Before React 16.8, we were forced to use class based component to have access in component lifecycle. And now with Hooks, we are now able to use functional component for state management, data fetching etc. We can now replace component lifecycle like componentDidMount, componentDidUpdate and componentWillUnmount with useEffect in our React component.

1. componentDidMount

The componentDidMount() method runs after the component output has been rendered to the DOM. This is a good place to cause side-effects.

// With ccomponentDidMount()
componentDidMount() {
    console.log('Hello World');
}

// with useEffect()
useEffect(() => {
    console.log('Hello World');
}, [])

If we don't pass an empty array to useEffect, it will run on every change. Therefore, we must give as second argument an empty array to mimic the componentDidMount behavior. It tells to React that your effect doesn’t depend on any values from props or state, so it never needs to re-run, useEffect will run only once after the component is created.

2. componentDidUpdate

The componentDidUpdate() method is invoked immediately after updating occurs.

// With ccomponentDidUpdate()
componentDidUpdate(prevProps) {
    console.log(`Hello World ${prevProps}`);
}

// with useEffect()
useEffect(() => {
    console.log('Hello World');
}, [prevProps])

When we pass a value(prevProps) to the array, it tells to useEffect to run only if the value change.

3. componentWillUnmount

The componentWillUnmount() method is called immediately before a component is unmounted and destroyed. It is a good place to do some cleanup works.

// With ccomponentDidUnmount()
componentWillUnmount() {
    console.log('Hello World');
}

// with useEffect()
useEffect(() => {
    console.log('Hello World');
    return () => {
        console.log('Do some cleanup');
    }
}, [])

By returning an anonymous function with useEffect, it will run before the component unmounting. And the empty array passed as second argument tells to useEffect to run when the component will be mounted or unmounted.