TCS React.js Interview Questions and Answers

Prepare for your TCS (Tata Consultancy Services) React.js interview with this comprehensive collection of interview questions and expert-curated answers. TCS, a leading global IT services and consulting company, often evaluates candidates on their proficiency in React.js, a popular JavaScript library for building user interfaces.

Q1. What is React.js? Why is it used? 

Ans: React.js, commonly referred to as React, is an open-source JavaScript library for building user interfaces or UI components. It was developed by Facebook and is maintained by Facebook and a community of individual developers and companies. React.js is specifically designed for creating efficient, modular, and scalable web applications with a focus on building user interfaces that can dynamically update and render efficiently.

Key Features of React.js:

1. Declarative Syntax:

• React uses a declarative syntax, making it easier to understand and debug. Developers describe how the UI should look, and React takes care of updating the DOM to match that description.

2. Component-Based Architecture:

• React follows a component-based architecture, where the UI is broken down into reusable and independent components. Components encapsulate their own state and logic, making it easier to manage and maintain complex UIs.

3. Virtual DOM (Document Object Model):

• React uses a virtual DOM to efficiently update the actual DOM. Instead of directly manipulating the DOM for every change, React creates a virtual representation of the DOM in memory and calculates the most efficient way to update the actual DOM.

4. One-Way Data Binding:

• React enforces a one-way data flow, which means data changes in a parent component will automatically update the child components, but not the other way around. This makes the application more predictable and easier to debug.

5. JSX (JavaScript XML):

• JSX is a syntax extension for JavaScript recommended by React. It allows developers to write HTML elements and components in a syntax similar to XML or HTML within JavaScript code. JSX is then transformed into regular JavaScript by a build process.

6. React Hooks:

• Introduced in React 16.8, hooks are functions that allow developers to use state and other React features in functional components. Hooks provide a more concise way to handle state and side effects in functional components.

Why React.js is Used:

1. Efficient Updates:

• React's virtual DOM and efficient reconciliation algorithm ensure that only the necessary parts of the DOM are updated, resulting in better performance.

2. Component Reusability:

• Components in React are modular and encapsulate their own logic and state. This promotes reusability and maintainability of code.

3. Declarative Syntax:

• The declarative syntax of React makes it easier to understand and reason about the UI. Developers can describe how the UI should look, and React takes care of the rest.

4. Large and Active Community:

• React has a large and active community, providing ample resources, documentation, and third-party libraries. This makes it easier for developers to find solutions and stay updated with best practices.

5. Ecosystem and Tooling:

• React has a rich ecosystem and excellent tooling that enhances the development experience. Tools like React DevTools and Create React App simplify development and debugging.

6. Flexibility and Compatibility:

• React can be easily integrated with other libraries and frameworks. It is also compatible with both front-end and server-side rendering.

7. Supported by Facebook:

• Being developed and maintained by Facebook, React has a high level of support and is actively improved and updated.

Overall, React.js is used to build modern and interactive user interfaces for web applications, providing developers with the tools and patterns to create scalable and maintainable code. It has become a popular choice for building UIs in single-page applications and is widely adopted in the development community.

 

Q2. What are the features of React.js?

Ans: React.js is a popular JavaScript library for building user interfaces. It comes with a range of features that contribute to its popularity and effectiveness in developing modern, dynamic web applications. Here are some key features of React.js:

1. Virtual DOM:

• React uses a virtual DOM to optimize rendering. Changes to the virtual DOM are compared with the actual DOM, and only the necessary updates are applied. This improves performance by minimizing direct manipulation of the DOM.

2. JSX (JavaScript XML):

• JSX is a syntax extension for JavaScript recommended by React. It allows developers to write UI components using a syntax similar to XML or HTML within JavaScript code. JSX is then transformed into regular JavaScript by a build process.

3. Component-Based Architecture:

• React follows a component-based architecture where the UI is broken down into reusable and independent components. Components encapsulate their own state and logic, making it easier to manage and maintain complex UIs.

4. One-Way Data Binding:

• React enforces a one-way data flow, meaning data changes in a parent component will automatically update the child components, but not the other way around. This makes the application more predictable and easier to debug.

