获得第一个履行承诺 [英] Get first fulfilled promise

问题描述

如果我有两个承诺A和B，其中只有一个会成功，我怎样才能获得成功完成的任何承诺？我正在寻找类似于 Promise.race 的东西，但这只会返回第一个履行的承诺。我正在使用来自ES6的承诺。

If I have two promises A and B, only one of which will succeed, how can I get whichever one fulfills successfully? I'm looking for something similar to Promise.race, but which will return only the first promise that fulfills. I'm using promises from ES6.

推荐答案

反转承诺的极性，然后你可以使用 Promise.all ，因为它拒绝了第一个被拒绝的承诺，它在反转后对应于第一个履行的承诺：

Invert the polarity of the promises, and then you can use Promise.all, because it rejects on the first rejected promise, which after inversion corresponds to the first fulfilled promise:

const invert  = p  => new Promise((res, rej) => p.then(rej, res));
const firstOf = ps => invert(Promise.all(ps.map(invert)));

// Utility routines used only in testing.
const wait    = ms => new Promise(res => setTimeout(() => res(ms), ms));
const fail    = f  => Promise.reject(f);
const log     = p  => p.then(v => console.log("pass", v), v => console.log("fail", v));

// Test.
log(firstOf([wait(1000), wait(500) ]));
log(firstOf([wait(1000), fail("f1")]));
log(firstOf([fail("f1"), fail("f2")]));

这将返回第一个履行承诺的价值，或者如果全部拒绝，一系列拒绝原因。

This will return the value of the first fulfilled promise, or if all reject, an array of rejection reasons.

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