Top 50 React Interview Questions and Answers

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Do you want to crack any react profile job interviews? If yes, You should know these Best React Interview Questions and answers for the React position. Keep in mind that even though React's job market is booming and many companies are hiring React Developers, you will still have to compete for this job.

Nowadays, React is one of the fastest-growing JavaScript frameworks in the market. If you want to become a good front-end developer, we have rolled up some important React interview questions and answers that will help you with interview preparation. Also,Check out our React JS course to become a master in the front-end field.So, here are the top React interview questions.

1. What is React?

   React is a JavaScript-based library that is used to create a SPA (Single Page Application) and different User Interfaces. React is developed and maintained by Facebook and other contributors. By using React, we can develop web and mobile applications by just following component-based architecture, and by using React we can get robust, scalable, and high-performance applications.

   React makes the task easier to create dynamic web applications because it requires less coding and offers more functionality along with quick application configuration, unlike opposed to JavaScript, where the coding often gets complex very quickly over a period of time.

2. What is Virtual DOM in React?

   DOM plays an important role in terms of web application performance, thus there are tons of frameworks available that manipulate the DOM directly by adding, modifying, or removing any existing element. When we perform any action on a page, at that time content should be updated based on the result of the action and it should be reflected in the DOM. In React, Virtual DOM plays an important role by updating DOM which is marked as dirty. React’s Virtual DOM converts the dirty component into a Virtual image of a component and compares it with the existing DOM, and based on the result of the comparison, the new DOM will be replaced by the existing DOM.

   The concept of a virtual DOM comes in and performs significantly better than the real or the actual browser DOM. The virtual DOM in React is only a virtual representation of the DOM which means every time the state of our application changes or any element in the DOM should be manipulated, the virtual DOM gets updated instead of the real DOM and this is how rendering will be better.

   For the execution of Virtual DOM in React, the diffing algorithm plays an important role in watching for the changes and does it requires a re-render of that section or not.

   More:React Courses: Learn React Step By Step

3. What is the difference between Real DOM and Virtual DOM?

   When we change any action in an application, at that time real DOM should be updated, in this case, it would be expected that all that conversion tasks should be faster to achieve higher performance. Basically, a DOM or Real DOM is a structured collection of abstracted elements and the Virtual DOM is the abstraction of HTML DOM. Real DOM is the structure of the existing elements, whereas Virtual DOM is the virtual tree of our application with the recent changes we have done. At the end of the comparison between Real DOM and Virtual DOM, the newly updated DOM will be rendered into the browser.

4. What is a Diffing Algorithm?

   Diffing Algorithm is used to compare different DOM trees, this algorithm compares the two different root elements, and based on the different results, it mounts the new tree and unmounts the old one. For example, we have an element inside <div>, and below is the old and new version of the change.

   
    // Old
    <div id="mydiv">
    <img src="" />
    </div>
    
    // New
    <div id="mydiv">
    <a href="">Click Me</a>
    </div>
    

   Here is an above code snippet of two different versions of a <div>, we have changed <img> control to <a> element, now React will tear down the old tree and build the new tree from scratch with the updated changes. And to keep in mind that the child element of the root element will also be unmounted.

5. What are the Hooks in React?

   As per the official definition, React Hooks means.“Hooks lets you use state and other React features without writing a class. You can also build your own Hooksto share reusable stateful logic between components” By using Hooks, we can share state using function component which acts as isolated reusable units which means now we don’t need to use class components for using state. One of the updates in Hooks is that we can write the state and update it even within a single line of code. Apart from using state in the function component, we can also access different lifecycle hooks using useEffect.

   With the release of React 16.7, multiple hooks functions were added to use stateful component features and state into the functional components that were not possible before the release of the React hooks.

6. What are the advantages of using create-react-app CLI?

   Creating a project of React is a very straightforward process and for that, we just need to install the create-react-app package. But why do we need to use it, there are a few advantages to using create-react-app which are listed below.

