react functional component unmount code example

Example 1: componentwillunmount hooks

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

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

Example 2: component did update hooks

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  },[count]); // use dependency array to watch for state changes on this part of state

  return (
    

      
You clicked {count} times
       setCount(count + 1)}>
        Click me
      
    
  );
}

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