useEffect中return函数的作用和执行时机解读

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

function FriendStatus(props) {
  const [isOnline, setIsOnline] = useState(null);

  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    // Specify how to clean up after this effect:
    return function cleanup() {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

import { useEffect, useState } from 'react';

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

  useEffect(() => {
    console.log("哈哈哈，useEffect 又执行了");
    return () => {
      console.log("看到我就知道执行了清除机制(～￣▽￣)～");
    };
  }, [count]);

  return (
    <div>
      <p>那啥，你点了我 {count} 次 ⏲️⏲️⏲️⏲️</p>
      {console.log("这是 dom 节点渲染了，小样╭(╯^╰)╮")}
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        你觉得你点击我之后会发生什么⛏️⛏️⛏️
      </button>
    </div>
  );
};

function App() {
  return (
    <div className="App">
      <Example />
    </div>
  );
}

export default App;