将异步等待与 Array.map 一起使用 [英] Use async await with Array.map

问题描述

给定以下代码:

var arr = [1,2,3,4,5];

var results: number[] = await arr.map(async (item): Promise<number> => {
        await callAsynchronousOperation(item);
        return item + 1;
    });

产生以下错误:

TS2322:类型Promise[]"不可分配给类型number[]".输入承诺<数量>"不能分配给输入数字".

TS2322: Type 'Promise<number>[]' is not assignable to type 'number[]'. Type 'Promise<number> is not assignable to type 'number'.

我该如何解决?如何让 async await 和 Array.map 一起工作?

How can I fix it? How can I make async await and Array.map work together?

推荐答案

这里的问题是你试图await一个承诺数组而不是一个承诺.这不符合您的预期.

The problem here is that you are trying to await an array of promises rather than a Promise. This doesn't do what you expect.

当传递给 await 的对象不是 Promise 时，await 只是立即按原样返回值，而不是尝试解析它.因此，由于您在 await 此处传递了一个(由 Promise 对象组成的)数组而不是 Promise，因此 await 返回的值只是该数组，其类型为 Promise[].

When the object passed to await is not a Promise, await simply returns the value as-is immediately instead of trying to resolve it. So since you passed await an array (of Promise objects) here instead of a Promise, the value returned by await is simply that array, which is of type Promise<number>[].

您可能想要做的是在 map 返回的数组上调用 Promise.all 以便在 await它.

What you probably want to do is call Promise.all on the array returned by map in order to convert it to a single Promise before awaiting it.

根据 MDN 文档 Promise.all:

Promise.all(iterable) 方法返回一个可解析的承诺当可迭代参数中的所有承诺都已解决时，或拒绝的原因是第一个通过的承诺拒绝.

The Promise.all(iterable) method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects.

所以在你的情况下:

var arr = [1, 2, 3, 4, 5];

var results: number[] = await Promise.all(arr.map(async (item): Promise<number> => {
    await callAsynchronousOperation(item);
    return item + 1;
}));

这将解决您在此处遇到的特定错误.

This will resolve the specific error you are encountering here.

根据您想要做什么，您也可以考虑使用 Promise.allSettled, Promise.any 或 Promise.race 而不是 Promise.all，尽管在​​大多数情况下(几乎可以肯定包括这个)Promise.all 将是你想要的.

Depending on exactly what it is you're trying to do you may also consider using Promise.allSettled, Promise.any, or Promise.race instead of Promise.all, though in most situations (almost certainly including this one) Promise.all will be the one you want.

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