   • One line command to set up the React web app.

   • No need to mug up all the configurations, because it provides a way to set up and minimize the production-ready app.

   • Application versioning can be well maintained.

   • It comes with great package support like ESLint, Babel, Webpack, and so on.

   • Upgrading the existing app to the latest version is less painful

7. What is JSX?

   JSX stands for “JavaScript XML”.JSX is a syntax extension of JavaScript, in other words, we can say JSX is a way to use JavaScript along with the HTML elements.

   The browser does not know that this is a JSX syntax because it's not valid JavaScript code but it's an extended version of JavaScript with XML. It is happening because we are assigning an HTML tag to a variable or various JavaScript expressions that are not a string but just an HTML code. So to convert the JSX-based codebase to the browser-understandable JavaScript code, we can use tools such as Babel which is a JavaScript compiler/transpiler.

   It can be used as a template engine but it’s not necessary to use it compulsorily thus it has some advantages to use along with React which are listed below.

   • Faster in rendering and performance

   • Can create a reusable component using JSX syntax

   • It will be very easy to create an HTML structure by using JSX

8. Why we should use JSX?

   Using JavaScript with XML makes it more useful while developing a web application using React. While using React for web development, we need to perform various tasks along with the UI like events, DOM manipulation, changes of states and props, managing reusable components,s and so on. By using JSX, we can embed code of JavaScript with HTML easily which allows us to manage errors, inline expressions, conditions, etc. Using Component, we can have HTML markup and business logic in a single file so that it can be a more manageable component in order to update anything. And of the main advantage is that JSX can be easily used by just using curly braces { } in our JSX file and we are good to go.

9. How does JSX work behind the scenes?

   JSX syntax is different than JavaScript, but while we write anything in JSX, at last, it will be converted into JavaScript using Babel. When we create any component, at that time every element in the component will be transpired into a React.createElement () call. It means that every element should be converted into the React element before rendering, for example, we have a simple <h1> element in our component like this.

   
   render() {
    return (
    <div>
    <h1>Hello World</h1>
    </div>
   
    );
    }
   

   Here we have <h1> element inside the parent <div> element, but <h1> element will be converted using React.createElement() like this.

    React.createElement("h1", {}, "Hello, World");
   

10. How to use custom JSX elements?

    We can create the component using JSX, the same way we can also render a reusable JSX element as a child element into the JSX file. For that, we just need to import the same component and can use it inside the parent JSX element as explained in the below example.

     render() {
     return (
     <div>
     // JSX Child file 1
     <Header />
     // JSX Child file 2
     <Content />
     // JSX Child file 3
     <Footer />
     </div>
     );
     }
    

    Here in this example, we have one parent element <div> and inside the parent element, we have three different elements which is basically a JSX element that resides at a different location. So, when we run our application, our child JSX element will be rendered in a particular order.

11. What is a Component?

    Component nowadays is a booming world, but we may have a question like what is Component. The component is a reusable, independent piece of code that contains the JavaScript and HTML component data. It can be exported from one place to another and using the import statement, we can access its various functionalities developed in the component file. Basically, the component acts as a part of a big application, and the component has to serve the specific functionality wherever it’s being used, one of the important advantages of using a component is that we can create a loosely coupled system by dividing the application module as chunk which called as “Component”.

    In React, the components can be architected according to the application requirement hence, creating reusable components makes it even better for getting better application maintenance and usability while developing it.

12. How to create a Component in React?

    In React, the component just returns the JSX code which is used to render something in the DOM. Creating a component is a very straightforward thing, for that, we just need to create a “.js” file and the code should look like this.

     import React from 'react';
     
     class Demo extends React.Component {
     render() {
     return (
     <div>
     <h1>Component in React</h1>
     </div>
     );
     }
     }
     
     export default Demo;
    

    As you can see we have an import statement where we have used ‘react’ which means we are going to use the react package for this component and its element “Component”.One thing to be noticed here is that we have used “React. Component” because we are going to use a Component from the React package itself. We cannot skip the “render ()” method which is used to render the JSX/HTML content from the current component. The last statement is to export the current component so that any component can access the current component by its class name.

