React interview Question

100 Reactjs Question to answer with code example

  1. What is React and how does it differ from other front-end frameworks?

React is a JavaScript library for building user interfaces. It uses a component-based architecture and virtual DOM to update the UI efficiently. Unlike other front-end frameworks, React does not have a built-in router or state management system, but it can be easily integrated with other libraries and tools to provide these features.

  1. How do you create a new React component?

You can create a new React component by defining a JavaScript function or class that returns a JSX element. Here’s an example:

function MyComponent() {
  return (
    <div>
      <h1>Hello, world!</h1>
    </div>
  );
}
  1. What is JSX and how does it work?

JSX is a syntax extension for JavaScript that allows you to write HTML-like code in your JavaScript files. It is not a separate templating language or a HTML parser, but a syntax transform that generates plain JavaScript code at compile time. Here’s an example of JSX:

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

When this code is compiled, it generates the following JavaScript code:

const element = React.createElement("h1", null, "Hello, world!");
  1. How do you render a React component in the browser?

To render a React component in the browser, you need to use the `ReactDOM.render()` method. Here’s an example:

const element = <h1>Hello, world!</h1>;
ReactDOM.render(element, document.getElementById("root"));
  1. How do you pass props to a React component?

You can pass props to a React component by including them as attributes in the JSX element. Here’s an example:

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

const element = <Greeting name="John" />;
  1. How do you access props inside a React component?

You can access props inside a React component by using the `props` object. Here’s an example:

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}
  1. What is state in React and how does it differ from props?

State in React is a way to store data that can change over time, typically as a result of user interaction or server responses. Unlike props, which are passed down from parent to child components and cannot be changed by the child, state is local to a component and can be updated using the `setState()` method. When state is updated, React re-renders the component and its children to reflect the new state.

  1. How do you set state in a React component?

You can set state in a React component by calling the `setState()` method with a new state object. Here’s an example:

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

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

  render() {
    return (
      <div>
        <p>Count: {this.state.count}</p>
        <button onClick={() => this.handleClick()}>Click me</button>
      </div>
    );
  }
}
  1. How do you update state in a React component?

You can update state in a React component by calling the `setState()` method with a new state object.

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

Props are used to pass data from a parent component to a child component, while state is used to manage data that can change over time within a component. Props are immutable and cannot be changed by the child component, while state can be changed using the `setState()` method. Additionally, props are passed down from parent to child components, while state is local to a component and cannot be accessed by its parent or siblings.

  1. What is the render() method in React?

The `render()` method is a required method in a React component that returns a React element, which describes what should be displayed on the screen.

It is called automatically when the component is mounted or updated, and should be a pure function that does not modify component state or interact with the outside world.

  1. What is a React component’s lifecycle?

A React component’s lifecycle refers to the different stages in its existence, from creation to destruction. There are three main phases in a component’s lifecycle:

1. Mounting: The component is created and added to the DOM.

2. Updating: The component receives new props or state and is re-rendered

. 3. Unmounting: The component is removed from the DOM. Each phase has its own lifecycle methods that can be used to perform actions at different points in the component’s existence.

  1. What is the componentDidMount() lifecycle method in React?

The `componentDidMount()` lifecycle method is called after a component has been mounted and added to the DOM. It is typically used to perform any initialization that requires access to the DOM, such as fetching data from a server or setting up event listeners. Here’s an example:

class MyComponent extends React.Component {
  componentDidMount() {
    console.log("Component mounted");
  }

  render() {
    return <div>Hello, world!</div>;
  }
}
  1. What is the componentDidUpdate() lifecycle method in React?

The `componentDidUpdate()` lifecycle method is called after a component has been updated and re-rendered. It is typically used to perform any side effects that need to happen after the component has been updated, such as fetching new data from a server or updating the DOM directly. Here’s an example:

class MyComponent extends React.Component {
  componentDidUpdate(prevProps, prevState) {
    console.log("Component updated");
  }

  render() {
    return <div>Hello, {this.props.name}!</div>;
  }
}
  1. What is the shouldComponentUpdate() lifecycle method in React?

The `shouldComponentUpdate()` lifecycle method is called before a component is updated and re-rendered. It is used to determine whether the component should be re-rendered or not, based on changes in its props or state.

By default, React will always re-render a component when its props or state change, but you can use `shouldComponentUpdate()` to optimize performance by preventing unnecessary re-renders. Here’s an example:

class MyComponent extends React.Component {
  shouldComponentUpdate(nextProps, nextState) {
    if (this.props.name !== nextProps.name) {
      return true;
    }
    return false;
  }

  render() {
    return <div>Hello, {this.props.name}!</div>;
  }
}
  1. What is a React fragment?

A React fragment is a way to group a list of children without adding extra nodes to the DOM. It is useful when you need to return multiple elements from a component, but you don’t want to add a wrapper element. Here’s an example:

function MyComponent() {
  return (
    <>
      <h1>Hello, World!</h1>
      <p>This is a React fragment example</p>
    </>
  );
}

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