useEffect then reactjs code example

Example 1: react use effect

useEffect(() => {
	
  },[]);

Example 2: 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