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

Question

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.

Answer 1

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.

Answer 2

setTimeout is a special case for promisify.

Per the node.js specs:

Note: This method has a custom variant for promises that is available using util.promisify()

const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);

setTimeoutPromise(40, 'foobar').then((value) => {
 // value === 'foobar' (passing values is optional)
 // This is executed after about 40 milliseconds.
});

With async/await, that becomes:

await promisify(setTimeout)(1000)