为什么async await和Promise.all具有相同的运行时间

Question

我创建了三个Promise，如下所示，并等待它们，预计它们需要6000毫秒：

let PromiseOne = new Promise(resolve => {
setTimeout(() => {
resolve('Promise One');
}, 2000);
});
let PromiseTwo = new Promise(resolve => {
setTimeout(() => {
resolve('Promise Two');
}, 2000);
});
let PromiseThree = new Promise(resolve => {
setTimeout(() => {
resolve('Promise Three');
}, 2000);
});
(async() => {
console.time();
let ResponseOne = await PromiseOne;
let ResponseTwo = await PromiseTwo;
let ResponseThree = await PromiseThree;
console.log(ResponseOne, ResponseTwo, ResponseThree);
console.timeEnd();
})();

但是，我收到了一条意外的控制台消息，如下所示。

Promise One Promise Two Promise Three
default: 2.004s

据我所知，每个等待都有2000米的运行时间。但是，事实并非如此。

在这种情况下，我上面使用async/await的代码和下面使用Promise.all的代码之间有什么区别？

let PromiseOne = new Promise(resolve => {
setTimeout(() => {
resolve('Promise One');
}, 2000);
});
let PromiseTwo = new Promise(resolve => {
setTimeout(() => {
resolve('Promise Two');
}, 2000);
});
let PromiseThree = new Promise(resolve => {
setTimeout(() => {
resolve('Promise Three');
}, 2000);
});
(() => {
Promise.all([PromiseOne, PromiseTwo, PromiseThree]).then(res => {
console.log(res);
});
})();

Answer 1

在这两种情况下，您都会立即构建所有Promise。在PromiseThree的声明结束后，对于这两种情况，您只剩下3个promise，每个promise在2秒钟后解析。无论脚本后面发生什么，每一个都将在2秒钟后解决。

如果使用Promise.all，则所有承诺将在2秒钟后得到解决。

如果使用async/await，等待的第一个问题将需要2秒才能解决。同时，其他两个问题将得到解决，因此执行await PromiseTwo和await PromiseThree几乎不需要任何时间——因此，总共需要2秒。

// at this point, each promise has been created and each will take 2 seconds
(async() => {
console.time();
// 2 seconds for the below line
let ResponseOne = await PromiseOne;
// after 2 seconds, all have resolved; below line takes no time at all
let ResponseTwo = await PromiseTwo;
// same as above
let ResponseThree = await PromiseOne;

如果在await之后构建其他promise，而不是之前的promise，则情况会有所不同——在这种情况下，它们会像您对await所期望的那样串行运行。

let makePromise = () => new Promise(resolve => {
setTimeout(() => {
resolve();
}, 2000);
});
console.log('starting');
(async () => {
console.time();
await makePromise();
await makePromise();
await makePromise();
console.timeEnd();
})();