ReactJS Interview Questions Part-2

What is the behavior of uncaught errors in react?

In React, uncaught errors that occur during rendering, lifecycle methods, or in the constructors of higher-order components (HOCs) are now treated as unhandled exceptions. This behavior is a result of React’s new error handling mechanism called “React Error Boundaries.”

When an uncaught error happens inside a component, React will now unmount the whole component tree that is affected by the error. This ensures that the application state doesn’t become inconsistent and prevents the user from seeing partially rendered or broken UI.

React will then present a fallback UI to the user, indicating that an error has occurred. The error message and stack trace can be displayed within this fallback UI element. Developers can customize the fallback UI by implementing an error boundary component.

Error boundaries are regular React components that have a special lifecycle method called componentDidCatch. When an error occurs within a child component, the closest error boundary component in the component hierarchy will be triggered. This allows developers to gracefully handle errors, log them, display a custom error message, or even recover from them if possible.

By adopting error boundaries, developers can encapsulate error handling logic for a portion of their application without affecting the rest of the UI. It’s worth noting that error boundaries only catch errors in the components beneath them during rendering. They won’t catch errors in event handlers, asynchronous code, or inside setTimeout or requestAnimationFrame callbacks.

Overall, React’s behavior with uncaught errors prioritizes application stability and provides developers with more control over error handling, resulting in a better user experience.

What is suspense component in ReactJS?

In ReactJS, suspense is a component that allows you to specify a fallback UI to render while waiting for some asynchronous data to load. It helps in handling the loading states of components and improves the user experience by showing a loading indicator or placeholder content until the data is ready.

Suspense is especially useful when working with code-splitting and lazy-loading techniques, where certain parts of your application are loaded on-demand. It allows you to define the loading behavior for those dynamically imported modules.

To use suspense, you can wrap your dynamically imported components or resources inside a component, providing a fallback UI within its props. Here’s an example:

import React, { Suspense } from 'react';const LazyComponent = React.lazy(() => import('./LazyComponent'));function App() {
  return (
      My App
}export default App;

In this example, the LazyComponent will be lazily loaded when it is rendered. While it is loading, the fallback UI (in this case, the simple “Loading…” text) will be displayed. Once the component is loaded, it will be rendered normally.

Suspense provides a simple and convenient way to handle loading states in React applications, making it easier to create smooth and responsive user interfaces.

Do we need to keep all my state into Redux? Should we ever use react internal state?

When it comes to managing state in a React application, Redux is often suggested as a centralized state management solution. However, it is not always necessary to keep all your state in Redux. 

React’s internal state can be a valuable tool for managing local component-specific state that doesn’t need to be shared with other components.

Here are some scenarios where using React’s internal state might be appropriate:

1. Component-specific state: If a particular state is only relevant to a specific component and doesn’t need to be shared globally, using React’s internal state is a lightweight and easy-to-implement solution.

2. UI-related state: React’s internal state can be useful for managing UI-related state that affects the presentation of a single component. For example, toggling a dropdown menu or controlling the visibility of a modal.

3. Form input state: When dealing with form inputs, React’s internal state can be used to manage the values and their changes before submitting the form. This allows for real-time user input validation and control over form behavior.

However, there are cases where using Redux might be more suitable:

1. Global application state: If state needs to be shared among different components across your application, Redux provides a centralized store that can be accessed from any component. This can help simplify data flow and keep your application’s state in a single source of truth.

2. Complex state management: If your application has complex state interactions, such as multiple components needing to update or subscribe to the same piece of data, using Redux can provide a clear and structured way to manage such complexities.

In summary, while Redux is a powerful tool for managing state in larger applications, it’s not always necessary to use it for every piece of state. 

React’s internal state should be used when the state is component-specific or UI-related, while Redux shines in managing global state or complex state interactions. The choice ultimately depends on the requirements and complexity of your application.

What are the different phases of React Components lifecycle in React JS?

In React JS, components go through several lifecycle phases, which are divided into three main categories: Mounting, Updating, and Unmounting. Here is a breakdown of each phase:

1. Mounting Phase: 

  • constructor(): This is the first method called when an instance of a component is being created and initialized. 
  • static getDerivedStateFromProps(): This method is invoked right before rendering and provides an opportunity to update the component’s state based on props. 
  • render(): This method is responsible for returning the component’s JSX representation. 
  • componentDidMount(): This method is called once the component has been mounted to the DOM, and it’s a good place to perform any side effects like API calls or event subscriptions.

