Top 50 React Native and React JS Interview Questions
ReactJS Interview Questions 2021 CRS Info Solutions

Top 50 React Native and React JS Interview Questions

On February 16, 2022, Posted by , In Interview Questions, With Comments Off on Top 50 React Native and React JS Interview Questions
ReactJS Interview Questions 2021 CRS Info Solutions
ReactJS Interview Questions

Table Of Content

React Native is one of the most popular mobile application development frameworks. Launched in 2015 by Facebook, React Native’s popularity has increased tremendously. It is one of the most demanded technologies today. Though it has many similarities with React.js, there are many differences in React Native development. In this article, we will list down fifty React Native questions with answers.

1. What is ReactJS?

ReactJS is a JavaScript library developed by Facebook that helps in building user interfaces, particularly for single-page applications (SPAs). The library allows developers to build reusable UI components, which makes the development process more efficient and maintainable. I use ReactJS to create dynamic and interactive web applications where the state of the application can change without reloading the entire page. React achieves this through a virtual DOM, which updates the real DOM efficiently.

What makes React stand out is its component-based architecture, where the UI is broken down into smaller, manageable parts. These components can be reused and nested, which significantly speeds up development. It also integrates well with other frameworks and libraries, allowing for flexibility in application development. As a developer, ReactJS offers me a smooth and effective way to create complex user interfaces with minimal code.

2. What are components and their type in React?

In React, components are the building blocks of any React application. They are independent and reusable pieces of code that define how a user interface (UI) should appear. Components can be either functional components or class components, and each serves its own purpose.

Functional components are simpler and are typically used when there is no need to manage the state or lifecycle methods. They are just JavaScript functions that return JSX. For example:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

On the other hand, class components are more powerful and offer more features like state management and lifecycle methods. I typically use class components when my application needs to handle more complex logic, such as interacting with a database or API.

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

This class component extends React.Component and uses the render() method to return JSX. The this.props.name accesses the name prop passed to the component. Class components allow me to take advantage of features like lifecycle methods and internal state.

3. Explain the building blocks of React.

The main building blocks of React are components, props, state, and the virtual DOM. These elements work together to create dynamic and interactive user interfaces. Components are the fundamental units of React, where the UI is divided into small, manageable parts. Each component can hold its own state and receive data through props.

Props (short for properties) are used to pass data from one component to another. They are immutable, meaning that once a prop is set, it cannot be changed by the component receiving it. I often use props to pass data from a parent component to a child component.

State is another critical part of React components. Unlike props, state is mutable, meaning it can be changed within the component. It allows the component to keep track of its internal data and render the UI based on this state. React also uses the virtual DOM, a lightweight representation of the actual DOM, to ensure efficient updates and rendering when the state or props change.

4. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript, which looks very similar to HTML but is used with React to describe what the UI should look like. JSX allows me to write HTML-like code inside JavaScript, making it easier to create React components. Even though JSX looks like HTML, it is actually transpiled into JavaScript by tools like Babel before it gets rendered on the browser.

One of the key benefits of JSX is that it lets me embed expressions inside the markup. For example, I can use curly braces {} to insert variables, functions, or even other components inside JSX. Here’s a simple example:

const name = 'John';
const element = <h1>Hello, {name}!</h1>;

This JSX expression will be rendered as a h1 element with the text Hello, John!. I find it much more intuitive and efficient than mixing JavaScript with HTML in separate files.

5. How do browsers read JSX?

Browsers cannot directly understand JSX because it is not valid JavaScript. This is where a tool like Babel comes in, which transpiles JSX into regular JavaScript code that the browser can execute. When I write JSX code, Babel converts it into React.createElement calls, which are understood by React.

For example, the following JSX code:

const element = <h1>Hello, world!</h1>;

gets transpiled to the following JavaScript code:

const element = React.createElement('h1', null, 'Hello, world!');

React then takes this output and updates the virtual DOM, eventually syncing it with the actual DOM to reflect the changes. This process is what enables React to efficiently update the UI without a full page reload.

6. Explain props and state in React with differences.

In React, props and state are both essential concepts that help manage data within components, but they serve different purposes.

  • Props (short for properties) are immutable data passed from a parent component to a child component. I use props to provide data and event handlers from one component to another. Since props are read-only, they cannot be modified by the child component that receives them. This ensures that data flows from parent to child in a one-way direction.
function ChildComponent(props) {
  return <h1>{props.message}</h1>;
}

In this example, the ChildComponent receives a message prop from its parent and renders it inside an h1 element. The message prop cannot be changed by the ChildComponent.

