Why is my code not awaiting in a forEach when it works when I use a for loop?

Question

I have a following code which creates remote folders locally if they do not exist:

await remoteFolders
  .filter(rf => !localFolders.some(lf => lf === rf))
  .forEach(async (rf) => await this.createFolder(rf));

Method createFolder looks like this:

async createFolder(folderPath) {
  if (!(await fs.exists(folderPath))) {
    await fs.mkdir(folderPath);
  }
}

The problem is that if I debug I can see that await in forEach is ignored and the loop does not wait until createFolder method finishes. So if I create directories which depend on previous ones, it obviously fails.

But if I use for-loop it works fine (await is taken into account and loop execution continues after createFolder finishes):

for (let i = 0; i < remoteFolders.length; i++) {
  if (!localFolders.some(lf => lf === remoteFolders[i])) {
    await this.createFolder(remoteFolders[i]);
  }
}

I can use this one, but using forEach looks more elegant to me. Why does not forEach take await into account?

See https://stackoverflow.com/questions/37576685/using-async-await-with-a-foreach-loop — jbialobr, Oct 02 '18 at 12:36
@jbialobr `Promise.all` allows to await for all folders creation, but does not chain them — Guerric P, Oct 02 '18 at 12:42

score 2 · Answer 1 · answered Oct 02 '18 at 12:32

2

An async function runs until it hits an await, and then returns a promise. ForEach doesn't do anything special when you return a promise. All it sees is that your function returned, and therefore it can move on to the next element of the array. So forEach synchronously steps through the array, calls a bunch of functions, and gets a bunch of promises back which it ignores. Later, the promises will start resolving and your functions will resume running, but this is after forEach has finished.

answered Oct 02 '18 at 12:32

Nicholas Tower

72,740
7
86
98

and is there any option how to write my code using forEach or I have to stick with classic for-loop? – michal.jakubeczy Oct 02 '18 at 12:36
1

@jabko87 look at https://stackoverflow.com/questions/5050265/javascript-node-js-is-array-foreach-asynchronous , you may use `async.each` – sagi Oct 02 '18 at 12:37
@sagi thanks will give it a try – michal.jakubeczy Oct 02 '18 at 12:38

score 0 · Answer 2 · answered Oct 02 '18 at 12:37

0

When using forEach, we specify a callback function. This callback will be executed on each element in the array. When await is used it returns a promise and nothing else. The loop continues and promise is resolved later.

answered Oct 02 '18 at 12:37

Shyam Kumar

148
9

Why is my code not awaiting in a forEach when it works when I use a for loop?

2 Answers2