componentdidmount in hooks

1import React, { useState, useEffect } from 'react';
2function Example() {
3  const [count, setCount] = useState(0);
4
5  // Similar to componentDidMount and componentDidUpdate:  
6  useEffect(() => {    
7    // Update the document title using the browser API    
8    document.title = `You clicked ${count} times`;  
9  });
10
11  );
12}

1useEffect(() => {
2  window.addEventListener('mousemove', () => {});
3
4  // returned function will be called on component unmount 
5  return () => {
6    window.removeEventListener('mousemove', () => {})
7  }
8}, [])

1useEffect(() => {
2  // Your code here
3}, []);

1useEffect(() => {
2    messagesRef.on('child added', snapshot => {
3    const message = snapshot.val();
4    message.key = snapshot.key;
5
6    setMessages(messages.concat(message)); // See Note 1
7}, []); // See Note 2