  • State, on the other hand, is a mutable data storage that is managed within a component. I use state to store dynamic data that changes over time, such as user input or responses from an API. Unlike props, state can be updated using the this.setState method, causing the component to re-render.
class Counter extends React.Component {
  constructor() {
    super();
    this.state = { count: 0 };
  }

  increment() {
    this.setState({ count: this.state.count + 1 });
  }

  render() {
    return <h1>{this.state.count}</h1>;
  }
}

In this example, the Counter component maintains its own state with a count variable. The state is updated when the increment() method is called, and the component re-renders to display the updated count.

7. What is state in React?

State in React is a mechanism used to store and manage data that can change over time in a component. Unlike props, which are passed down from parent to child, state is local to a component and can be changed by that component. When the state changes, React automatically re-renders the component to reflect the updated state.

For example, in a class component, I can define state in the constructor like this:

class MyComponent extends React.Component {
  constructor() {
    super();
    this.state = {
      name: "John",
    };
  }

  render() {
    return <h1>{this.state.name}</h1>;
  }
}

The state here holds a name value, and if I change it using this.setState, React will update the UI. This enables dynamic interaction with the user and real-time updates.

In functional components, React hooks like useState allow me to add state functionality. For example:

function MyComponent() {
  const [name, setName] = useState("John");

  return <h1>{name}</h1>;
}

Here, useState creates a state variable name, and setName is the function used to update its value.

8. Explain props in React?

Props (short for properties) are read-only data passed from a parent component to a child component in React. They allow me to pass information down the component tree. Props are used to transfer values like strings, numbers, objects, and even functions, which the child component can use to render content or perform actions. However, props cannot be modified by the child component that receives them.

For example, if I want to display a greeting message in a child component, I can pass a message prop from the parent:

function ParentComponent() {
  return <ChildComponent message="Hello, React!" />;
}

function ChildComponent(props) {
  return <h1>{props.message}</h1>;
}

In this example, the ParentComponent passes the message prop to ChildComponent, and the ChildComponent renders the message inside an h1 tag.

Props enable a one-way data flow in React, making it predictable and easy to debug.

9. What is virtual DOM in React?

The virtual DOM is a lightweight copy of the actual DOM that React maintains in memory. When the state of a component changes, React updates the virtual DOM first. It then compares this updated virtual DOM with the previous one using a process called reconciliation. Once the differences (or “diffs”) are identified, React updates only the changed parts of the real DOM. This process significantly improves performance, as it avoids full page reloads.

Here’s how the virtual DOM works:

  • When a component’s state or props change, React creates a new virtual DOM.
  • React then compares the new virtual DOM with the old one using an algorithm called diffing.
  • Finally, it updates only the parts of the real DOM that have changed.

This approach is more efficient than directly manipulating the real DOM, as it reduces the number of reflows and repaints, leading to a faster and more responsive UI.

10. What is this.setState function in React?

The this.setState function in React is used to update the state of a component. When I call this.setState, React automatically re-renders the component with the new state values. This allows the UI to reflect changes dynamically.

For example, in a class component, if I want to update the count state, I would use this.setState like this:

class Counter extends React.Component {
  constructor() {
    super();
    this.state = { count: 0 };
  }

  increment() {
    this.setState({ count: this.state.count + 1 });
  }

  render() {
    return <h1>{this.state.count}</h1>;
  }
}

In this example, the increment() method updates the count state, and React triggers a re-render of the component with the new count value. The UI will display the updated count after each increment.

11. Explain one way data binding in React?

One-way data binding in React refers to the flow of data from a parent component to a child component via props. The key idea behind one-way data binding is that data flows in one direction, making it easier to track changes and understand how data moves through an application.

For example, if I have a parent component with state and a child component that receives this state as a prop, the data flows from the parent to the child:

function ParentComponent() {
  const [name, setName] = useState("John");
  return <ChildComponent name={name} />;
}

function ChildComponent(props) {
  return <h1>Hello, {props.name}</h1>;
}

In this example, the name state in the ParentComponent is passed down as a prop to the ChildComponent. The child component cannot modify the name prop, ensuring a predictable flow of data. This is the essence of one-way data binding.

12. Explain the use of render method in React?

The render method is a crucial part of class components in React. It is responsible for returning the JSX (or React elements) that describe what should be rendered on the UI. Whenever a component’s state or props change, React calls the render method again to update the UI accordingly.

Here’s an example:

class MyComponent extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

In this example, the render() method returns a JSX element that displays a greeting message using the name prop. Each time the state or props change, React re-renders the component by calling the render method and updating the DOM.

The render method must return a single React element, which can be a JSX element or another component.

