componentDidMountcomponentWillUnmount

ComponentDidMount and componentWillUnmount are lifecycle methods in React, a popular JavaScript library for building user interfaces. These methods are used to manage side effects in functional components, which are operations that affect something outside of the component, such as fetching data, subscribing to events, or manually changing the DOM.

ComponentDidMount is invoked immediately after a component is mounted, meaning it has been inserted into the DOM. This method is commonly used to perform initial setup tasks, such as fetching data from an API or setting up subscriptions. For example, in a class component, you might use componentDidMount to fetch data and update the component's state:

class MyComponent extends React.Component {

componentDidMount() {

fetchData().then(data => {

this.setState({ data });

});

}

render() {

// ...

}

}

ComponentWillUnmount is called just before a component is unmounted and destroyed. This method is used to perform cleanup tasks, such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in componentDidMount. For example, you might use componentWillUnmount to cancel a data fetch if the component is unmounted before the data is received:

class MyComponent extends React.Component {

componentDidMount() {

this.dataFetch = fetchData().then(data => {

this.setState({ data });

});

}

componentWillUnmount() {

if (this.dataFetch) {

this.dataFetch.cancel();

}

}

render() {

// ...

}

}

In functional components, the useEffect hook can be used to achieve similar results. The first argument to useEffect is a function that performs the side effect, and the second argument is an array of dependencies. If the array is empty, the effect will only run once, similar to componentDidMount. To perform cleanup, the effect function can return a cleanup function:

function MyComponent() {

useEffect(() => {

const dataFetch = fetchData().then(data => {

setData(data);

});

return () => {

if (dataFetch) {

dataFetch.cancel();

}

};

}, []);

// ...

}