5. React Hooks:

• Introduced in React 16.8, hooks are functions that allow developers to use state and other React features in functional components. Hooks provide a more concise way to handle state and side effects in functional components.

6. Unidirectional Data Flow:

• React follows a unidirectional data flow, which makes it easier to understand how data changes propagate through the application. This architecture enhances predictability and maintainability.

7. Reusable Components:

• Components in React are designed to be reusable. This reusability simplifies development and reduces redundancy, as components can be used across different parts of the application.

8. Declarative Syntax:

• React uses a declarative syntax, allowing developers to describe how the UI should look, and React takes care of updating the DOM to match that description. This makes the code more readable and easier to maintain.

9. Efficient Updates:

• React optimizes updates by using a diffing algorithm on the virtual DOM, ensuring that only the necessary parts of the actual DOM are updated. This results in improved performance and a smoother user experience.

10. Large and Active Community:

• React has a large and active community of developers. This community contributes to the continuous improvement of React, provides valuable resources, and shares best practices and solutions.

11. Ecosystem and Tooling:

• React has a rich ecosystem of libraries and tools, including state management solutions (e.g., Redux), routing libraries (e.g., React Router), and development tools (e.g., React DevTools). Create React App is a popular tool for setting up a new React project quickly.

12. Server-Side Rendering (SSR):

• React supports server-side rendering, enabling faster initial page loads and better search engine optimization (SEO) by rendering components on the server before sending them to the client.

These features collectively make React.js a powerful and flexible library for building dynamic and interactive user interfaces in web applications.

 

Q3. How to embed two components in one component?

Ans: In React, you can embed or nest multiple components within another component by rendering them within the JSX of the parent component. This is achieved by placing the child components as tags or expressions within the JSX of the parent component. Here's an example to illustrate how you can embed two components within one component:

import React from 'react';

// Child Component 1
const ComponentOne = () => {
  return (
    <div>
      <h2>This is Component One</h2>
      {/* Add content or additional child components here */}
    </div>
  );
};

// Child Component 2
const ComponentTwo = () => {
  return (
    <div>
      <h2>This is Component Two</h2>
      {/* Add content or additional child components here */}
    </div>
  );
};

// Parent Component embedding ComponentOne and ComponentTwo
const ParentComponent = () => {
  return (
    <div>
      <h1>This is the Parent Component</h1>
      <ComponentOne /> {/* Embed ComponentOne */}
      <ComponentTwo /> {/* Embed ComponentTwo */}
    </div>
  );
};

export default ParentComponent;

In this example:

• ComponentOne and ComponentTwo are two child components.
• ParentComponent is the parent component that embeds both ComponentOne and ComponentTwo.
• Within the JSX of ParentComponent, <ComponentOne /> and <ComponentTwo /> are used to embed the child components.

When ParentComponent is rendered, it will include the content and structure of both ComponentOne and ComponentTwo. You can customize the parent and child components according to your application's requirements.

Remember to import and use the appropriate React library at the beginning of your files if you're not using JSX in a file with a .jsx extension.

import React from 'react';

This structure allows for easy composition and reuse of components, which is one of the key benefits of React's component-based architecture.

 

Q4. How to use forms in React.js?

Ans: In React.js, handling forms involves using controlled components, which means that the form elements, such as input fields and text areas, are controlled by the state of the React component. Here's a step-by-step guide on how to use forms in React:

Step 1: Create a Form Component

Create a new React component that represents your form. This component will have a state to manage the form data.

import React, { useState } from 'react';

const MyForm = () => {
  const [formData, setFormData] = useState({
    // Initialize form fields in state
    username: '',
    password: '',
  });

  // Handle form field changes
  const handleInputChange = (event) => {
    const { name, value } = event.target;
    setFormData({
      ...formData,
      [name]: value,
    });
  };

  // Handle form submission
  const handleSubmit = (event) => {
    event.preventDefault();
    // Perform actions with form data, e.g., submit to a server
    console.log('Form submitted:', formData);
  };

  return (
    <form onSubmit={handleSubmit}>
      {/* Form fields */}
      <label>
        Username:
        <input
          type="text"
          name="username"
          value={formData.username}
          onChange={handleInputChange}
        />
      </label>
      <br />
      <label>
        Password:
        <input
          type="password"
          name="password"
          value={formData.password}
          onChange={handleInputChange}
        />
      </label>
      <br />
      {/* Submit button */}
      <button type="submit">Submit</button>
    </form>
  );
};

export default MyForm;

Step 2: Controlled Components

• The form fields are controlled components, meaning their values are controlled by the state (formData in this case).
• The handleInputChange function is called whenever the value of an input field changes, updating the corresponding property in the state.

