid-level or intermediate developers, be on the lookout for the following skills in your react JS developer job description:

    • They should have at least 3 to 4 years of experience
    • They should be able to execute routine tasks with greater skills and efficiency
    • They should be able to mentor junior developers

    Moving on, lets look at React JS interview questions for intermediate developers.

    Q1. What are the advantages and limitations of using ReactJS?

    Why asking this question is crucial for hiring managers: This question will help hiring managers to understand the candidates ReactJS knowledge.

    Key points an interviewee should include in their response:


    • JSX and inline templating make coding complex
    • Boasts of a large community and is more SEO-friendly
    • Developers must possess a different kind of learning curve and thinking to be able to use React

    Q2. What is Virtual DOM, and why is it important?

    Why asking this question is crucial for hiring managers: This question will help hiring managers to assess the candidates expert-level ReactJS knowledge.

    Key points an interviewee should include in their response:

    • As one of the key concepts in React, the candidate must be able to explain the main problems with DOM manipulation and how virtual DOM can help

    Q3. What is JSX, and how does it relate to ReactJS? Provide an example.

    Key points an interviewee should include in their response:

    Q4. What do components refer to in React?

    Q5. What does render do?

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    What Interviewer Wants To See: Practical Usage Of Hooks And Understanding Of How It Works

    Despite hooks are still new API, a lot of people already use it in production and they expect you to know it.

    Lets recreate a useWindowSize its easy-to-read hook and quite straightforward:

    import  from 'react' const useWindowSize =  =>  {  const getSize =  =>  ({

    You might expect some questions like:

    1. Is that necessary to call your function useWindowSize , what about just getWindowSize?

    Yes, without it, we wouldnt be able to automatically check for violations of rules of Hooks because we couldnt tell if a certain function contains calls to Hooks inside of it.

    2. Will it work if we remove argument from useEffect?

    Yes, but well call useEffect hook on every render which may cause performance issues.

    3. How React knows when to re-render App component if we handle window resizing in useWindowSize?

    When you call setSize inside the custom hooks, React knows that this hook is used in App component and will re-render it.

    There is some portion of magic with React hooks. Check out Why Do React Hooks Rely on Call Order? article for more info.

    4. How to make this hook ready for Server-Side Rendering?

    Something like this:

    import  from 'react' const useWindowSize =  =>  

    What Are The Features Of React

    Frontend Interview Experience (Cars24) – Javascript and React JS Interview Questions

    JSX: JSX is a syntax extension to JavaScript. It is used with React to describe what the user interface should look like. By using JSX, we can write HTML structures in the same file that contains JavaScript code.

    Components: Components are the building blocks of any React application, and a single app usually consists of multiple components. It splits the user interface into independent, reusable parts that can be processed separately.

    Virtual DOM: React keeps a lightweight representation of the real DOM in the memory, and that is known as the virtual DOM. When the state of an object changes, virtual DOM changes only that object in the real DOM, rather than updating all the objects.

    One-way data-binding: Reacts one-way data binding keeps everything modular and fast. A unidirectional data flow means that when designing a React app, you often nest child components within parent components.

    High performance: React updates only those components that have changed, rather than updating all the components at once. This results in much faster web applications.

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    What Are The Different Phases Of React Components Lifecycle

    • Initialization: This is the stage where the component is constructed with the given Props and default state. This is done in the constructor of a Component Class.
    • Mounting: Mounting is the stage of rendering the JSX returned by the render method itself.
    • Updating: Updating is the stage when the state of a component is updated and the application is repainted.
    • Unmounting: As the name suggests Unmounting is the final step of the component lifecycle where the component is removed from the page.

    What Is A Higher

    Higher-order components are a widely used technique in React for applying concepts that involve the component reusability logic. They are not a native part of the React API and allow users to easily reuse the code and bootstrap abstraction.

    HOCs are also used to allow simple sharing of behaviors across all of the components in React, adding more advances to the efficiency and functioning of the application.

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    Is It Possible To Nest Jsx Elements Into Other Jsx Elements

    It is possible. The process is quite similar to that of nesting the HTML elements. However, there are certain things that are different in this. You must be familiar with the source and destination elements to perform this task simply.

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    Why Do We Need Keys For React Lists

    Setting up react js development environment using Create React App ...

    Keys are a unique value that we must pass to the key prop when we are using the .map function to loop over an element or a component.

    If we are mapping over an element, it would look like this:

    Or like this if we are mapping over a component:

    And in both case, we need to add a key that is a unique value, otherwise React will warn us.

    Why? Because keys tell React which element or component is which in a list.

    Otherwise, if we were to try to change items in this list by inserting more or editing them in some way, React wouldnât know the order to put them in.

    This is because React takes care of all of the business of updating the DOM for us , but keys are necessary for React to update it properly.

