缺少回调时，"await promisify(setTimeout)(ms)"如何工作? [英] How does `await promisify(setTimeout)(ms)` work when the callback is missing?

问题描述

对于JavaScript中的简单异步睡眠功能，await promisify(setTimeout)(ms)可以使用！

For a simple async sleep function in JavaScript, await promisify(setTimeout)(ms) works!

但是如何?参数看起来错误.

But how? The arguments look wrong.

• promisify 传递错误回调，所以
• setTimeout 调用将是setTimeout(ms, errorCallback)

• promisify passes an error callback, so the
• setTimeout call would be setTimeout(ms, errorCallback)

这不起作用，但确实可以.怎么样?

which should not work, yet it does. How?

import { promisify } from 'util'
(async () => {
  // const start = Date.now()
  await promisify(setTimeout)(1000)
  // console.log(Date.now() - start)
})()

node <<HEREDOC
  (async () => {
    // const start = Date.now();
    await require('util').promisify(setTimeout)(1000);
    // console.log(Date.now() - start);
  })()
HEREDOC

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背景:await setTimeout(() => {}, 1000) 无法正常工作. 单线:await new Promise(resolve => setTimeout(resolve, 1000))对我不起作用(为什么?).我们可以手动对其进行承诺:const sleep = ms => new Promise(resolve => setTimeout(resolve, ms)); await sleep(1000)，但这是

---

Background: await setTimeout(() => {}, 1000) doesn't work. This one-liner: await new Promise(resolve => setTimeout(resolve, 1000)) did not work for me (why?). We can promisify it manually: const sleep = ms => new Promise(resolve => setTimeout(resolve, ms)); await sleep(1000), but that is an extra function. We can do better.

推荐答案

这是因为setTimeout具有

This is because setTimeout has a custom promisified function now. You can find it when you print setTimeout.

> console.log(setTimeout)
{ [Function: setTimeout] [Symbol(util.promisify.custom)]: [Function] }
undefined

以非标准方式进行回调的公用函数可能具有自定义的承诺函数，而util.promisify在函数具有自定义的承诺函数时将其返回.

Common functions that take a callback not in the standard way may have a custom promisified function, and util.promisify returns it when a function has a custom promisified function.

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