ReactJS Interview Questions Part-1

What are the benefits of using TypeScript with ReactJS?

Using TypeScript with ReactJS provides several key benefits:

1. Type Safety: TypeScript adds static typing to JavaScript, which helps catch errors and bugs early in the development process. With TypeScript, you can define types for props, state, and function parameters, ensuring that your code is more robust and less prone to runtime errors.

2. Improved Developer Experience: TypeScript provides excellent tooling support, including autocompletion, type checking, and intelligent code suggestions. This greatly enhances the developer experience by reducing debugging time, improving code readability, and making it easier to refactor code.

3. Scalability: As ReactJS applications grow larger and more complex, maintaining code quality becomes increasingly challenging. TypeScript helps mitigate these challenges by enabling clear and self-documenting APIs, making it easier for developers to understand and navigate the codebase. Additionally, TypeScript’s strong typing helps prevent subtle issues when working with multiple contributors or integrating external libraries.

4. Enhanced Refactoring: Refactoring is an integral part of software development. Adding TypeScript to ReactJS projects makes refactoring code significantly easier, as the compiler can identify where changes need to be made across the codebase. This helps prevent potential bugs and reduces the risk of introducing regressions during refactoring.

5. Better Collaboration: TypeScript’s static typing provides a common language and shared understanding of code across teams, making collaboration and code reviews more efficient. It increases the maintainability of a codebase and reduces the learning curve for new team members.

6. Ecosystem and Tooling: TypeScript has a rich ecosystem of libraries and tooling, including popular development tools like ESLint, Prettier, and Jest, which have excellent TypeScript support. This ecosystem ensures that you have access to a wide range of libraries and tools to enhance your ReactJS development experience.

Overall, using TypeScript with ReactJS offers numerous advantages, including increased code quality, improved developer productivity, better scalability, and enhanced collaboration among team members.

What are the possible return types of render method in ReactJS?

In ReactJS, the render method is responsible for rendering the component’s output, which will be displayed on the screen. The return type of the render method can be one of the following:

1. JSX (JavaScript XML): JSX is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. The render method can return JSX elements, which will be transformed into regular JavaScript functions and objects by the JSX compiler before being rendered to the DOM.

2. React Fragments: Sometimes, you may need to return multiple elements from the render method without adding any extra wrappers ,in such cases, you can use React Fragments. Fragments let you group a list of children elements without adding unnecessary markup to the DOM. Fragments are also useful when you want to return an array of elements from the render method.

3. Strings or numbers: Although not commonly used, the render method can also return plain strings or numbers. However, keep in mind that these values will be treated as text content and won’t be able to have dynamic behavior or interactivity like JSX elements.

4. Portals: Portals provide a way to render children components into a different DOM node outside of the parent component’s hierarchy. The render method can return a Portal, which can render its children into a different location in the DOM.

It’s important to note that the render method should always return a single root element. However, this root element can have any number of nested child elements.

What are Pure Components in ReactJS?

Pure Components in ReactJS are a type of components that are optimized for performance by reducing unnecessary re-renders. In React, when a component’s state or props change, it triggers a re-render by calling the render() method. However, sometimes the component doesn’t need to be re-rendered if its state or props haven’t actually changed.

Pure Components solve this problem by implementing a shouldComponentUpdate() lifecycle method with a default shallow comparison of the component’s props and state. This method determines whether the component should re-render or not. If the props and state have the same values as before, React skips the re-rendering process, saving valuable computing resources.

To create a pure component, you can extend the React.PureComponent class instead of the usual React.Component class. The class-based pure component automatically performs the shallow comparison in shouldComponentUpdate() for you. 

For functional components, React provides a higher-order component called React.memo() to achieve the same behavior. It’s used as a wrapper around a functional component and performs the shallow comparison of props.

Using pure components can often lead to significant performance improvements, especially if your components have large render trees or heavy computations inside render()

However, it’s important to note that pure components rely on shallow comparisons, so they may not catch every case where a component should update. In such cases, you might still need to manually optimize the component or use other techniques like memoization or shouldComponentUpdate overrides.

What are loadable components in ReactJS?

