component did mount in react function code example

Example 1: how to use componentdidmount in functional component

// passing an empty array as second argument triggers the callback in useEffect
// only after the initial render thus replicating `componentDidMount` lifecycle behaviour
useEffect(() => {
  if(!props.fetched) {
 	 props.fetchRules();
  }
  console.log('mount it!');
}, []);

// componentDidUpdate
useEffect({
	your code here
}) 

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

// For componentWillUnmount
useEffect(() => {
  // componentWillUnmount
  return () => {
     // Your code here
  }
}, [yourDependency]);

Example 2: componentwillunmount hooks

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

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

Example 3: useeffect componentdidmount

useEffect(() => {
    messagesRef.on('child added', snapshot => {
    const message = snapshot.val();
    message.key = snapshot.key;

    setMessages(messages.concat(message)); // See Note 1
}, []); // See Note 2