13. Explain the difference between React and Angular?

React and Angular are both popular JavaScript frameworks, but they differ significantly in terms of architecture and usage.

  • React is a library focused mainly on building user interfaces, particularly the view layer in the MVC (Model-View-Controller) architecture. It is component-based, and I can integrate it with other libraries for routing, state management, and more. React is more flexible and allows me to choose the tools that fit my needs. It uses a virtual DOM to enhance performance.
  • Angular, on the other hand, is a full-fledged framework that offers an end-to-end solution for building web applications. It provides a set of tools for routing, state management, and even HTTP requests. Angular uses two-way data binding, which means the model and view are always synchronized. This makes Angular more opinionated, offering a more structured approach to building applications.

React is more focused on the UI, while Angular provides a complete solution for building large-scale applications.

14. What is a key in React?

In React, the key prop is used to identify elements in a list and helps React efficiently update and render components. When rendering a list of elements, I should assign a unique key to each item to enable React to track and manage them during updates. Using keys helps improve performance by minimizing unnecessary re-renders.

For example:

const list = ['Apple', 'Banana', 'Cherry'];

function ListComponent() {
  return (
    <ul>
      {list.map((item, index) => (
        <li key={index}>{item}</li>
      ))}
    </ul>
  );
}

In this example, each li element has a key prop set to its index in the list. This allows React to track the items and efficiently update the list when necessary.

15. How to write a comment in React?

In React, comments are written in a similar way to regular JavaScript comments. However, when writing comments inside JSX, I must use curly braces {} to embed JavaScript expressions.

  • For single-line comments:
// This is a single-line comment in React
  • For multi-line comments inside JSX:
function MyComponent() {
  return (
    <div>
      {/* This is a multi-line comment in JSX */}
      <h1>Hello, World!</h1>
    </div>
  );
}

In this example, the comment inside the curly braces is ignored during rendering and does not affect the UI. It’s a useful way to document and explain parts of your JSX code.

16. Explain the lifecycle methods of components.

Lifecycle methods in React are special methods that allow me to run code at specific points in a component’s life cycle. These methods are used in class components to manage tasks like setting up data, making API calls, or cleaning up resources.

The main lifecycle phases are:

  1. Mounting: This phase involves setting up the component. The methods that run in this phase are:
    • constructor(): Initializes the component.
    • getDerivedStateFromProps(): Used to update state based on props.
    • render(): Returns JSX for the component.
    • componentDidMount(): Called after the component has been rendered to the screen.
  2. Updating: Happens when a component’s state or props change. The methods in this phase are:
    • getDerivedStateFromProps()
    • shouldComponentUpdate(): Determines if the component should re-render.
    • render()
    • componentDidUpdate(): Runs after a re-render, useful for performing tasks after changes.
  3. Unmounting: When the component is removed from the DOM. The method in this phase is:
    • componentWillUnmount(): Used to clean up resources like event listeners.

These lifecycle methods help in optimizing performance and managing component state effectively.

17. Explain the methods used in the mounting phase of components.

The mounting phase is when a React component is created and inserted into the DOM. During this phase, several lifecycle methods are executed:

  1. constructor(props): The constructor method is called first when the component is created. It is used to initialize the state and bind event handlers.
constructor(props) {
  super(props);
  this.state = { name: "React" };
}
  1. getDerivedStateFromProps(props, state): This method is called before every render, including the first one. It is used to update the state based on the props that the component receives.
static getDerivedStateFromProps(nextProps, nextState) {
  if (nextProps.name !== nextState.name) {
    return { name: nextProps.name };
  }
  return null;
}
  1. render(): This is the most important lifecycle method and is required in every class component. It returns JSX, which describes what the UI should look like.
  2. componentDidMount(): After the component has been mounted to the DOM, this method is called. It’s useful for making API calls, setting up subscriptions, or triggering animations.
componentDidMount() {
  console.log("Component has mounted.");
}

These methods allow me to initialize the component, receive props, and run code after the component is mounted, such as making API calls or subscribing to data streams.

18. What is React Fragments?

React Fragments allow me to group multiple elements without adding extra nodes to the DOM. In normal React, a component must return a single root element. However, sometimes it’s convenient to return multiple elements without wrapping them in a parent div or other element.

For example:

function MyComponent() {
  return (
    <>
      <h1>Hello</h1>
      <p>Welcome to React Fragments!</p>
    </>
  );
}

In this example, I use the shorthand syntax <> and </> to return multiple elements. React automatically wraps them in a fragment without creating an additional div. This is useful for keeping the DOM structure clean and avoiding unnecessary wrapper elements.

19. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript that allows me to write HTML-like code within JavaScript. It makes the process of creating UI components easier and more intuitive. JSX is not required to use React, but it is widely adopted because of its clear and concise syntax.

JSX allows me to write UI elements using HTML-like syntax, and then React transforms these JSX elements into actual DOM elements. For example:

function MyComponent() {
  return <h1>Hello, World!</h1>;
}

In this example, I write JSX code directly inside the return statement of a function component. React converts this JSX into JavaScript code that manipulates the DOM. One important thing to remember is that JSX elements must be wrapped in a single parent tag, either a div, a Fragment, or a custom component.

20. What are hooks in React?

Hooks are functions in React that allow me to use state and other React features in functional components. Prior to hooks, state and lifecycle methods were only available in class components. Hooks make it easier to reuse logic and keep functional components more powerful and concise.

There are several built-in hooks in React, including:

  • useState: Adds state to functional components.
  • useEffect: Allows side effects like data fetching and subscriptions in functional components.
  • useContext: Enables the use of context in functional components.
  • useRef: Provides a way to access DOM elements directly.

Hooks allow me to use the power of state, side effects, and context in a simple and functional way, making the code more readable and maintainable.

21. Explain the useState hook in React?

The useState hook is a built-in hook in React that allows me to add state to functional components. It provides a way to define state variables and update their values, triggering re-renders when the state changes.

Here’s how I use it:

import React, { useState } from 'react';

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

  const increment = () => setCount(count + 1);

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

In this example, I use useState(0) to initialize the count state variable to 0. The setCount function updates the state whenever the user clicks the “Increment” button. Each time setCount is called, React re-renders the component with the updated state.

22. Explain the useEffect hook in react?

The useEffect hook in React is used to perform side effects in functional components. Side effects can include operations like fetching data from an API, subscribing to events, or modifying the DOM. The useEffect hook runs after the render phase and can be set to run after every render or only when certain dependencies change.

Here’s how to use it:

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

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

  useEffect(() => {
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(data => setData(data));
  }, []); // Empty dependency array ensures the effect runs only once (like componentDidMount)

  return <div>{data ? data : "Loading..."}</div>;
}

In this example, the useEffect hook fetches data from an API when the component is first mounted. The empty dependency array [] ensures the effect runs only once, similar to componentDidMount. If I included specific variables inside the array, the effect would run whenever those variables change.

23. What is the difference between functional and class components in React?

The key differences between functional and class components in React are as follows:

  1. State Management:
    • Class components manage state using the this.state object and this.setState() method.
    • Functional components use hooks like useState to manage state.
  2. Lifecycle Methods:
    • Class components have lifecycle methods such as componentDidMount, componentDidUpdate, and componentWillUnmount.
    • Functional components rely on hooks like useEffect to handle side effects and component life cycle events.
  3. Syntax:
    • Class components require the class keyword and extend React.Component.
    • Functional components are simpler functions that directly return JSX.
// Class Component Example
class MyComponent extends React.Component {
  render() {
    return <h1>Hello from Class Component</h1>;
  }
}

// Functional Component Example
function MyComponent() {
  return <h1>Hello from Functional Component</h1>;
}

In modern React development, functional components are preferred due to the simplicity and power of hooks.

24. How to create an event in React?

In React, events are handled using a syntax that is similar to HTML but with some key differences. React events are camelCase, and I pass a function as the event handler, rather than a string.

Here’s an example of handling a click event:

import React from 'react';

function MyList() {
  const items = ["Apple", "Banana", "Cherry"];

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

In this example, the handleClick function is called when the button is clicked. The event handler is passed to the onClick attribute in camelCase. This allows me to handle events efficiently in React.

26. What is conditional rendering in React?

Conditional rendering in React refers to the technique where different components or elements are rendered based on certain conditions. This allows me to dynamically change the UI by rendering different elements based on the state or props of a component. In React, I can achieve conditional rendering using JavaScript operators like if, ternary, or logical && operators.

For example, here’s how I can use a ternary operator for conditional rendering:

function MyComponent({ isLoggedIn }) {
  return (
    <div>
      {isLoggedIn ? <h1>Welcome Back!</h1> : <h1>Please Log In</h1>}
    </div>
  );
}

In this example, the UI shows a message based on the value of isLoggedIn. If the user is logged in, the message “Welcome Back!” is rendered; otherwise, it shows “Please Log In.” This flexibility in rendering allows React components to be highly dynamic based on user interactions or data.

27. What is React Router?

React Router is a standard library for routing in React applications. It enables navigation between different components or views within a single-page application (SPA). React Router helps in managing URL paths and displaying appropriate components based on the current route.

