示例
console.log(1);
// The first line executes immediately, it outputs `1`.
// Macrotask and microtask queues are empty, as of now.
setTimeout(() => console.log(2));
// `setTimeout` appends the callback to the macrotask queue
// - macrotask queue content:
// `console.log(2)`
Promise.resolve().then(() => console.log(3));
// The callback is appended to the microtask queue
// - microtask queue content:
// `console.log(3)`
Promise.resolve().then(() => setTimeout(() => console.log(4)));
// The callback with `setTimeout(...4..)` is appended to microtasks
// - microtask queue content:
// `console.log(3); setTimeout(...4..)`
Promise.resolve().then(() => console.log(5));
// The callback is appended to the microtask queue
// - microtask queue content:
// `console.log(3); setTimeout(...4..); console.log(5)`
setTimeout(() => console.log(6));
// `setTimeout` appends the callback to macrotasks
// - macrotask queue content:
// `console.log(2); console.log(6)`
console.log(7);
// Outputs 7 immediately.同步代码执行完后，会先清空微任务队列，再进入事件循环
输出：1 7 3 5 2 6 4
示例
console.log(1);
// 同步代码先执行，输出"1"。
await new Promise(res => setTimeout(res, 0));
// new Promise(...)构造函数里的代码会立即执行，将setTimeout(res,0)被放入宏任务队列
// 然后await让出主线程，进入事件循环，取下一个宏任务执行，也就是执行res()
// Promise已resolve，await返回
console.log(2);
// await后的代码（也就是`Promose.then`）会被放入微任务队列
// 然后清空执行微任务队列也就是说，要强行把res()放到下一宏任务执行，用：
await new Promise(res => setTimeout(res, 0))