make componentdidmount with react hook example

Example 1: useeffect only on mount

import React, { useEffect } from 'react';

function App() {
  useEffect(() => {
    // Run! Like go get some data from an API.
  }, []);
}

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