    Other than the class based on stateful component, we can also create the functional function using the "function" followed by the function name which is similar to the JavaScript function that accepts multiple arguments as given below.

    function App(props) { 
     // Use props values here
     return (
     // return html content as required 
     )
    }

13. How many types of Components can be created using React?

    React primarily supports two types of Component structures which are listed below.

    • Stateful/Class-based Component

    • Stateless/Function-based Component

14. What is a Stateful/Class-based Component in React?

    A stateful Component or Class-based component can be created using ES6 class which is one of the features of ES6. Basically stateful component in React contains the internal state object which is used to manage component-level data for rendering purposes. Being a Class-based component, we can access different components from each other because there is communication possible between two components by passing the state from one to another. So whenever the state of the class is changed, at that time the render () method also re-renders its JSX/HTML content. Below is a simple example of a Stateful Component.

     import React from 'react';
     
     class Demo extends React.Component {
     constructor() {
     super();
     this.state = {
     message: "This is Stateful Component !!!"
     }
     }
     
     render() {
     return (
     <div>
     <h1>Hello {this.state.message}</h1>
     </div>
     );
     }
     }
     
     export default Demo;
    

    As you can see in the above example, we have used a class that is part of ES6, and apart from that, we have a constructor () which is again the functionality of an ES6. One thing to be noticed is that inside the constructor, we have used “this.state” which is a component-level state management mechanism to populate data while rendering. We can manipulate the state object whenever we want for the current component.

15. What is a Stateless/Functional Component in React?

    A stateless or Functional component does not contain the state object and it follows purely a functional approach while creating the component. It is a kind of simple function-based component that contains the “container” part, it means when we want to show data based on the request but don’t require data to be changed then the function component will be used. For example, we have data of 10 employees and we just need to show them as a list, then we can create a functional component and can return the list of the employees along with the design part. Keep in mind that a functional component cannot maintain an internal state object unlike a Stateful component, but it can use props in order to populate data in a webpage. Below is a simple example of the Stateless component.

     const functionalComponent = (props) => {
     <h1>Hello World !!!</h1>;
     }
     
     export default functionalComponent;
    

    Here in the above example, we have a simple function called “functional component” along with the props as an argument, and this function just returns the static message. We can use the function-based component while we want to use only rendering logic.

    With the latest version of React, function components are not more limited to just rendering the logic but it has now all capabilities to use state and lifecycle methods as we do in stateful components and that is only possible using React hooks.

16. What is the higher-order component in React?

    As per the official definition of HOC is. “A higher-order component (HOC) is an advanced technique to React for reusing component logic” In other words, we can say that it accepts the component and returns the new processed component. HOC is not a part of React’s Component API, it is a kind of third-party component where we can use external component functionality by just importing it. The syntax of the HOC will look like this.

     const newComponent = myhoc(thisComponent);
    

17. What is a State in React?

    The State in React is an instance of the React Component Class and it can be defined as an object of a set of observable properties that control the behavior of the component by mutating its value. The state in React is the heart of the application, which is used to maintain the component-level data as well as the global state for the whole application. The object of the State is mutable which means we can update the state value over a period of time. The important thing to be noted is that the values of the State can be passed from the parent component to a child for the rendering purpose and the parent component can update the state of the child.

    The component state can be manipulated from the component itself but to manage the global state object across the application, we may use other ways such as context API, Redux, or similar libraries that manage the store and global state object that can be manipulated while interacting with the components.

