javascript - 异步生成器 : Yielding a rejected promise

Question

我一直在玩异步生成器，试图制作一个“ promise 排序”生成器，它接受一系列 promise ，并按照它们解决或拒绝的顺序一个接一个地产生 promise 。所以像:

async function* orderProms(prom_arr) {

    // Make a copy so the splices don't mess it up.
    const proms = [...prom_arr];

    while (proms.length) {
        // Tag each promise with it's index, so that we can remove it for the next loop.
        const {prom, index} = await Promise.race(proms.map((prom, index) => prom.then(
            () => ({prom, index}),
            () => ({prom, index})
        )));

        proms.splice(index, 1);
        yield prom;
    }
}

像这样使用这个生成器的想法:

const resAfter = (val, delay) => new Promise(res => setTimeout(() => res(val), delay));
const rejAfter = (val, delay) => new Promise((_, rej) => setTimeout(() => rej(val), delay));

const promises = [
    resAfter("Third", 3000),
    resAfter("First", 1000),
    rejAfter("Second", 2000), // NOTE: this one rejects!
];

(async () => {

    let ordered = orderProms(promises);

    let done = false;
    for (let next_promise = ordered.next(); !done; next_promise = ordered.next()) {
        const next = await next_promise
            .catch(err => ({done: false, value: `Caught error: ${err}`}));

        done = next.done;
        if (!done) console.log(next.value);
    }
})()

但是，我注意到这将达到第二个 promise ，然后生成器将停止。似乎是因为拒绝了“第二个” promise 。调用 yield prom当 prom被拒绝。

但这是我困惑的根源。我不想在这里创建异常，我只想将被拒绝的 promise 作为 value迭代器的结果。我不希望它被打开。这几乎就像被视为 yield await prom; ，但如您所见，没有 await称呼。

这里发生了什么，我怎样才能简单地从这个生成器中产生一个被拒绝的 promise 。

这是可运行片段中的上述代码:

async function* orderProms(prom_arr) {

    // Make a copy so the splices don't mess it up.
    const proms = [...prom_arr];

    while (proms.length) {
        // Tag each promise with it's index, so that we can remove it for the next loop.
        const {prom, index} = await Promise.race(proms.map((prom, index) => prom.then(
            () => ({prom, index}),
            () => ({prom, index})
        )));

        proms.splice(index, 1);
        yield prom;
    }
}

const resAfter = (val, delay) => new Promise(res => setTimeout(() => res(val), delay));
const rejAfter = (val, delay) => new Promise((_, rej) => setTimeout(() => rej(val), delay));

const promises = [
    resAfter("Third", 3000),
    resAfter("First", 1000),
    rejAfter("Second", 2000), // NOTE: this one rejects!
];

(async () => {

    let ordered = orderProms(promises);

    let done = false;
    for (let next_promise = ordered.next(); !done; next_promise = ordered.next()) {
        const next = await next_promise
            .catch(err => ({done: false, value: `Caught error: ${err}`}));

        done = next.done;
        if (!done) console.log(next.value);
    }
})()

Answer 1

It's almost like this is being treated as yield await prom. What is going on here?

Exactly that是异步生成器的行为方式。

how can I simply yield a rejected promise as-is from this generator.

你不能。请注意，异步迭代器预计将被

try {
    for await (const value of orderProms(promises)) {
        console.log(value);
    }
} catch(err) {
    console.error('Caught error: ', err);
}

语法中没有对单个错误处理的便利。当出现异常时，循环停止，生成器完成。观点。

所以，你可以做什么？我看到三个选择:

保持原样并尽早将失败视为一项功能(类似于 Promise.all )

处理错误(在 orderProms 中或在向其中传递 promise 之前)并产生 promise 状态和值的元组

for await (const value of orderProms(promises.map(prom =>
    prom.catch(err => `Caught error: ${err}`)
))) {
    console.log(value);
}

使用普通(非 async )生成器，您可以从中手动生成一个又一个 promise ，以便能够以您想要的方式使用它