Lab 8.2 - Promisify setTimeout(s) "A B C"

jayjohnston

This lab question poses an interesting issue for me: I am typically trying to avoid doing anything in serial.

I'm unsure if the lab is testing my lack of understanding on js setTimeout. I've re-read the chapter and seen mention of a different setTimeout. Still, i think my issue is something else about promisify.

It is worth noting that I am familiar with test and labs forbidding any alteration above where it says "PUT YOUR CODE HERE", and i may be trying too hard to not alter the provided code.

Anyway, please consider this (failed) code snippet:

// Call the functions in such a way that A then B ... is printed 
// PROVIDED CODE:

'use strict'
const { promisify } = require('util')

const print = (err, contents) => {
  if (err) console.error(err)
  else console.log(contents)
}

const opA = (cb) => {
  setTimeout(() => {
    cb(null, 'A')
  }, 500)
}

const opB = (cb) => {
  setTimeout(() => {
    cb(null, 'B')
  }, 250)
}

// PUT YOUR CODE HERE:

const a = promisify(opA)
const b = promisify(opB)

const callAB = async function () {
  [a, b].forEach(async (op) => {
    await op(print)
  })
}
callAB()

This outputs the unexpected order (due to the longer timeout of A, of course):

jj->~/nodejs/JSNAD/labs/ch-8/labs-2$ node serial2.js
B
A

While the output looks like it's executed in parallel, my intuition tells me that setTimeout is being called synchronously. The reason the output is out of order is the "setTimeout fork" messing up the expected output.

---

Here is a snippet that my review of the chapter suggests should have worked. It does not work because then isn't being called. This is where my lack of clarity lies.

What and where do we need to return (or resolve) in order for then to be called?

...

const a = promisify(opA)
const b = promisify(opB)

const callAB = async function () {
  await a(print)
    .then(() => {
      b(print)
    })
}
callAB()

This outputs only A (probably because i'm not resolving, returning something):

jj->~/nodejs/JSNAD/labs/ch-8/labs-2$ node serial.js
A

jayjohnston

I think i found the answer:

When you pass an argument (print in this case) into a promisified function it offsets the callback that it calls internally. The confusion here is you think you have to pass the promisified function the callback, you don't - that happens internally in the promise returned from promisify.

https://forum.linuxfoundation.org/discussion/861587/lab-8-2-with-async-await#latest

  const a = promisify(opA)
  const b = promisify(opB)

  const callAB = async () => {
    console.log(await a())
    console.log(await b())
  }
  callAB()

Outputs:

jj->~/nodejs/JSNAD/labs/ch-8/labs-2$ n serial2.js
A
B

xdxmxc

yep

lesimoes

Another solution, with Promise.all, could be:

  const a = promisify(opA)
  const b = promisify(opB)

  Promise.all([a(), b()]).then(print);