useeffect will unmount

1  useEffect(() => {
2    return () => {
3      console.log("cleaned up");
4    };
5  }, []);

1useEffect( () => console.log("mount"), [] );
2useEffect( () => console.log("will update data1"), [ data1 ] );
3useEffect( () => console.log("will update any") );
4useEffect( () => () => console.log("will update data1 or unmount"), [ data1 ] );
5useEffect( () => () => console.log("unmount"), [] );
6

1  useEffect(() => {
2    return () => {
3      console.log("cleaning up --> unmount ");
4    };
5  }, []);
6

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( () => console.log("mount"), [] );
2useEffect( () => console.log("will update data1"), [ data1 ] );
3useEffect( () => console.log("will update any") );
4useEffect( () => () => console.log("will update data1 or unmount"), [ data1 ] );
5useEffect( () => () => console.log("unmount"), [] );

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