18. What are Props in React?

    In React, Props and State are the heart of every application, and Props is the same as State but there is some difference between them. Props in React is a way to pass the data to the Component which is exactly like the properties that contain a single value or object as a value. One of the important points about the Props is that it is “immutable”, which means we cannot update it from the component anyhow, but we can make use of it for the rendering purpose.

    When we pass any data such as state values or other values to the child component for the child component, those values act as a prop and it can be used by the child component by using "this.props.prop_name" while working with the React component.

19. How many types of Form input are supported?

    Forms in React are completely different than other libraries or frameworks, here In React, we need to manage everything from scratch like creating forms, validations, and so on. There are two types of Form input that are supported which are listed below.

    1. Controlled form inputs

    2. Uncontrolled form inputs

20. What is react-router-dom?

    The react-router-dom is a library that is used to implement web app-based routing functionality. Previously it was called “react-router” but now it’s split into three different packages which are listed below.

    1. react-router-dom:Used for implementing web application-based routing.

    2. react-router-native:Used for implementing routing only for mobile applications (i.e. React Native)

    3. react-router-core(deprecated):It is used to implement the core routing mechanism anywhere for the core React application

    “Everything is a component in React.” Do you agree?

    The UI of a React application is made up of building blocks called components. The components break the entire user interface into independent, reusable pieces. These different fragments are then rendered independently of the rest of the UI.

    What is the significance of Reducers?

    It specifies how the application’s state would change in response to specific actions. Based on the type of activity, reducers analyze what updates are required and then return new values. They return to the same previous state if no change is necessary.

    What is a React Router?

    The React Router is a routing library that helps add new screens and flows to JavaScript applications. We add this library to our app to create multiple routes, each leading to a unique page. The URL matches what is being displayed on the web page.

    How to render an array of elements?

    To render an array of elements, you can use the map() method to iterate over the array and return a new array of React elements.

    What is the difference between controlled and uncontrolled components?

    The difference between controlled and uncontrolled components depends on how they manage and update their state.

    Controlled components: These are components where the state is controlled by React. The component gets its current value and updates it through props. Actually, It also triggers a callback function when the value changes which means the component doesn't store its own internal state. Instead of it the parent component manages and passes the value down to the controlled component.

    Uncontrolled components: Uncontrolled components, manage their own state internally using refs or other methods. They always store and update their state independently, without relying on props or callbacks. The parent component has very little control over the state of uncontrolled components.

    How are forms created in React?

    Forms allow the users to interact with the application as well as gather information from the users. Forms can perform many tasks such as user authentication, adding users, searching, filtering, etc. A form can contain text fields, buttons, checkboxes, radio buttons, etc.

    What are the different phases of the React component's lifecycle?

    The different phases of the React component's lifecycle are:

    1. Initial Phase: It is the birth phase of the React lifecycle when the component starts its journey on its way to the DOM. In this phase, a component contains the default Props and initial State. These default properties are done in the constructor of a component.

    2. Mounting Phase: In this phase, the instance of a component is created and added to the DOM.

    3. Updating Phase: It is the next phase of the React lifecycle. In this phase, we get new Props and change the State. This phase can potentially update and re-render only when a prop or state change occurs. The main aim of this phase is to ensure that the component is displaying the latest version of itself. This phase repeats again and again.

    4. Unmounting Phase: It is the final phase of the React lifecycle, where the component instance is destroyed and unmounted(removed) from the DOM.

    What are Pure Components?

21. Pure components are introduced in the React 15.3 version. The React.Component and React.PureComponent differs in the shouldComponentUpdate() React lifecycle method. This method decides the re-rendering of the component by returning a boolean value (true or false). In React. Component.The pure component enhances the simplicity of the code and the performance of the application.

    What are Higher Order Components(HOC)?

    The higher-order component is an advanced technique for reusing component logic. It is a particular function that takes a component and returns a new component. In other words, it is a function that accepts another function as an argument.

    What can you do with HOC?