With React Router, I can define routes and link them to different components. For example, I can use Route and Link components to navigate between pages:

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

function App() {
  return (
    <Router>
      <div>
        <nav>
          <Link to="/">Home</Link>
          <Link to="/about">About</Link>
        </nav>
        <Route path="/" exact component={Home} />
        <Route path="/about" component={About} />
      </div>
    </Router>
  );
}

In this example, the Link component is used to navigate between the “Home” and “About” pages, and the Route component renders the corresponding component based on the URL path.

28. Explain the components of a react-router.

React Router consists of several key components that work together to enable routing in a React application. The main components include:

  1. Router: The main container that holds the routes and provides routing context. It determines how to render the UI based on the URL path.
    • Example: <BrowserRouter> is commonly used as the router for web apps.
  2. Route: This component is used to define the mapping between a URL path and the component that should be rendered.
    • Example: <Route path="/home" component={Home} />
  3. Link: This component provides declarative navigation between different routes without reloading the page, thus enabling single-page application behavior.
    • Example: <Link to="/about">Go to About</Link>
  4. Switch: This component is used to group multiple Route components and render only the first route that matches the current URL path.
    • Example: <Switch><Route path="/about" component={About} /></Switch>
  5. Redirect: This component allows me to redirect users to a different route programmatically, typically used for handling authentication or redirects.

These components work together to create a seamless navigation experience in a React application, making it easy to manage URL paths and render corresponding components.

29. What is Flux architecture in Redux?

Flux is an application architecture used for building client-side web apps. It works on a unidirectional data flow, meaning the data flows in one direction from the dispatcher to the view. Redux, a popular state management library, is based on Flux principles but with a simpler and more predictable approach.

The core components of Flux architecture include:

  1. Actions: These are simple JavaScript objects that contain the data and type of the event (such as ADD_TODO).
  2. Dispatcher: A central hub that manages the flow of data and sends actions to stores.
  3. Stores: These are containers for the application’s state and logic. They listen for actions and update themselves based on those actions.
  4. Views (Components): React components that listen to store changes and re-render when the state is updated.

Flux provides a predictable structure that helps in managing complex state across large applications.

30. What is React-Redux?

React-Redux is a library that provides bindings to integrate React with Redux. It allows React components to connect to the Redux store, enabling them to access and update the application state stored in Redux.

React-Redux provides two main functions to connect components with Redux:

  1. connect(): This higher-order component (HOC) is used to connect React components to the Redux store. It allows components to receive state and dispatch actions.
  2. Provider: This component makes the Redux store available to all components in the application via context, allowing components to interact with the store.
import { Provider } from 'react-redux';
import { createStore } from 'redux';
import App from './App';

const store = createStore(reducer);

function Root() {
  return (
    <Provider store={store}>
      <App />
    </Provider>
  );
}

In this example, the Provider component wraps the application and provides the store to all components, allowing them to access the Redux state and dispatch actions.

31. What are benefits of using React-Redux?

React-Redux simplifies state management in React applications, especially in large-scale projects. Some benefits of using React-Redux include:

  1. Centralized State: Redux stores all the application state in a single, centralized store, which makes managing and debugging state easier.
  2. Predictable State: With Redux’s unidirectional data flow, it is easy to track and predict how state changes in response to actions, improving maintainability.
  3. Separation of Concerns: React-Redux keeps UI logic and state management separate, making the codebase cleaner and more modular.
  4. Optimized Performance: By using connect() and mapStateToProps(), React-Redux optimizes the re-rendering process, ensuring that only the components that need to update do so.

By using React-Redux, I can manage state in a more predictable and efficient manner, which is especially important for large applications with complex state interactions.

32. Explain the core components of React-Redux?

React-Redux consists of several core components that facilitate the connection between React components and the Redux store:

  1. Provider: The Provider component is the top-level component in a React-Redux app. It makes the Redux store accessible to all components within the app.
    • Example: <Provider store={store}><App /></Provider>
  1. connect(): This higher-order component connects a React component to the Redux store. It allows the component to access state and dispatch actions.
    • Example:
const mapStateToProps = (state) => ({
  count: state.count
});

export default connect(mapStateToProps)(Counter);
  1. store: The Redux store holds the application state and dispatches actions to update it. It is created using the createStore() function.
    • Example:
const store = createStore(rootReducer);

These core components help manage the state of the application and enable React components to interact with the store efficiently.

33. How can we combine multiple reducers in React?

In Redux, when the state is large and complex, I can split the state into multiple smaller reducers, each managing its own slice of the state. To combine multiple reducers into a single root reducer, I use the combineReducers() function provided by Redux.

