Cognizant Top 20 React JS Interview Questions and Answers

React JS is a powerful JavaScript library widely used in front-end development. As a React JS developer at Cognizant, you will be expected to demonstrate proficiency in building dynamic user interfaces, understanding React concepts, and managing application states efficiently. Here are the top 20 Cognizant React JS interview questions and answers to help you prepare for the interview.

1. What is React JS, and why is it popular?

React JS is a JavaScript library developed by Facebook for building user interfaces. It’s popular because of its simplicity, flexibility, reusable components, and the efficient handling of dynamic data with a virtual DOM.

2. What are the major features of React?

Some key features of React include:

• JSX (JavaScript XML): A syntax extension that allows writing HTML elements in JavaScript.
• Components: Reusable building blocks in React that make it easy to manage UI elements.
• Virtual DOM: React uses a virtual DOM to improve performance by reducing the number of direct manipulations to the real DOM.
• One-way data binding: Ensures unidirectional data flow, making the application easier to debug and maintain.

3. What is JSX?

JSX is a syntax extension for JavaScript that allows writing HTML-like code within JavaScript files. It makes it easier to create React elements and is transformed into React createElement() calls behind the scenes.

4. What is the virtual DOM, and how does it improve performance?

The virtual DOM is an in-memory representation of the real DOM. React updates the virtual DOM first and compares it with the previous version (called reconciliation). It then efficiently updates only the changed elements in the real DOM, improving performance.

5. What are React components?

React components are the building blocks of a React application. There are two types of components:

• Functional Components: Stateless components that are simple JavaScript functions returning JSX.
• Class Components: Stateful components that extend React.Component and use lifecycle methods.

6. What is the difference between a functional and a class component in React?

• Functional Components: Do not have state or lifecycle methods. They are simple functions that take props and return JSX.
• Class Components: Can hold state and have access to lifecycle methods. They are more feature-rich but have been somewhat replaced by functional components with React Hooks.

7. What are React hooks?

React Hooks are functions introduced in React 16.8 that allow you to use state and lifecycle features in functional components. Common hooks include:

• useState(): Manages state in functional components.
• useEffect(): Handles side effects such as data fetching or subscriptions.
• useContext(): Provides access to the React context API.

8. Explain the use of the useState() hook.

The useState() hook is used to declare state variables in functional components. It returns an array with two values:

• The current state value.
• A function to update the state.

Example:

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

9. What is the purpose of useEffect() in React?

The useEffect() hook is used to handle side effects in functional components, such as fetching data, subscribing to events, or manually updating the DOM. It is a combination of several lifecycle methods (e.g., componentDidMount, componentDidUpdate, and componentWillUnmount).

10. What is the difference between useEffect() and useLayoutEffect()?

• useEffect(): Runs asynchronously after the render and is used for tasks like fetching data or updating state based on props.
• useLayoutEffect(): Runs synchronously after the DOM updates but before the browser paints. It's used when you need to make visual updates immediately after rendering.

11. What is props drilling, and how do you avoid it?

Props drilling occurs when you pass props through multiple layers of components, even if intermediate components do not need them. You can avoid props drilling by using React Context API or state management libraries like Redux.

12. What is the React Context API?

The React Context API is used to manage global state and share data across components without props drilling. It provides a way to pass data through the component tree without passing props down manually at every level.

13. Explain the concept of a higher-order component (HOC).

A higher-order component (HOC) is a function that takes a component as input and returns a new component with enhanced functionality. It’s a pattern used to share logic across multiple components.

Example:

const withLogger = (Component) => {
  return (props) => {
    console.log("Logging props:", props);
    return <Component {...props} />;
  };
};

14. What are controlled and uncontrolled components in React?

• Controlled Components: The form data is handled by the React component’s state. Input fields have their values controlled by state.
• Uncontrolled Components: The form data is handled by the DOM itself. You access the input values using refs instead of state.

15. How do you optimize performance in a React application?

Performance optimization techniques in React include:

• Using the React.memo higher-order component to prevent unnecessary re-renders.
• Using the shouldComponentUpdate() lifecycle method in class components.
• Avoiding inline functions and object creations in JSX.
• Using the useCallback and useMemo hooks to memoize functions and values.

16. What is the significance of keys in React lists?

Keys help React identify which items in a list have changed, been added, or removed. They must be unique among siblings to ensure that React can efficiently update and reorder items.

17. Explain the component lifecycle methods in React.

Class components in React have lifecycle methods that are invoked at different phases of a component’s life:

• Mounting: constructor(), componentDidMount()
• Updating: shouldComponentUpdate(), componentDidUpdate()
• Unmounting: componentWillUnmount()

18. What is the difference between state and props in React?

• State: Local data owned and managed by the component. It can change over time, typically in response to user actions.
• Props: Data passed from a parent component to a child component. They are read-only and cannot be modified by the child component.

19. How do you handle forms in React?

In React, forms are handled using controlled components where the form input fields' values are stored in the component's state. Each form element should have a corresponding state variable and an onChange handler to update the state.

20. What is React Router, and how do you implement routing in a React application?

React Router is a library used to handle routing in a React application. It allows developers to define different routes for various components of the application. Implementing routing involves:

• Installing React Router using npm.
• Defining routes using the Route component.
• Navigating between routes using the Link component.

Example:

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

<Router>
  <Switch>
    <Route path="/home" component={HomePage} />
    <Route path="/about" component={AboutPage} />
  </Switch>
</Router>

Conclusion

These top 20 React JS interview questions will help you prepare effectively for your Cognizant interview. Understanding the core concepts, hooks, component lifecycle, and state management is essential to succeeding in a React JS role.