    You can do many tasks with HOC Such asCode Reusability,Props manipulation,State manipulation, andRender highjacking.

    Why are fragments better than container divs?

    It is faster and consumes less memory because it does not create an extra DOM node.

    When do we prefer to use a class component over a function component?

    If a component needs state methods, you should use the class component; otherwise, use the function component. However, after React 16.8, with the addition of Hooks, we could use state, lifecycle methods, and other features that were only available in the class component right in our function component.

    Is it possible for a web browser to read JSX directly?

    Generally, Web browsers can't read JSX directly. This is just because the web browsers are built to read the regular JS objects only, and JSX is not a regular JavaScript object.

    What do you understand by props in React?

    The props are input to components. Props are single values or objects containing a set of values passed to components on creation using a naming convention similar to HTML tag attributes. They are data passed down from a parent component to a child component.

    What is the main purpose of props in React?

    1. Pass custom data to your component.

    2. Trigger state changes.

    3. Use via this.props.reactProp inside component's render() method.

    List down the advantages of React Router.

    1. In this, it is not necessary to set the browser history manually

22. 2. Link is used to navigate the internal links in the application. It is similar to the anchor tag.

23. 3. It uses the Switch feature for rendering.

24. 4. The Router needs only a Single Child element.

25. 5. In this, every component is specified in <Route>.

26. 6. The packages are split into three packages, which are Web, Native, and Core. It supports the compact size of the React application.

    What were the major problems with the MVC framework?

    The major problems with the MVC framework are:

27. 1. DOM manipulation was very expensive.

28. 2. It makes the application slow and inefficient.

29. 3. There was a huge memory wastage.

30. 4. It makes the application debugging hard.

    When Did We Begin to Use Hooks in React?

    It first introduced React Hooks to the world in late October 2018, during React Conf, and then in early February 2019, with React v16. 8.0.

    How to create elements in a loop in React?

    The Applications usually involve displaying a dynamic set of components based on an array or object. The student or candidate should be able to explain what a v-for directive is along with their applications. Let's useJavaScript’s map function to loop through an array or object – rendering the returned template for each entry.

    What are portals in React?

    It is React’s recommended way to render children components into a DOM node that exists outside the DOM hierarchy of the parent component.

    What are error boundaries?

    These are React components that catch JavaScript errors anywhere in its child component tree. The Error Boundaries component can then log those errors and display a fallback UI instead of crashing the entire component tree. We could think of error boundaries as a catch block for components.

    What is React Profiler and what is it used for?

    It is a means to measure the cost of rendering in a React application. The purpose of this is to help developers identify parts of the application that are slow and may benefit from further optimizations.

    What is StrictMode in React?

    It is a tool to highlight potential problems in an application. StrictMode is used as a component, it doesn’t create a visible UI in the DOM. It enables additional checks for its descendants.

    How often does the React useState update? Why?

    Since Programmers use useState to enhance performance by creating queues, React doesn’t update changes Quickly. The coder should know that useState doesn’t implement changes to the state object directly; instead, the updates occur asynchronously.

    In which situation would you use useMemo() in React?

    Developers can use useMemo() to cache a variable’s value along with dependency lists. They would use useMemo() to help them avoid unnecessary re-renders. The useMemo() can be useful in situations where there are high processing amounts.

    Why would you use super constructors with props arguments?

    The programmer passes props to super constructors to access and use this. props in the constructor. They can mention that when they implement a constructor() function within a React component, they use super() to call the parent constructor.

    How would you avoid binding in React?

    Developers who have advanced React skills should be aware that they can use arrow functions in class properties to avoid binding in React.

    Which method would you use to handle events in React?

    We can use JSX, which passes a function as the event handler (instead of a string).

    Does React Use HTML?

    No, the front-end library does not use HTML. Instead, it uses JSX which is quite identical to HTML.

    How Can You Handle Errors With React?

    We can use Error Boundaries for handling errors with React.

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