Here’s an example:

import { combineReducers } from 'redux';

const rootReducer = combineReducers({
  counter: counterReducer,
  user: userReducer,
});

const store = createStore(rootReducer);

In this example, the combineReducers() function combines the counterReducer and userReducer into a single rootReducer, which can then be passed to the createStore() function to create the Redux store. This helps in managing different parts of the state separately while keeping the code modular.

34. What is Context API?

The Context API is a feature in React that allows me to share data across the component tree without having to pass props manually through every level. It provides a way to create global variables that can be accessed by any component, regardless of its position in the component tree.

To use the Context API, I need to:

  1. Create a context using React.createContext().
  2. Provide the context to the component tree using the Provider component.
  3. Consume the context in any component using useContext() or the Consumer component.

35. Explain provider and consumer in ContextAPI?

The Provider and Consumer are two essential components of the React Context API.

  1. Provider: The Provider component makes the context value available to all components in the component tree. I wrap my entire application (or part of it) with the Provider to share the context with all nested components.
const MyContext = React.createContext();

function App() {
  return (
    <MyContext.Provider value="Hello, World!">
      <MyComponent />
    </MyContext.Provider>
  );
}
  1. Consumer: The Consumer component allows me to access the context value within a component. It provides a function that takes the context value as an argument and returns the component that should be rendered.
<MyContext.Consumer>
  {value => <h1>{value}</h1>}
</MyContext.Consumer>

Alternatively, I can use the useContext() hook in functional components to directly access the context value:

const value = useContext(MyContext);

The Provider is used to provide the context, and the Consumer (or useContext()) is used to access the context in components.

36. Explain CORS in React?

CORS (Cross-Origin Resource Sharing) is a security feature implemented by browsers that restricts web applications from making requests to a domain other than the one that served the web page. When building a React application that interacts with an API hosted on a different domain or port, the browser may block the request due to CORS policy. This can be resolved by ensuring that the API server includes the appropriate CORS headers to allow the request from the React app’s origin.

For example, when using the fetch API in React, if the server does not enable CORS, it might result in an error like:

Access to XMLHttpRequest at 'http://api.example.com/data' from origin 'http://localhost:3000' has been blocked by CORS policy.

To fix this, the server needs to send the Access-Control-Allow-Origin header with a proper value (like * or the specific domain making the request).

37. What is Axios and how to use it in React?

Axios is a popular JavaScript library for making HTTP requests. It simplifies the process of sending requests and handling responses, including handling CORS and request/response transformations. Axios is often used in React for fetching data from external APIs.

Here’s an example of how I can use Axios in React:

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

function App() {
  const [data, setData] = useState([]);

  useEffect(() => {
    axios.get('https://api.example.com/data')
      .then(response => {
        setData(response.data);
      })
      .catch(error => {
        console.error('There was an error fetching data:', error);
      });
  }, []);

  return (
    <div>
      <h1>Data</h1>
      <ul>
        {data.map(item => <li key={item.id}>{item.name}</li>)}
      </ul>
    </div>
  );
}

In this example, Axios sends a GET request to fetch data from an API, and the data is then stored in the component’s state and displayed. Axios makes it easy to handle errors and asynchronous requests.

38. How to use styles in ReactJS?

There are several ways to use styles in ReactJS, such as:

  • Inline Styles: I can apply styles directly in JSX by passing an object to the style attribute.
const divStyle = {
  backgroundColor: 'blue',
  color: 'white',
  padding: '10px',
};

function App() {
  return <div style={divStyle}>Hello, world!</div>;
}
  • CSS Stylesheets: I can import external CSS files into my React components.
import './App.css';

In App.css, I can write traditional CSS:

.app {
  background-color: blue;
  color: white;
  padding: 10px;
}

Then, apply it to the component:

function App() {
  return <div className="app">Hello, world!</div>;
}
  • CSS Modules: This helps scope styles to a particular component, preventing global styles from affecting other components.
import styles from './App.module.css';

function App() {
  return <div className={styles.app}>Hello, world!</div>;
}
  • Styled-components: For more complex styling, I can use a CSS-in-JS solution like styled-components (explained in the next question).

39. Explain styled components in React?

Styled-components is a CSS-in-JS library that allows me to write traditional CSS syntax directly in my JavaScript code. It helps in creating styled React components that are scoped to a specific component. This approach eliminates the need for external CSS files and provides better componentization.

