Infosys React JS Interview Questions

Table of contents

• What is React
• What are props in React
• Significance of keys in React
• What is Redux
• Write a React component
• React JS Interview Questions

Infosys, a global leader in technology services and consulting, is renowned for its commitment to innovation, excellence, and client satisfaction. Founded in 1981, Infosys has grown to become one of the largest IT services companies in the world, with a presence in over 50 countries and a workforce of more than 250,000 employees. The company’s extensive range of services includes software development, IT consulting, and business process outsourcing, making it a one-stop solution for many Fortune 500 companies. Infosys places a strong emphasis on hiring top talent to maintain its competitive edge in the rapidly evolving tech landscape. As part of its recruitment strategy, Infosys seeks individuals proficient in cutting-edge technologies, ensuring they remain at the forefront of industry trends and continue to deliver exceptional value to their clients.

CRS Info Solutions stands out for its exceptional React.js training in Hyderabad, tailored specifically for students. Their program focuses on practical, hands-on learning, ensuring that students not only understand React.js training Bangalore concepts but also apply them effectively in real-world scenarios. This approach has established CRS Info Solutions as a go-to destination for aspiring React.js developers in the region.

In particular, Infosys is actively recruiting professionals skilled in React JS, a popular JavaScript library for building user interfaces. React JS has gained immense popularity due to its efficiency, flexibility, and ability to create dynamic and responsive web applications. Mastery of React JS is crucial for candidates aspiring to secure a role in Infosys, as it enables them to develop seamless and interactive user experiences. The interview process for React JS roles at Infosys is designed to assess a candidate’s technical knowledge, problem-solving abilities, and practical experience with the library. By preparing for Infosys React JS interview questions, candidates can enhance their chances of landing a coveted position at one of the world’s leading tech companies, thereby advancing their careers and contributing to the future of digital innovation.

Explore: React JSX

Infosys React JS Interview Questions and Answers

1. What is React, and how does it differ from other JavaScript frameworks?

React is a JavaScript library developed by Facebook for building user interfaces, specifically for single-page applications. It allows developers to create reusable UI components that manage their own state, leading to efficient and dynamic web applications. Unlike other JavaScript frameworks such as Angular or Vue, React focuses on the view layer only, making it simpler and more flexible. React’s primary strength lies in its ability to update and render components efficiently using a virtual DOM, which minimizes direct manipulation of the actual DOM and results in faster performance.

2. Explain the concept of the Virtual DOM and how it enhances React’s performance.

The Virtual DOM is a lightweight representation of the actual DOM that React uses to optimize updates and rendering. When the state of a React component changes, a new Virtual DOM tree is created. React then compares this new tree with the previous one using a process called “diffing.” Only the parts of the DOM that have changed are updated, rather than re-rendering the entire DOM. This selective updating process significantly enhances performance, especially for complex and dynamic web applications, by reducing the number of direct manipulations of the real DOM, which can be slow and resource-intensive.

Explore: How Can You Pass Props to Children Components in React?

3. What are props in React, and how are they different from state?

Props, short for properties, are read-only attributes that are passed from a parent component to a child component. They allow data to flow down the component tree and are used to configure and customize components. Props are immutable, meaning they cannot be modified by the child component. On the other hand, state is a mutable, local data storage specific to a component that can change over time, often as a result of user actions or network responses. While props facilitate communication between components, state is used to manage dynamic data within a component.

4. Describe the lifecycle methods in React and their purposes.

React lifecycle methods are hooks that allow developers to run code at specific points in a component’s lifecycle. These methods are divided into three phases: mounting, updating, and unmounting. During the mounting phase, componentDidMount is called after the component is inserted into the DOM, typically used for initial data fetching. In the updating phase, methods like shouldComponentUpdate, componentDidUpdate, and getDerivedStateFromProps are used to control re-rendering and respond to prop or state changes. During unmounting, componentWillUnmount is called just before the component is removed from the DOM, allowing for cleanup operations like invalidating timers or canceling network requests.

Explore: Event Handling in Reactjs

5. What is the significance of keys in React, and how are they used?

Keys are unique identifiers assigned to elements in a list or array of children in React. They help React identify which items have changed, been added, or removed, thus optimizing the re-rendering process. When rendering lists of elements, each element should have a unique key, often derived from the element’s ID or a stable attribute. This allows React to maintain the correct state of each element and apply updates efficiently. Without keys, React would re-render the entire list, leading to performance issues and potential bugs.