2. Updating Phase: 

  • static getDerivedStateFromProps(): Similar to the mounting phase, this method allows you to update the state based on props changes. 
  • shouldComponentUpdate(): This method determines whether the component should be re-rendered or not. It can help optimize performance by preventing unnecessary updates. 
  • render(): This method re-renders the updated component. 
  • getSnapshotBeforeUpdate(): This method is called right before changes from the virtual DOM are actually reflected in the DOM. It can capture some information from the current DOM state, like scroll position, before an update. 
  • componentDidUpdate(): This method is invoked immediately after the component’s DOM has been updated. It’s useful for performing post-update side effects.

3. Unmounting Phase: 

  • componentWillUnmount(): This method is called right before a component is unmounted and destroyed. It is used for cleanup tasks like cancelling timers or removing event listeners.

 In addition to these phases, there are Error Handling methods like componentDidCatch() that can be used to handle errors during rendering.

 It’s worth mentioning that React Hooks have provided an alternative way to manage component lifecycle with the useEffect() hook, making it easier to handle side effects and component updates.

What is your favorite React stack?

However, there are several popular React stacks that developers often choose based on their specific needs and preferences. Here are a few notable options:

1. MERN Stack (MongoDB, Express.js, React, Node.js): This stack combines a MongoDB database, Express.js framework for server-side development, React for building user interfaces, and Node.js for server-side runtime. It is widely used for building full-stack JavaScript applications.

2. MEAN Stack (MongoDB, Express.js, Angular, Node.js): Similar to the MERN stack, but instead of React, it uses Angular as the front-end framework. Angular is a comprehensive framework maintained by Google, while React is a more lightweight library.

3. Next.js: Next.js is a popular framework built on top of React that adds server-side rendering and other features, making it easier to build scalable and production-ready React applications.

4. Gatsby.js: Gatsby is another popular framework focused on creating fast and optimized websites. It uses React for building the user interface and provides various performance optimizations out of the box.

5. React Native: Although not strictly a React stack for web development, React Native allows you to build native mobile applications using React. It shares some concepts with React for web development, making it easier for developers to transition from one platform to another.

These are just a few examples, and the choice of a React stack depends on various factors such as project requirements, team expertise, and personal preferences. Each stack has its advantages and disadvantages, so it’s essential to evaluate them according to your specific needs.

Give a code example to modularize code in React?

Certainly! Modularizing code in React is a recommended practice that promotes reusability, maintainability, and scalability. One common way to achieve this is by using the concept of components.

Here’s a code example to demonstrate how you can modularize your code in React:

1. Create a new folder called “components” within your React project. 

2. Inside the “components” folder, create a new file called “Header.js” and add the following code:

// Header.js
import React from 'react';
function Header() 
  return (
   My Website
   {/* ... */}
export default Header;

3. Now, let’s create another component. Create a new file called “Footer.js” inside the “components” folder and add the following code:

// Footer.js
import React from 'react';
function Footer() {
  return (
      © {new Date().getFullYear()} My Website
}export default Footer;

4. Finally, in your main App.js file or any other relevant component, you can import and use these newly created components as follows:

// App.js
import React from 'react';
import Header from './components/Header';
import Footer from './components/Footer';
function App() {
  return (
     {/* ... Your main content */}
 export default App;

By separating your code into reusable components like this, you can easily manage and update different parts of your application. Additionally, if you need to reuse the same components in other parts of your application or even in different projects, you can simply import them and reuse them without duplicating the code.

How to define constants in React?