In ReactJS, loadable components are a way to implement code splitting, which allows you to load and render components asynchronously. This helps in reducing the initial bundle size of your application and improves its performance.

Loadable components are used to dynamically import components only when they are needed, rather than including them in the main bundle. This can be especially useful for large applications with many components, where loading all components upfront would result in a slower initial load time.

To use loadable components, you first need to install the necessary package. You can do this by running the following command:

npm install @loadable/component

Once you have installed the package, you can create loadable components using the loadable function provided by the package. Here’s an example of how you can use it:

import Loadable from '@loadable/component';
const MyComponent = Loadable(() => import('./MyComponent'));

function App() {
  return (
      My App  

In the above example, the MyComponent is dynamically imported only when it is rendered in the App component. The @loadable/component package takes care of loading the component asynchronously and rendering it once it is loaded.

Loadable components also provide additional features like handling loading and error states, preloading components in the background, and server-side rendering support.

By using loadable components, you can optimize the performance of your ReactJS application by reducing the initial load time and improving the user experience.

What is the difference between React and React DOM?

React and React DOM are both libraries developed by Facebook for building user interfaces. While they are closely related, there are some key differences between the two.

React is a JavaScript library that provides a way to create reusable and composable UI components. It allows developers to build applications using a declarative syntax, where the UI is described in terms of the desired output and React handles the underlying updates efficiently. It uses a virtual DOM (Document Object Model) to efficiently update only the parts of the actual DOM that have changed, making UI updates fast and efficient. React provides the core functionality for building UI components, managing state, and handling events.

On the other hand, React DOM is a separate package specifically designed for rendering React components into the browser’s DOM (Document Object Model). It provides the necessary tools and APIs to interact with the browser environment. React DOM is responsible for taking the React components and creating the corresponding HTML elements that can be injected into the web page. It handles the process of updating the actual DOM to reflect the changes made to React components.

In summary, React is the core library for building UI components and managing their behavior, while React DOM is responsible for rendering these components into the browser DOM. They work together to provide an efficient and powerful way to build interactive user interfaces in web applications.

How to fetch data with React Hooks?

To fetch data with React Hooks, you can use the useEffect hook in combination with the fetch API or any other HTTP library like Axios. 

Here’s a step-by-step guide on how to do it:

1. Import the necessary dependencies:

<code>import React, { useState, useEffect } from 'react';</code>

2. Create a functional component:

function MyComponent() {
  // State to store the fetched data
  const [data, setData] = useState(null);  // Effect hook to fetch data
  useEffect(() => {
    // Function to make the API request
    async function fetchData() {
      try {
        const response = await fetch('https://api.example.com/data');
        const data = await response.json();
      } catch (error) {
        console.error('Error:', error);
    }    fetchData(); // Call the function
  }, []);
  // JSX code to render the component
  return (
      {/* Check if data is loaded */}
      {data ? (
      ) : (

In this example, we define a functional component called MyComponent. We use the useState hook to create a state variable called data to store the fetched data. The useEffect hook is used to fetch the data when the component mounts, by making an API request to ‘https://api.example.com/data’. Once the data is fetched, we update the state using the setData function. Finally, we render the fetched data or a loading message based on the state.

Remember to replace ‘https://api.example.com/data’ with the actual API endpoint you want to fetch data from.

How do you set default value for uncontrolled component in ReactJS?

In ReactJS, to set a default value for an uncontrolled component, you can use the defaultValue attribute. For example, let’s say you have an input element that you want to set a default value for:

<input type="text" />

In this case, the initial value of the input field will be set to “Default value” when the component renders. However, keep in mind that once the component has rendered, any changes to the input value will not be reflected in the component state.

On the other hand, if you need to achieve a controlled component behavior, where the value of the component is managed by state, you can use the value attribute instead of defaultValue. This way, the value of the input field will be bound to the component’s state, enabling you to update it.Here’s an example of how that can be done:

<code>import { useState } from 'react';function InputComponent() {
  const [value, setValue] = useState('Default value');  const handleChange = (event) => {
  };  return (

With this approach, the initial value of the input field will be set to “Default value” and can be updated by the user, while keeping the component’s state in sync.

