Convert async await in while loop to promises

Question

I can't figure out how to convert async await functionality in a while loop to a promise based implementation.

repl showing the async await version

https://repl.it/repls/IdealisticPepperyCoding

var dependency = false;

function checkDependency() {
  return new Promise(resolve => {
    setTimeout(() => {
      dependency = true;
      return resolve();
    }, 1000)
  });
}

async function isReady() {
    while(!dependency) {
      console.log('not loaded');
      await checkDependency();
    }
    console.log('loaded')
}

isReady();

Uh, async-await *is* promise-based, so I'm not sure what you're asking about? Does the code work or not, what's the problem? — Bergi, Mar 05 '18 at 20:38
Btw, your `checkDependency` function should resolve the returned promise with the boolean value, not mutate some global `dependency` variable. — Bergi, Mar 05 '18 at 20:39
I'm guessing this is far from "actual code", because there are about 100 better ways to write this :p — Jaromanda X, Mar 05 '18 at 20:57
@ZackLucky, it seems that you have a stream for `false/true` values. You could consider using [RxJS stream extensions](https://rxjs-dev.firebaseapp.com/). — AlexMelw, Mar 03 '20 at 10:28

score 6 · Accepted Answer · answered Mar 05 '18 at 20:58

If I'm understanding you correctly, you're wanting to use Promises without async functions instead of Promises with async functions, and the sample checkDependency may actually not set dependency = true in all cases, so you want to "loop."

Equivalent functionality could look something like this, where you "recursively" call the check function. It won't actually be recursive and lead to a stack overflow, though, since the call is done in an async callback:

var dependency = false;

function checkDependency() {
  return new Promise(resolve => {
    setTimeout(() => {
      dependency = true;
      return resolve();
    }, 1000)
  });
}

function isReady() {
  if (!dependency) {
    return checkDependency().then(isReady);
  }

  return Promise.resolve(true);
}

isReady().then(() => {
  console.log('loaded')
});

score 4 · Answer 2 · answered Mar 05 '18 at 20:27

4

You don't need the while loop.

checkDependency().then(() => { console.log('loaded');});

answered Mar 05 '18 at 20:27

Jelmer Jellema

1,072
10
16

My fault the `dependency = true` in the promise isn't always true in which case it needs to check again when it is – Zack Lucky Mar 05 '18 at 20:33
1

@ZackLucky, it seems that you have a stream for `false/true` values. You could consider using [RxJS stream extensions](https://rxjs-dev.firebaseapp.com/). – AlexMelw Mar 03 '20 at 10:26

score 0 · Answer 3 · answered Mar 05 '18 at 20:30

You don't need to return recolve() just call it.
Call the function then within the function isReady

function checkDependency() {
  return new Promise(resolve => {
    setTimeout(resolve, 1000);
  });
}

function isReady() {
  console.log('not loaded');
  checkDependency().then(() => {
    console.log('loaded')
  });
}

isReady();

Convert async await in while loop to promises

3 Answers3