Step 3: Form Submission

• The form has an onSubmit event that triggers the handleSubmit function.
• In the handleSubmit function, you can perform actions like submitting data to a server.

Step 4: Rendering the Form Component

Include your form component in another component or in the main component where you want to render the form.

import React from 'react';
import MyForm from './MyForm'; // Import your form component

const App = () => {
  return (
    <div>
      <h1>React Form Example</h1>
      <MyForm /> {/* Render your form component */}
    </div>
  );
};

export default App;

Now, your form is ready to be used. As users input data, the state of the form component is updated, and you can handle the form submission as needed.

 

Q5. How to use Events in React.js? Give an example of using Events?

Ans: In React.js, handling events involves using React's synthetic event system. React wraps the native browser events with its synthetic events to provide a consistent interface across different browsers. Here's an example of how to use events in React:

import React, { useState } from 'react';

const EventExample = () => {
  const [message, setMessage] = useState('');

  // Event handler function for button click
  const handleClick = () => {
    setMessage('Button Clicked!');
  };

  // Event handler function for input change
  const handleChange = (event) => {
    setMessage(`Input Value: ${event.target.value}`);
  };

  return (
    <div>
      <h1>React Event Example</h1>

      {/* Button with a click event */}
      <button onClick={handleClick}>Click Me</button>

      <br />

      {/* Input with a change event */}
      <input type="text" onChange={handleChange} placeholder="Type something" />

      <br />

      {/* Display message based on events */}
      <p>{message}</p>
    </div>
  );
};

export default EventExample;

In this example:

• The handleClick function is an event handler for the click event on the button. It sets a message in the component's state.
• The handleChange function is an event handler for the change event on the input field. It updates the message based on the input value.
• Both event handlers are attached to their corresponding elements in the JSX using React's synthetic event system.

When the button is clicked or the input value changes, the event handlers are invoked, and the component's state is updated accordingly. The updated state triggers a re-render, and the updated message is displayed.

Remember that event names in React are camelCased (e.g., onClick, onChange) to align with JavaScript conventions. React abstracts away the differences in browser event systems, providing a unified way to handle events across various browsers.

 

Q6. What is Flux concept in React.js?

Ans: Flux is a design pattern used in React.js applications to manage the flow of data and state in a unidirectional manner. It was introduced by Facebook to address challenges in managing state in large and complex React applications. Flux complements the React component architecture and provides a clear and predictable way to handle application state.

The key components of the Flux architecture are:

1. Dispatcher:

• The Dispatcher is responsible for coordinating actions within the application. It acts as a central hub that receives actions and dispatches them to registered stores.

2.Action:

• Actions are simple objects that represent events or user interactions within the application. They carry data and a type that describes the action's purpose.

3. Store:

• Stores are responsible for managing the application state and logic. They listen to actions dispatched by the Dispatcher and update their state accordingly. Each store is associated with a specific domain of the application.

4. View (React Components):

• React components represent the views in the Flux architecture. They receive data from stores and send user actions to the Dispatcher. Components are kept simple and stateless, with their state managed by the stores.

The flow of data in Flux follows a unidirectional cycle:

1. A user interacts with a view, triggering an action.

2. The action is dispatched to the Dispatcher.

3. The Dispatcher notifies registered stores about the action.

4. Stores update their state based on the action.

5. Views receive updated data from the stores and re-render.

This unidirectional flow ensures predictability and maintainability in large applications, as changes in the application state are managed centrally through the Dispatcher and stores.

Libraries like Redux, inspired by the Flux architecture, have become popular in the React ecosystem. Redux simplifies the Flux pattern by combining the Dispatcher and stores into a single store with a more streamlined API. It is widely used for state management in React applications.

 

Q7. What is React.js-jsx?What are the advantages of using jsx?

