[SOLVED] How does `await promisify(setTimeout)(ms)` work when the callback is missing?

How does `await promisify(setTimeout)(ms)` work when the callback is missing?

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

But how? The arguments look wrong.

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

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

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.

Solution

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

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.