async本质是什么? 其实就是generator的语法糖是自动执行的generator，虽然ES6已经实现了async和generator，但是在生产环境中都是经过babel编译成promise.

简单的async

async function p1(){    console.log(1)    return 1}// 等效function p1(){    console.log(1)    return Promise.resolve(1)}复制代码

一般情况的async

async function p2(){    console.log(2)    let a = await new Promise(resolve => setTimeout(() =>resolve(1), 3000)  )    console.log(a);    return 2}; // 等效function p2(){    console.log(2)    return Promise.resolve(new Promise(resolve => setTimeout(() =>resolve(1), 3000)).then(res => {        let a = res;        console.log(a)        return Promise.resolve(2)    }))} 复制代码

循环中的async

通过队列实现

let p1 = () => new Promise((resolve => setTimeout(() => resolve(1), 1000)));let p2 = () => new Promise((resolve => setTimeout(() => resolve(2), 2000)));let p3 = () => new Promise((resolve => setTimeout(() => resolve(3), 3000)));let ps = [p1, p2, p3];async function p(){    for (let i = 0; i < 3; i++) {        let a = await ps[i]();        console.log(a)    }}// 等效async function p(){    let queue = [];    for (let i = 0; i < 3; i++) {         queue.push(() => ps[i]().then(res => {            let a = res;            console.log(a)        }))     }    queue.reduce((p1, p2) => p1.then(res => p2()) , Promise.resolve());}; 复制代码

可以看出来async简化了Promise,大部分场景下Promise其实也够用了，但是在链式调用的场景下使用async非常简洁. 下面抛出一个问题

let p1  = new Promise((resolve) => resolve(1))let p2 =  new Promise((resolve) => {    resolve(p1);    new Promise(resolve => resolve()).then(() => console.log(2));})p2.then(res => console.log(1))// 为什么先执行2然后执行1? resolve(p1)其实等效于 Promise.resolve().then(res => p1.then(resolve))所以事件队列的进入顺序为// res => p1.then(resolve)// () => console.log(2)// resolve复制代码