What are the Pointer Events supported in ReactJS?

In ReactJS, you can handle various pointer events using synthetic touch and mouse events. Here are the pointer-events supported in ReactJS:

1. onPointerDown: This event is triggered when a pointing device (such as a mouse or touch) is pressed down on an element.

2. onPointerUp: This event is triggered when a pointing device is released from an element.

3. onPointerMove: This event is triggered when a pointing device is moved while it is over an element.

4. onPointerEnter: This event is triggered when a pointing device enters an element.

5. onPointerLeave: This event is triggered when a pointing device leaves an element.

6. onPointerCancel: This event is triggered when a pointing device is interrupted (e.g., due to system events like alerts or modal dialogs).

7. onGotPointerCapture: This event is triggered when an element receives pointer capture.

8. onLostPointerCapture: This event is triggered when an element loses pointer capture.

Additionally, you can also use the equivalent touch and mouse events in ReactJS to handle pointer interactions:

  • onTouchStart 
  • onTouchEnd 
  • onTouchMove 
  • onTouchCancel 
  • onMouseDown 
  • onMouseUp 
  • onMouseMove 
  • onMouseEnter 
  • onMouseLeave  

By using these events in ReactJS, you can create interactive and dynamic UI components that respond to user interactions across different pointing devices.

What are the differences between call() and put() in redux-saga in ReactJS?

In Redux-Saga, the call() and put() functions are used to control the flow and execution of asynchronous operations. Here are the differences between these two functions:

1. call(): 

The call() function is used to invoke a function asynchronously. It allows you to call a function and wait for its result before proceeding further in the saga. The syntax for using call() is as follows:

<code>yield call(fn, ...args);</code>

Here, fn is the function that you want to call, and args are the arguments to be passed to that function.The call() function returns an Effect object, which instructs the middleware to perform the specified function call. This allows Redux-Saga to handle the execution of the function and manage any errors that may occur.

2. put(): 

The put() function is used to dispatch an action to the Redux store. It creates an Effect object that instructs the middleware to dispatch the specified action. The syntax for using put() is as follows:

<code>yield put(action);</code>

Here, action is the action object that you want to dispatch.put() is commonly used to trigger state changes in the Redux store after completing an asynchronous operation. For example, you can use put() to dispatch a success or failure action after making an API request.

In summary, call() is used to invoke a function asynchronously and wait for its result, while put() is used to dispatch an action to the Redux store.

What are the major advantages of React JS?

React JS has several major advantages that have contributed to its popularity among developers:

1. Virtual DOM: React uses a virtual representation of the Document Object Model (DOM), known as the Virtual DOM. This allows for efficient rendering and updating of components by minimizing actual DOM manipulations. It results in improved performance and a smoother user experience.

2. Component-based architecture: React follows a component-based approach, allowing developers to build reusable and modular UI components. Each component encapsulates its own logic and state, making it easier to manage and test.

3. Reusable code: With React, you can create reusable components that can be used across different parts of your application. This reusability saves development time and effort, and promotes code consistency and maintainability.

4. Easy integration: React is flexible and can be easily integrated with existing projects or libraries. It can be combined with other frameworks or libraries, such as Redux for state management or React Router for routing.

5. Declarative syntax: React uses a declarative syntax, where you describe how your UI should look based on its current state. This makes it easier to understand and reason about your code, as well as debug and maintain it.

6. Community and ecosystem: React has a vibrant community and a vast ecosystem of libraries and tools that support its development. This includes popular libraries like React Router, Redux, and Material-UI, as well as tools like Create React App and Next.js.

7. Performance optimization: React’s virtual DOM and smart diffing algorithm help optimize rendering performance. Additionally, React provides tools like React.memo and React.lazy that allow for further performance optimizations by reducing unnecessary re-renders and code-splitting.

8. SEO-friendly: React can be made SEO-friendly by using server-side rendering (SSR) or static site generation (SSG) techniques. This ensures that search engines can index your app’s content effectively.

Overall, React JS offers developers a powerful and efficient way to build complex, scalable, and interactive user interfaces. Its advantages make it a popular choice for many web developers.