What's it all about, Alfie?

This morning I was thinking about the interrogation of the Promise in Node, and it was a little bothering to me that there was no way to easily see if it was complete. Then I thought - Maybe there is a different way? So I decided to give this a try.

The use-case is this: I want to be able to fire off a request to another Service, and maybe it'll get back to me in time, and maybe it won't. Either way, I don't want to wait. I have other work to do that must get done. So I want to fire this Promise off, and then, if it's done when I need its result - Great! If not, then Also Great!

But the normal:

  const foo = await callAsyncFunction()

isn't going to work, because that will wait until it's done. So how to work this out?

It turns out that it's not too hard.

  const { setTimeout } = require('timers/promises')
 
  const runTest = async () => {
    let done = false
    const foo = setTimeout(5000).then((val) => done = true)
    for (let i = 0; i < 10; i++) {
      console.log('FOO', foo, done)
      await setTimeout(1000)
    }
  }
 
  runTest()
    .then(() => console.log('All done running Timing test.'))
    .catch(console.error)
    .finally(() => process.exit())

and when I run this:

  $ node foo.js
  FOO Promise { <pending> } false
  FOO Promise { <pending> } false
  FOO Promise { <pending> } false
  FOO Promise { <pending> } false
  FOO Promise { <pending> } false
  FOO Promise { true } true
  FOO Promise { true } true
  FOO Promise { true } true
  FOO Promise { true } true
  FOO Promise { true } true
  All done running Timing test.

So all we need to do is to have a variable that indicates the state of the completeness, and then return that in the .then() call. Sure, it may make a lot more sense to have:

    const foo = setTimeout(5000).then((val) => {
      done = true
      return val
    })

so that we get the value back into foo, but that's easy... the point is to toggle the variable in that .then() and query that, as needed.

This way, I don't have to worry about any unsupported ways of finding out, it's simple. 🙂