Here’s an example:

import styled from 'styled-components';

const Button = styled.button`
  background-color: blue;
  color: white;
  padding: 10px;
  border-radius: 5px;
`;

function App() {
  return <Button>Click Me</Button>;
}

In this example, the Button component is styled directly using template literals. The benefit is that the styles are scoped to that specific component, ensuring they don’t leak out to other components. Styled-components also allow for dynamic styling based on props and theme.

40. What is prop drilling and its disadvantages?

Prop drilling refers to the process of passing data from a parent component to a deeply nested child component via intermediate components. While this technique works well in simple applications, it can become cumbersome and inefficient when the component tree is large.

For example, if I have a component A and I need to pass a prop to a child component C, but B is in between, I have to pass the prop through B even though B doesn’t use it. This can lead to:

  1. Unnecessary re-renders: Intermediate components may re-render unnecessarily.
  2. Harder maintenance: As the application grows, prop drilling makes it harder to track which components are receiving props and leads to more complex code.
  3. Reduced readability: It becomes unclear where data is coming from when multiple layers of props are passed through.

Solutions like the Context API or state management libraries (like Redux) can mitigate the problem by providing a more direct way to share data across components.

41. What is custom hooks in React?

Custom hooks in React are functions that allow me to reuse stateful logic across multiple components. React’s built-in hooks like useState and useEffect can be combined to create custom hooks that encapsulate complex behavior and make it easier to share across different components.

Here’s an example of a custom hook that fetches data from an API:

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

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

  useEffect(() => {
    axios.get(url)
      .then(response => {
        setData(response.data);
        setLoading(false);
      })
      .catch(error => {
        console.error('Error fetching data:', error);
      });
  }, [url]);

  return { data, loading };
}

I can use this custom hook in any component like so:

function App() {
  const { data, loading } = useFetchData('https://api.example.com/data');

  if (loading) return <p>Loading...</p>;

  return <div>{JSON.stringify(data)}</div>;
}

Custom hooks allow me to encapsulate logic and keep components clean and readable.

42. What is React Developer Tool?

React Developer Tools is a browser extension (available for both Chrome and Firefox) that provides an interface for inspecting the React component tree. It allows me to:

  1. Inspect the component hierarchy, including props and state.
  2. Track component re-renders and performance.
  3. Modify state and props in real-time for debugging and testing.

React Developer Tools is invaluable for debugging React applications as it provides insights into the structure and behavior of the React components during runtime.

43. What is Flux architecture in Redux?

Flux is an architectural pattern for building client-side web applications with a unidirectional data flow. Redux is a simplified version of Flux, but the core principles are similar:

  1. Actions: These are objects that represent events or changes in the application (e.g., ADD_TODO).
  2. Dispatcher: A central entity that handles the actions and forwards them to the stores.
  3. Stores: These maintain the application’s state and logic, listening for actions and updating the state accordingly.
  4. Views: React components that subscribe to the stores and render the UI based on the state.

Flux ensures that data flows in a single direction, improving predictability and maintainability. Redux is based on Flux but simplifies the dispatcher and state management.

44. How to optimize React code?

To optimize React code for performance, I can apply several strategies:

  1. Memoization: Using React.memo() for functional components and PureComponent for class components to prevent unnecessary re-renders.
const MyComponent = React.memo(function MyComponent(props) {
  return <div>{props.name}</div>;
});
  1. Lazy loading: Implementing code splitting using React.lazy() and Suspense to load components only when needed.
const LazyComponent = React.lazy(() => import('./LazyComponent'));
  1. Efficient Lists: Using virtualization libraries like react-window or react-virtualized to efficiently render large lists of items without rendering all of them at once.
  2. Avoid Inline Functions: Inline functions in JSX can cause unnecessary re-renders because they are recreated every time the component renders. It’s better to define functions outside of JSX.
  3. Batching State Updates: Grouping multiple state updates into a single render to reduce re-renders.

45. What is the difference between useRef and createRef in React?

Both useRef and createRef are used to create references to DOM elements or class components, but they differ in their usage:

  1. createRef: This is used in class components. It creates a reference that persists across renders but doesn’t change over time.
class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.myInput = React.createRef();
  }

  render() {
    return <input ref={this.myInput} />;
  }
}
  1. useRef: This is used in functional components and works similarly to createRef but persists across re-renders. It’s commonly used for accessing DOM elements or storing mutable values. 
function MyComponent() {
  const myInput = useRef();

  return <input ref={myInput} />;
}

The key difference is that useRef is used in functional components and can be used to hold mutable values (that do not trigger re-renders), while createRef is used in class components and is generally meant for DOM elements.

