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)
}

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)
    }))
}

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());
};

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