In React, you can define constants using several approaches. Here are three common methods:

1. Using a JavaScript file: 

  • Create a JavaScript file (e.g., constants.js) to store your constants. 
  • Inside the file, export the constants using the export keyword. 
  • Import the constants in your React component files using the import statement. Example:
   // constants.js
   export const API_URL = 'https://api.example.com';
   export const MAX_RESULTS = 10;   // MyComponent.js
   import React from 'react';
   import { API_URL, MAX_RESULTS } from './constants';   function MyComponent() {
     return (
         API URL: {API_URL}
         Max Results: {MAX_RESULTS}
   export default MyComponent;

2. Using an object: 

  • Define an object to store your constants. 
  • Import the object in your React component files. 
  • Access the constants using dot notation. Example:
   // constants.js
   const Constants = {
     API_URL: 'https://api.example.com',
     MAX_RESULTS: 10,
   export default Constants;   
   // MyComponent.js
   import React from 'react';
   import Constants from './constants';   function MyComponent() {
     return (
         API URL: {Constants.API_URL}
         Max Results: {Constants.MAX_RESULTS}
   export default MyComponent;

3. Using the useContext hook: 

  • Create a context to hold your constants using the createContext function from react
  • Wrap your component hierarchy with a context provider. 
  • Set the constants as values in the provider. 
  • Use the useContext hook in your component to access the constants. Example:
   // ConstantsContext.js
   import React, { createContext } from 'react';   
   export const ConstantsContext = createContext();   
   export const ConstantsProvider = ({ children }) => {
     const API_URL = 'https://api.example.com';
     const MAX_RESULTS = 10;     return (
   // MyComponent.js
   import React, { useContext } from 'react';
   import { ConstantsContext } from './ConstantsContext';   
   function MyComponent() {
     const { API_URL, MAX_RESULTS } = useContext(ConstantsContext);     
     return (
         API URL: {API_URL}
         Max Results: {MAX_RESULTS}
   export default MyComponent;

Choose the method that best suits your needs and preferences. All the methods mentioned above allow you to define and use constants in your React components effectively.

What is a higher order component in ReactJS?

In ReactJS, a Higher Order Component (HOC) is a function that takes a component and returns an enhanced version of that component. HOCs are used to encapsulate common logic or behavior and to easily share this logic across multiple components.

By wrapping a component with an HOC, you can enhance its functionality without modifying its original implementation. This allows for better code reusability and separation of concerns.

Here’s a simple example:

const withLogger = (WrappedComponent) => {
  return class extends React.Component {
    componentDidMount() {
      console.log(Component ${WrappedComponent.name} rendered.);
    }    render() {
      return ;

const MyComponent = () => {
  return Hello, World!;

const EnhancedComponent = withLogger(MyComponent);

In the example above, the withLogger HOC logs a message whenever a component wrapped by it gets rendered. The EnhancedComponent is a new component that includes this logging functionality.

By using HOCs, you can easily apply additional features to your components, such as authentication, data fetching, or even Redux integration, without cluttering the component’s original code.

Why React uses class Name over class attribute in ReactJS?

React uses the className attribute instead of the regular class attribute in JSX to avoid conflicts with JavaScript’s reserved keyword, class. JSX is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript files.

 In JavaScript, class is used to define a class, which is a blueprint for creating objects with a specific set of properties and methods. Since React components are also JavaScript classes, using the class attribute in JSX would cause confusion because it could be interpreted as the JavaScript keyword class.

To maintain consistency and readability, React chooses to use the className attribute instead. This way, it’s clear that we are specifying a CSS class name for styling purposes, rather than referring to a JavaScript class.Here’s an example to illustrate the difference:

// Using className
// This is a component with the class name "my-class".
// Using class (incorrect)
// This would cause an error because "class" is a reserved keyword.

By using className, React ensures that the attribute is treated as a CSS class name and not mistaken for JavaScript functionality.

When do you need to use refs in ReactJS?

In ReactJS, you typically use refs when you need to access the DOM node or a React component instance. Refs provide a way to get a reference to a particular element or component, allowing you to interact with it directly.

Here are some common scenarios where you might need to use refs in ReactJS:

1. Managing focus: If you want to focus on an input field or a button programmatically, you can use refs to obtain a reference to the DOM node and call the focus() method on it.

2. Accessing form values: When working with forms, refs can be used to retrieve the values of form elements, especially when you don’t want to rely solely on the state.

3. Integrating with third-party libraries: Sometimes, you may need to interact with third-party libraries that expect direct access to a DOM node. By using refs, you can provide the necessary references to those libraries.

4. Animations and imperative logic: In certain cases, you might need to perform imperative operations or create animations directly on specific DOM elements. Refs can help you obtain the necessary references to achieve this.

It’s important to note that while refs can be useful in these cases, they should be used sparingly and carefully since they bypass React’s declarative nature. In most cases, you can rely on props and state to manage your application’s data flow.