46. Write a program to create a counter with increment and decrement?

Here’s an example of how I can create a counter in React that allows me to increment and decrement the value:

import React, { useState } from 'react';

function Counter() {
  // State to keep track of the counter value
  const [count, setCount] = useState(0);

  // Function to increment the counter
  const increment = () => setCount(count + 1);

  // Function to decrement the counter
  const decrement = () => setCount(count - 1);

  return (
    <div>
      <h1>Counter: {count}</h1>
      <button onClick={increment}>Increment</button>
      <button onClick={decrement}>Decrement</button>
    </div>
  );
}

export default Counter;

In this example, I use the useState hook to track the counter value. I define two functions, increment and decrement, to update the counter value. The buttons call these functions on click, updating the displayed counter value.

47. Explain why and how to update the state of components using a callback?

Updating state using a callback is important because state updates in React are asynchronous. React batches state updates for performance optimization, meaning that the state may not immediately reflect changes after calling setState or useState in functional components.

Using a callback function helps to manage state updates more predictably when the new state depends on the previous state. This ensures that I’m working with the most current state when making updates.

Here’s an example using a callback with setState in a class component:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }

  increment() {
    this.setState((prevState) => ({ count: prevState.count + 1 }));
  }

  render() {
    return (
      <div>
        <h1>Count: {this.state.count}</h1>
        <button onClick={() => this.increment()}>Increment</button>
      </div>
    );
  }
}

In this example, the callback (prevState) => ({ count: prevState.count + 1 }) ensures that the state update depends on the previous state, and React uses the most up-to-date value to calculate the new state.

48. What is React-Material UI?

React-Material UI (now MUI) is a popular React component library that implements Google’s Material Design principles. It provides a set of pre-built, customizable, and accessible components like buttons, sliders, dialogs, grids, and many more, allowing developers to build beautiful, consistent user interfaces efficiently.

Here’s an example of using MUI in React to create a simple button:

import React from 'react';
import Button from '@mui/material/Button';

function App() {
  return (
    <div>
      <Button variant="contained" color="primary">
        Click Me
      </Button>
    </div>
  );
}

export default App;

This code imports the Button component from MUI, which comes with default styles based on Material Design principles. MUI helps in reducing development time while ensuring that the UI follows modern design patterns.

49. Explain the steps to create a React application and print “Hello World”?

Here are the steps to create a React application and print “Hello World”:

  • Install Node.js and npm: Make sure that Node.js and npm are installed on your machine. You can download them from the official website Node.js.
npx create-react-app hello-world
cd hello-world
  • Create the “Hello World” component: In the src folder, open the App.js file. Replace its content with the following code to display “Hello World”:
function App() {
  return (
    <div>
      <h1>Hello World</h1>
    </div>
  );
}

export default App;
  • Start the React application: In the terminal, run the following command to start the development server: 
npm start

This will open the app in your default browser, and you should see “Hello World” displayed on the screen.

50. What is MVC architecture?

MVC (Model-View-Controller) is a software design pattern that separates an application into three interconnected components. This separation helps in organizing code efficiently, especially for large applications, and improves maintainability.

  1. Model: Represents the data and business logic of the application. It manages the data, responds to requests for information, and updates the View if the data changes.
    • For example, a Model could be a User class that stores and manipulates user information.
  2. View: Represents the UI (User Interface) of the application. It displays data from the Model to the user and sends user input to the Controller.
    • The View could be a page that displays user details or products in an online store.
  3. Controller: Acts as an intermediary between the Model and View. It handles user input, processes it (possibly updating the Model), and returns the updated data to the View.
    • The Controller could be a function that processes user login and updates the User model based on user input.

Here’s a simplified flow:

  • The Controller receives input from the user (e.g., clicking a button).
  • It updates the Model (e.g., saves data or performs a calculation).
  • The Model notifies the View of any changes.
  • The View is updated to reflect the new data.

MVC helps to keep concerns separate, making the application easier to develop, test, and maintain.

Conclusion

Mastering the top 50 React Native and React JS interview questions is a game-changer for anyone looking to excel in the world of modern web and mobile development. These questions dive deep into the heart of React and React Native, covering everything from foundational concepts like JSX, components, and state management to advanced features such as React Router, Redux, and React Hooks. By preparing with these topics, you not only gain the knowledge to answer interview questions confidently but also build the skills necessary to tackle real-world development challenges efficiently.

A strong grasp of these React Native and React JS interview questions positions you as a highly capable candidate in the competitive job market. The more you familiarize yourself with these concepts, the more you stand out in interviews and the better equipped you’ll be to handle complex development tasks. Whether you’re aiming for a React JS front-end role or a React Native mobile development position, this preparation will significantly enhance your ability to impress employers and succeed in any React-related job.

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