Ans: JSX (JavaScript XML) is a syntax extension for JavaScript commonly used with React.js. JSX allows you to write HTML-like code within JavaScript, making it easier to describe and render UI components in a more declarative way. JSX code is transpiled into regular JavaScript before it is executed in the browser.

Here's an example of JSX code:

import React from 'react';

const MyComponent = () => {
  return (
    <div>
      <h1>Hello, JSX!</h1>
      <p>This is a React component using JSX.</p>
    </div>
  );
};

export default MyComponent;

Advantages of using JSX in React:

1. Readability:

JSX makes the code more readable and closely resembles the HTML structure of the UI. This helps developers understand the component's structure and intent at a glance.

2. Expressiveness:

JSX allows you to express complex UI structures and component hierarchies in a concise and expressive manner. It provides syntactic sugar for creating React elements, making code more intuitive.

3. Integration of JavaScript Expressions:

JSX seamlessly integrates with JavaScript expressions. You can embed JavaScript expressions within curly braces {} to dynamically compute values, making it easy to inject dynamic content into the UI.

const name = 'John';
return <p>Hello, {name}!</p>;

4. Compile-Time Checking:

JSX code is transpiled into JavaScript at compile time. This allows for early detection of syntax errors and provides better compile-time checking compared to concatenating strings or using template strings.

           5. Tooling Support:

JSX is well-supported by React development tools, providing features like syntax highlighting, code completion, and error checking. Popular code editors and integrated development environments (IDEs) often have dedicated support for JSX.

            6. JSX Prevents Injection Attacks:

JSX by default escapes any values embedded in it, preventing injection attacks. This helps mitigate security vulnerabilities like Cross-Site Scripting (XSS) attacks.

            7. Component Structure:

JSX aligns with the component-based structure of React. Each JSX element corresponds to a React component, making it easy to visualize and organize the structure of a React application.

While JSX is not mandatory in React, it has become a standard and widely adopted syntax due to its advantages in terms of code readability, expressiveness, and integration with JavaScript. It plays a crucial role in making React development more efficient and developer-friendly.

 

Q8. What are fragments in React.js?

Ans: In React.js, a fragment is a way to group multiple elements without introducing an additional parent container in the DOM. Fragments allow you to group children elements without adding unnecessary nodes to the HTML structure when rendering.

Prior to the introduction of fragments, developers often used div elements or other container elements to group multiple children in a component. However, this approach could lead to unnecessary DOM nesting and could affect styling and layout.

Fragments provide a cleaner and more lightweight solution to group elements without introducing an extra node in the DOM.

Using Fragments:

1. With the <React.Fragment> Syntax:

import React from 'react';

const MyComponent = () => {
  return (
    <React.Fragment>
      <h1>Heading 1</h1>
      <p>Paragraph 1</p>
      <p>Paragraph 2</p>
    </React.Fragment>
  );
};

export default MyComponent;

2. With the Short Syntax <>...</> (available in React 16.2 and later):

import React from 'react';

const MyComponent = () => {
  return (
    <>
      <h1>Heading 1</h1>
      <p>Paragraph 1</p>
      <p>Paragraph 2</p>
    </>
  );
};

export default MyComponent;

Using fragments with the short syntax <>...</> is more concise and often preferred, especially when you don't need to add any additional attributes to the fragment.

Advantages of Fragments:

1. Reduced DOM Nesting:

• Fragments help avoid unnecessary div elements or other containers in the DOM, reducing the nesting level and making the rendered HTML cleaner.

2. Avoiding CSS Styling Conflicts:

• Without unnecessary wrapper elements, styling and layout are less likely to be affected by additional container nodes.

3. Improved Readability:

• Fragments contribute to cleaner and more readable JSX code by allowing you to group elements without introducing unnecessary elements.

4. Reduced Memory Usage:

• Since fragments don't create additional nodes in the DOM, they contribute to a more memory-efficient rendering process.

Fragments are particularly useful when you want to return multiple elements from a component without introducing an artificial parent container. They help improve the overall structure and maintainability of your React components.

 

Q9. What is a Hook in React.js?

Ans: In React.js, a hook is a special function that allows functional components to access and interact with React features, such as state and lifecycle methods, that were traditionally only available in class components. Hooks were introduced in React 16.8 to provide a more concise and readable way to manage state and side effects in functional components.