6. Explain how React hooks work, particularly useState and useEffect.

React hooks are functions that let developers use state and other React features in functional components. useState is a hook that allows you to add state to a functional component. It returns an array with two elements: the current state and a function to update it. For example:

const [count, setCount] = useState(0);

useEffect is a hook for managing side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM. It takes a function and an optional array of dependencies. The effect runs after the component renders and whenever the dependencies change. For example:

useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]);

These hooks allow functional components to manage state and lifecycle events without needing to convert to class components.

Explore: Form Handling in React JS

7. What is Redux, and how does it integrate with React for state management?

Redux is a predictable state management library for JavaScript applications, often used with React to manage complex application states. It centralizes the application’s state in a single store, allowing components to access state and dispatch actions through the store. Integration with React is facilitated by the react-redux library, which provides the Provider component and connect function. The Provider component makes the Redux store available to the entire app, while connect maps state and dispatch to component props, enabling components to read from the Redux store and dispatch actions to update the state.

8. How would you optimize the performance of a React application?

Optimizing a React application involves several strategies to enhance its performance. These include:

• Using PureComponent or React.memo: These components perform shallow comparisons of props and state to avoid unnecessary re-renders.
• Code splitting: This involves splitting the application into smaller chunks and loading them on demand using dynamic import or libraries like React.lazy and Suspense.
• Memoizing expensive calculations: Using useMemo and useCallback hooks to memoize expensive functions and avoid re-calculations on every render.
• Virtualizing long lists: Libraries like react-window or react-virtualized render only visible items in large lists, reducing the number of DOM nodes.
• Optimizing images and assets: Compressing images and using modern image formats can significantly reduce loading times.
• Avoiding inline functions and objects: Inline functions and objects create new references on every render, leading to unnecessary re-renders. Implementing these techniques helps in creating faster, more responsive React applications.

9. What is React, and how does it differ from other JavaScript frameworks?

React is a JavaScript library for building user interfaces, primarily for single-page applications. Unlike some other JavaScript frameworks like Angular or Vue, React focuses solely on the view layer of an application, which means I can integrate it into any part of my app. One of the biggest differences is that React uses a declarative programming approach, making it easier to predict and debug user interfaces by describing what I want the UI to look like at any point in time. Additionally, React is component-based, which allows me to break down complex UIs into small, reusable components.

In contrast to other frameworks, React doesn’t have a built-in system for routing or state management, although it can work seamlessly with libraries like React Router and Redux. Also, React’s ecosystem is vast and constantly evolving, allowing me to use additional packages for features not included in the core library. This flexibility makes it easier for me to customize and scale my projects.

10. Explain the concept of the Virtual DOM and how it enhances React’s performance.

The Virtual DOM is a concept where React creates a lightweight in-memory representation of the real DOM. This virtual copy allows React to track changes to the UI without directly manipulating the actual DOM until necessary. When the state of my application changes, React compares the updated Virtual DOM with the previous version (a process called reconciliation). This comparison helps determine the minimal number of updates required to the actual DOM, making updates more efficient.

By using the Virtual DOM, React optimizes performance, as directly manipulating the DOM can be slow, especially in complex applications. Instead of re-rendering the entire UI, React ensures that only the necessary parts are updated. This approach improves the application’s responsiveness and helps prevent unnecessary rendering, which is crucial when dealing with dynamic data and large datasets.

11. What are props in React, and how are they different from state?

Props are short for properties and are used to pass data from one component to another in React. They are read-only, meaning that once I pass them to a component, that component cannot modify them. This makes props a great way to pass down data from a parent component to a child component, ensuring that the data flows in one direction. I can use props to display dynamic content, control behavior, or pass callback functions to trigger actions in the parent component.

On the other hand, state is mutable and represents dynamic data that can change over time. While props are controlled externally, state is managed within the component itself. This means that I can update the state based on user actions or external data sources, which in turn will cause the component to re-render. In short, I use props to pass static data between components, while state is used to handle dynamic, internal data within a component.

12. Describe the lifecycle methods in React and their purposes.

React lifecycle methods are special functions that get called during different phases of a component’s life. There are three main phases: mounting, updating, and unmounting. During the mounting phase, I typically use componentDidMount, which runs after the component has been rendered in the DOM. It’s commonly used for API calls or setting up subscriptions. For example:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => this.setState({ items: data }));
}

This method ensures that I can update the state with data fetched from an external source after the component is already visible on the page.

In the updating phase, the most commonly used method is componentDidUpdate, which runs after every update, allowing me to perform actions based on prop or state changes. Lastly, componentWillUnmount is called right before a component is removed from the DOM, making it ideal for cleanup tasks like removing event listeners or canceling API requests. These methods help me control what happens to the component as it interacts with the DOM, ensuring optimal performance and behavior.

