1、JS是单线程语言，包括同步任务、异步任务，异步任务又包括宏观任务和微观任务
2、执行顺序：同步任务——>微观任务——>宏观任务
3、宏观任务的方法有：script(整体代码)、setTimeout、setInterval、I/O、UI交互事件、postMessage、MessageChannel、setImmediate(Node.js 环境)
4、微观任务的方法有：Promise.then、MutaionObserver、process.nextTick(Node.js 环境)，async/await实际上是promise+generator的语法糖，也就是promise，也就是微观任务
1. setTimeout
console.log('script start') //1. 打印 script start
setTimeout(function(){
console.log('settimeout') // 4. 打印 settimeout
}) // 2. 调用 setTimeout 函数，并定义其完成后执行的回调函数
console.log('script end') //3. 打印 script start
// 输出顺序：script start->script end->settimeout
2. Promise
console.log('script start')
let promise1 = new Promise(function (resolve) {
console.log('promise1')
resolve()
console.log('promise1 end')
}).then(function () {
console.log('promise2')
})
setTimeout(function(){
console.log('settimeout')
})
console.log('script end')
// 输出顺序: script start->promise1->promise1 end->script end->promise2->settimeout
3. async/await
async function async1(){
console.log('async1 start');
await async2();
console.log('async1 end')
}
async function async2(){
console.log('async2')
}
console.log('script start');
async1();
console.log('script end')
// 输出顺序：script start->async1 start->async2->script end->async1 end