The most commonly used hooks include:

1. useState:

• Allows functional components to manage local state.

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>
    </div>
  );
};

2. useEffect:

• Enables performing side effects in functional components, such as fetching data, subscribing to external events, or manually interacting with the DOM

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

const DataFetcher = () => {
  const [data, setData] = useState(null);

  useEffect(() => {
    // Fetch data from an API
    fetchData().then((result) => {
      setData(result);
    });
  }, []); // Empty dependency array runs the effect only once on mount

  return (
    <div>
      <p>Data: {data}</p>
    </div>
  );
};

3. useContext:

• Provides access to the value of a React context within a functional component.

import React, { useContext } from 'react';
import MyContext from './MyContext';

const MyComponent = () => {
  const contextValue = useContext(MyContext);

  return (
    <div>
      <p>Context Value: {contextValue}</p>
    </div>
  );
};

4. useReducer:

• A more advanced alternative to useState for managing complex state logic. It's often used when the next state depends on the previous state and requires more control over the update logic.

import React, { useReducer } from 'react';

const initialState = { count: 0 };

const reducer = (state, action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    default:
      return state;
  }
};

const Counter = () => {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
    </div>
  );
};

5. useMemo and useCallback:

• useMemo is used to memoize values to avoid unnecessary recalculations, and useCallback is used to memoize functions to prevent unnecessary re-creations.

import React, { useMemo, useCallback } from 'react';

const MemoizedComponent = () => {
  const memoizedValue = useMemo(() => calculateExpensiveValue(a, b), [a, b]);
  const memoizedFunction = useCallback(() => { /* function logic */ }, [dependency]);

  return (
    <div>
      <p>Memoized Value: {memoizedValue}</p>
      <button onClick={memoizedFunction}>Click Me</button>
    </div>
  );
};

These hooks, and others, provide a more functional and declarative approach to working with React features, making it easier to manage state, side effects, and other aspects of component behavior in functional components.

 

Q10. What is a single page application?

A Single Page Application (SPA) is a web application or website that interacts with the user by dynamically rewriting the current page, rather than loading entire new pages from the server. SPAs aim to provide a smoother and more seamless user experience by eliminating the need for full page reloads.

Key characteristics of Single Page Applications include:

1. Dynamic Content Loading:

• SPAs load content dynamically, typically using AJAX or Fetch API, to update the current page without requiring a full reload. Only the necessary data is fetched from the server, and the UI is updated in response to user interactions.

2. Client-Side Routing:

• SPAs often implement client-side routing, where navigation between different views or components is handled on the client side without triggering a request to the server. This is achieved using JavaScript frameworks or libraries that manage the application's state and routing.

3. Smooth User Experience:

• SPAs provide a more fluid and responsive user experience by avoiding the flicker and delay associated with traditional multi-page applications. Once the initial page is loaded, subsequent interactions typically feel faster and more immediate.

4. Rich User Interfaces:

• SPAs often leverage advanced client-side frameworks like React, Angular, or Vue.js to build rich and interactive user interfaces. These frameworks enable the creation of complex UI components, state management, and efficient updates.

5. Data Binding:

• SPAs commonly use two-way data binding or reactive programming to keep the UI in sync with the underlying data model. Changes in the model automatically update the view, and vice versa, without the need for explicit DOM manipulation.

6. Progressive Web App (PWA) Features:

• SPAs can incorporate Progressive Web App features, such as offline capabilities, push notifications, and responsiveness across various devices. PWAs enhance the overall user experience and provide an app-like feel.

7. Backend as a Service (BaaS) or API-Driven:

• SPAs often rely on Backend as a Service (BaaS) solutions or API-driven architectures. The server typically exposes a set of APIs that the client uses to fetch or update data, allowing for a clear separation between the frontend and backend.

8. Initial Loading and Caching:

• SPAs load a minimal HTML, CSS, and JavaScript bundle initially. Subsequent interactions may involve fetching additional assets or data as needed. Modern build tools and techniques like code splitting and caching contribute to optimizing performance.

Popular frameworks for building SPAs include React.js, Angular, Vue.js, and frameworks based on JavaScript libraries like React, such as Next.js. SPAs have become increasingly prevalent due to their ability to provide a more interactive and responsive user experience