100 Reactjs Question to answer with code example
- 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.
- 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>
);
}
- 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!");
- 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"));
- 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" />;
- 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>;
}
- 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.
- 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>
);
}
}
- 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.
- What is the difference between
props
andstate
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.
- 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.
- 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.
- 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>;
}
}
- 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>;
}
}
- 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>;
}
}
- 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>
</>
);
}