13. What is the significance of keys in React, and how are they used?

In React, keys play an important role when rendering lists of elements. They provide a way for React to identify which items have changed, been added, or been removed. By assigning a unique key to each item, React can optimize the rendering process by only updating the items that have actually changed. This improves performance, especially in scenarios where I have large datasets being rendered in the UI.

A good rule of thumb is to use a unique identifier for the key, such as an ID. Here’s a quick example:

{items.map(item => (
  <li key={item.id}>{item.name}</li>
))}

In this case, using item.id as the key ensures that React can track each list item efficiently. Without keys, React would need to re-render the entire list, which could lead to performance issues.

14. Explain how React hooks work, particularly useState and useEffect.

React hooks allow me to use state and other React features in functional components, without needing to write class components. The two most commonly used hooks are useState and useEffect. The useState hook enables me to add local state to functional components. For example, I can use it to create a counter:

const [count, setCount] = useState(0);

return (
  <div>
    <button onClick={() => setCount(count + 1)}>Increment</button>
    <p>{count}</p>
  </div>
);

Here, useState initializes count to 0, and setCount updates the state whenever the button is clicked.

The useEffect hook allows me to perform side effects in functional components, such as fetching data or subscribing to events. I can specify dependencies so that the effect only runs when certain values change, improving performance and ensuring the effect isn’t executed unnecessarily. For example, I might use useEffect to fetch data when a component first mounts:

useEffect(() => {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => setItems(data));
}, []);

In this case, the empty array [] ensures the effect only runs once when the component mounts.

15. What is Redux, and how does it integrate with React for state management?

Redux is a popular state management library that helps manage the state of my application in a predictable and centralized way. Unlike React’s local state, which is managed within individual components, Redux creates a global state that can be shared across all components. This is especially helpful for larger applications where state needs to be accessible by many components at different levels of the component tree. Redux follows three principles: having a single source of truth (the store), state being read-only (modified only via actions), and changes to the state being handled by pure functions (reducers).

To integrate Redux with React, I need to install the react-redux library, which provides tools like the Provider and connect functions. The Provider component wraps my entire React app, allowing any component within the app to access the Redux store. The connect function maps the Redux state and dispatch actions to the props of my components, making it easier to interact with the store without manually passing down state or actions. By using Redux, I can manage complex states more efficiently and ensure that changes are predictable and easy to track.

16. How would you optimize the performance of a React application?

Optimizing a React application is important to ensure smooth user experience and avoid performance bottlenecks, especially as the app grows. One technique I use is memoization, such as using React.memo to prevent unnecessary re-renders of functional components. If a component’s props haven’t changed, React skips the re-render, improving performance. For class components, I can use the shouldComponentUpdate method to achieve similar results.

Another optimization technique is code-splitting using React’s lazy and Suspense. This allows me to load parts of my app only when they’re needed, rather than loading everything upfront. For example:

const LazyComponent = React.lazy(() => import('./LazyComponent'));

return (
  <Suspense fallback={<div>Loading...</div>}>
    <LazyComponent />
  </Suspense>
);

By splitting my code, I reduce the initial load time and improve the user experience.

In addition to memoization and code-splitting, I also focus on reducing unnecessary renders by making sure components only re-render when their props or state actually change. Tools like React Developer Tools help me identify components that are rendering unnecessarily, allowing me to refactor them for better performance.

17. (Coding) Write a React component that fetches data from an API and displays it in a list format.

Here’s a simple React component that fetches data from an API and displays the results in a list:

import React, { useState, useEffect } from 'react';

const DataList = () => {
  const [items, setItems] = useState([]);

  useEffect(() => {
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(data => setItems(data));
  }, []);

  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
};

export default DataList;

In this component, I use the useState hook to store the fetched items and useEffect to trigger the data fetching when the component mounts. The data is then rendered as a list of <li> elements, with a unique key assigned to each item for optimal rendering. This pattern ensures that my data is fetched and displayed only when the component is loaded.

18. (Coding) Implement a simple counter component using hooks that increments and decrements a number.

Here’s a simple counter component that increments and decrements a number using React hooks:

import React, { useState } from 'react';

const Counter = () => {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(count - 1)}>Decrement</button>
    </div>
  );
};

export default Counter;

In this component, I use the useState hook to initialize and update the count state. The Increment button increases the count by 1, while the Decrement button reduces it by 1. This example showcases how easy it is to manage state in a functional component using React hooks, making the code more concise and easier to follow.