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    What Is The Difference Between Http2 And Http

    What answer to expect:

    HTTP, which stands for Hypertext Transfer Protocol, is the foundation of data communication for the World Wide Web. HTTP has existed since the very dawn of the Internet, but five years ago, it was revolutionized, and HTTP2 was introduced. The goals of HTTP2 include a protocol negotiation mechanism, improved page load speed, compressed request headers, request multiplexing, request pipelining, Head-of-line blocking, etc..

    What Will Happen If You Use Props In Initial State

    If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.

    The below component won’t display the updated input value:

    classMyComponentextendsReact.Component }render< /div> }}

    Using props inside render method will update the value:

    classMyComponentextendsReact.Component}render< /div> }}

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    What Are Keys In React

    React needs keys to be able to identify which elements were changed, added, or removed. Each item in an array needs to have a key that provides a stable identity.

    It’s not recommended to use indexes for keys if the order of items may change as it can have a negative impact on the performance and may cause state issues. React will use indexes as keys if you do not assign an explicit key to list items.

    Start Creating Your List Of Advanced Reactjs Interview Questions To Hire Top Talent

    Frontend Interview Experience (Unacademy) – Javascript and React JS Interview Questions

    With this list of advanced React.js interview questions, hiring top talent will be a stress-free process. Use the questions you require and build your own list to evaluate your candidates.

    Remember that skills testing is an essential part of the interview process that can make hiring easier.

    Build and distribute skills assessments before the interview to hire the best talent for your organization. To accomplish this, you can use TestGorillas skills-testing platform, which has the most diverse range of skills tests.

    Nothings stopping you now. Hire an exceptional developer with skills assessments and advanced React.js interview questions.

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    What Are The Rules You Should Follow For The Hooks In React

    We have to follow the following two rules to use hooks in React:

    • You should call hooks only at the top level of your React functions and not inside the loops, conditions, or nested functions. This is used to ensure that hooks are called in the same order each time a component renders, and it also preserves the state of hooks between multiple useState and useEffect calls.
    • You should call hooks from React functions only. Don’t call hooks from regular JavaScript functions.

    Advanced Behavioral Reactjs Interview Questions

    This section includes eight advanced behavioral React.js interview questions you can ask to find out which methods your candidates use when working on React.js projects.

  • Which method would you use to handle events in React?
  • In which situation would you use refs in React?
  • Why would you use super constructors with props arguments?
  • How would you use validation on props?
  • Which method would you use to add attributes to components conditionally?
  • What methods would you use to check and improve slow app rendering in React?
  • In which situation would you use useMemo in React?
  • How would you avoid binding in React?
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    What Is The Meaning Of Redux

    Redux is used to store the state of the application in a single entity. This simple entity is usually a JavaScript object. Changing states can be done by pushing out actions from the application and writing corresponding objects for them that are used to modify the states.

    For example:


    All of the data is retained by Redux .

    Familiarity With Controlled Components

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    ReactJS developers should know the concept of controlled components very well. These are specific types of input form elements. In the case of these elements, a ReactJS component would render the form. It also controls the events in the form upon receiving the next user input.

    The React state will drive the value of the input. ReactJS developers should know that they need to write more lines of code to use controlled components. They should know the advantages though, i.e., passing the values to other UI elements.

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    What Are Error Boundaries In React V16

    Error boundaries are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.

    A class component becomes an error boundary if it defines a new lifecycle method called componentDidCatch or static getDerivedStateFromError :

    classErrorBoundaryextendsReact.Component}componentDidCatchstaticgetDerivedStateFromError }render< /h1> }returnthis.props.children}}

    After that use it as a regular component:

    < ErrorBoundary> < MyWidget/> < /ErrorBoundary> 

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    Why Function Is Preferred Over Object For Setstate

    React may batch multiple setState calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

    This counter example will fail to update as expected:

    // Wrongthis.setState

    The preferred approach is to call setState with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.

    // Correctthis.setState=> )

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    Is It Possible To Use The Word Class In Jsx Why Or Why Not

    No, it is not possible in the JSX. This is because the word âClassâ is a reticent word in JavaScript. However, you can use you are free to use the word âClassNameâ. If you use the word âClassâ the JSX will be translated to JavaScript immediately.

    a) High-Level Component Lifecycle:

    At the highest level, React components have lifecycle events that fall into 3 general classifications:

  • State/Property Updates
  • Each React component defines these events as a system for dealing with its properties, state, and rendered output. Some of these events just happen once, others happen more as often as possible understanding these 3 general classes should help you clearly visualize when certain logic required to be applied.

    For instance, a component may need to add an event audience to the DOM when it initially mounts. In any case, it ought to likely expel those event listeners when the component unmounts from the DOM with the goal that not relevant handling doesn’t occur.

    class MyComponent extends React.Component//when the component is removed from the DOM...componentWillmountonResizeHandler}

    b) Low-Level Component Lifecycle:

    Inside these 3 general buckets exist various particular lifecycle hooks â basically unique techniques – that can be used by any React component to all the more precisely manage updates. Seeing how and when these hooks fire is vital to building stable components and will empower you to control the rendering procedure .

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