How to use componentWillMount() in React Hooks?

You cannot use any of the existing lifecycle methods (componentDidMount, componentDidUpdate, componentWillUnmount etc.) in a hook. They can only be used in class components. And with Hooks you can only use in functional components. The line below comes from the React doc:

If you’re familiar with React class lifecycle methods, you can think of useEffect Hook as componentDidMount, componentDidUpdate, and componentWillUnmount combined.

suggest is, you can mimic these lifecycle method from class component in a functional components.

Code inside componentDidMount run once when the component is mounted. useEffect hook equivalent for this behaviour is

useEffect(() => {
  // Your code here
}, []);

Notice the second parameter here (empty array). This will run only once.

Without the second parameter the useEffect hook will be called on every render of the component which can be dangerous.

useEffect(() => {
  // Your code here
});

componentWillUnmount is use for cleanup (like removing event listeners, cancel the timer etc). Say you are adding a event listener in componentDidMount and removing it in componentWillUnmount as below.

componentDidMount() {
  window.addEventListener('mousemove', () => {})
}

componentWillUnmount() {
  window.removeEventListener('mousemove', () => {})
}

Hook equivalent of above code will be as follows

useEffect(() => {
  window.addEventListener('mousemove', () => {});

  // returned function will be called on component unmount 
  return () => {
    window.removeEventListener('mousemove', () => {})
  }
}, [])