Explore: React Router Interview Questions

19. (Coding) Write a React component that fetches data from an API and displays it in a list format.

import React, { useState, useEffect } from 'react';

function DataFetchingComponent() {
  const [data, setData] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(data => {
        setData(data);
        setLoading(false);
      })
      .catch(error => console.error('Error fetching data:', error));
  }, []);

  if (loading) {
    return Loading...;
  }

  return (
    
      {data.map(item => (
        {item.name}
      ))}
    
  );
}

export default DataFetchingComponent;

Explore: Lifecycle Methods in React

20. (Coding) Implement a simple counter component using hooks that increments and decrements a number.

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(count - 1)}>Decrement</button>
    </div>
  );
}

export default Counter;

Explore: Step-by-Step Guide to React’s Context API

21. (Coding) Write a function component that accepts an array of items as props and displays each item in a separate div.

import React from 'react';

function ItemList({ items }) {
  return (
    <div>
      {items.map((item, index) => (
        <div key={index}>{item}</div>
      ))}
    </div>
  );
}

export default ItemList;

Scenario-Based React JS Interview Questions and Answers

1. Imagine you are developing a React application, and you need to handle user authentication. How would you implement a login flow using React and Redux?

To implement a login flow using React and Redux, first, create actions for login and logout. Then, create a reducer to handle these actions and update the state accordingly. Use redux-thunk or redux-saga for handling asynchronous login API calls. In the React component, dispatch the login action on form submission, passing the user credentials. On successful login, store the user data in the Redux state and redirect the user to a protected route. Use PrivateRoute components to guard the protected routes, checking if the user is authenticated by reading the state from the Redux store.

Explore: React Hooks: Revolutionizing Functional Components

2. You are tasked with creating a dynamic form in React that changes based on user inputs. Describe your approach and how you would manage the form state.

To create a dynamic form in React, start by defining the form structure in the component state. Use a state object to keep track of form field values and a function to handle changes. Render the form fields conditionally based on the state. For example, if a specific option is selected, display additional fields related to that option. Use controlled components for form inputs, binding their values to the state and updating the state on user input. This approach allows you to dynamically add or remove form fields and manage their values efficiently.

3. Suppose you need to integrate a third-party library for charts in your React application. How would you go about incorporating it and ensuring it works seamlessly with your existing components?

To integrate a third-party chart library in a React application, start by installing the library using npm or yarn. Import the necessary components from the library into your React component. Use the library’s API to configure and render the chart, passing the required data and options as props. Ensure the chart component is reusable by accepting props for data and configuration. Test the integration by rendering the chart with sample data. If needed, wrap the chart component in a higher-order component (HOC) or a custom hook to manage the data fetching and state updates, ensuring it integrates seamlessly with your existing components.

Explore: Component Composition in React

4. Consider a scenario where you have a large list of items to display, and performance is a concern. How would you implement efficient rendering and pagination in React?

To efficiently render a large list of items in React, use virtualization libraries like react-window or react-virtualized. These libraries render only the visible items in the viewport, reducing the number of DOM nodes and improving performance. Implement pagination by dividing the data into chunks and loading a subset of items at a time. Use a state variable to keep track of the current page and a function to load the next set of items when the user scrolls to the bottom or clicks a “Load More” button. This approach ensures efficient rendering and smooth user experience even with large datasets.

Explore: Conditional Rendering in React

5. You encounter a situation where two sibling components need to communicate and share data. How would you achieve this in React without using Redux or any other state management library?

To enable communication and data sharing between sibling components in React without using Redux, lift the shared state up to their common parent component. The parent component maintains the shared state and passes it down to the sibling components as props. The sibling components can call functions passed from the parent to update the shared state. This approach ensures that both components are synchronized with the shared state managed by their parent, facilitating communication and data sharing without needing a global state management library.

Conclusion:

In conclusion, preparing for Infosys React JS interview questions requires a thorough understanding of both fundamental concepts and advanced topics within the React ecosystem. Candidates should focus on mastering core principles such as component lifecycle, state management, and the virtual DOM, as well as familiarizing themselves with modern tools like hooks and context API. Additionally, practical experience in building projects can significantly enhance one’s ability to answer questions effectively and demonstrate real-world problem-solving skills.

Moreover, staying updated with the latest developments in React and related technologies can provide candidates with a competitive edge. Engaging in coding challenges, contributing to open-source projects, and practicing common interview scenarios will not only bolster technical knowledge but also improve confidence during the interview process. By approaching their preparation strategically, candidates can position themselves as strong contenders for roles at Infosys and other tech companies, showcasing their ability to adapt and thrive in a rapidly evolving industry.