Is it a good practice to run async functions using for loop?

Question

I have a function that uses spawn and calls ffmpeg to trim a file, which returns a Promise. I need to trim an audio file into multiple parts, so I iterate with a for loop which launches the trimming function, I only need the trimming process to complete (which generates the trimmed file) in the for loop (so I don't need to use await) but still need to make sure the trimming function did not return error.

for (let i = 0; i < myArray.length; i++) {
        trim().catch(console.error); //<--- Trim asynchronously
        linkedList.insertAtEnd(i); 
}

My problem is what is a good way to run the for loop trimming the audio asynchronously?

Edit: I don't want the trim() function to complete to iterate the next element in the array.

Edit: The difference between my question and this one is I don't want the for loop to wait for the trim() function in order to go on to the next element in the array.

Does this answer your question? [Callback after all asynchronous forEach callbacks are completed](https://stackoverflow.com/questions/18983138/callback-after-all-asynchronous-foreach-callbacks-are-completed) — Klaycon, Dec 10 '19 at 17:54
`trim()` takes no parameters, so how does it know what it is working on? Also, since there is no `await`, `trim()` likely does not complete "in the for loop" but after. On top of that, you *code* has no way to know if there was an error, as you only output that to the console. — crashmstr, Dec 10 '19 at 18:02
@Klaycon I think the difference is that I don't need to wait for the async function to finish in order to iterate the next element in the array. — hkisthebest, Dec 10 '19 at 18:02
@crashmstr I simplified the code a lot, as you mentioned `trim()` doesn't complete "in the for loop" but after. That is what I am trying to achieve. — hkisthebest, Dec 10 '19 at 18:04
To know that there were no errors, you to 1. save success or failure for each call and 2. check those values after all are completed (i.e. `Promise.all`). — crashmstr, Dec 10 '19 at 18:31
@TGrif I don't think it is relevant in this case, since it only returns string res("the function completed") and rej(new Error("something wrong")). — hkisthebest, Dec 10 '19 at 18:32
@crashmstr And in order to store the promise that it returns, I need to wait for the result before going to the next element, am I right? So how could I achieve what I mentioned in the post? — hkisthebest, Dec 10 '19 at 18:39
@hkisthebest The second solution under "Using ES6 Promises" in the answer to the linked question contains an example of using `Promise.all()` to run all promises in the loop alongside each other. It will resolve when all subtasks have resolved -or- reject as soon as one subtask rejects. You can `.catch()` the `Promise.all()` to handle the latter case — Klaycon, Dec 10 '19 at 19:10

score 1 · Accepted Answer · answered Dec 10 '19 at 18:57

To know if there is any error, you need to deal with it at some point, and to do that, you need to know when they are all done, even if you do not want to wait on each before the next iteration of the loop.

To do this, either store each promise into an array (or use .map). You can then Promise.all to trigger code when each call of trim has completed.

I've added trim and myArray definitions and commented out the insertAtEnd for a "run-able" example (though having the linked list insert not being dependent on the trim makes little sense, nor does trim not taking any variants from the loop itself).

function trim() {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(), 5)
  });
}

let myArray = [1, 2, 3, 4];
let results = [];
for (let i = 0; i < myArray.length; i++) {
  results.push(trim().then(() => true).catch(() => false))
  console.log(`iteration ${i} finished`);
  //linkedList.insertAtEnd(i); 
}

Promise.all(results).then((r) => {
  console.log(`trim() results: ${r}`);
  if (r.every((x) => x)) {
    console.log('success');
  } else {
    console.log('an error occurred');
  }
});

Is it a good practice to run async functions using for loop?

1 Answers1