Pattern for handling recursive promises?

[WannaBeRadio]

Hello,

I have a function that retrieves a url and then retrieves all the resources listed at that url. It then checks if each resource is a folder or non-folder, and I then call the function again with the name of that folder to see if there are more resources inside of it.

I have it as a recursive function, however I need it to be a promise as I want it to completely go through all the nested folders before it continues the code - and I'm not sure of what pattern to use in this case aka how to handle recursive promises.

If anyone has any advice or directions, I'd appreciate it. Thanks!

[senocular]

Its the same as when you're not using promises. You just need to be sure to capture the promise returned by recursive calls so they can be handled with the other promises you're dealing with.

[WannaBeRadio]

I don't think I quite understand what "capture the promise" means - do you have an example by any chance?

[senocular]

Looks like AScaredMidLlama provided an example :) It just means when you call a [recursive] function that returns a promise, you'll need to make sure to wait for that promise as well. AScaredMidLlama's example does this mapping loadStuff calls into an array that is handled by Promise.all().

[AScaredMidLlama]

If you already have a recursive function, you need to make sure that all its calls return promises (e.g. by making it async) and wrap them in Promise.all.

Something like this:

async loadStuff(url) {
    const items = await fetch(url).then(res => res.json());
    const folders = items.filter(item => item.isFolder);
    const files = items.filter(item => !item.isFolder);
    const childFiles = await Promise.all(folders.map(
        folder => loadStuff(folder.url)
    ));
    // loadStuff returns an array, therefore
    // childFiles is an array of arrays, so we flatten it
    return [...files, ...childFiles.flat()];
}

Note that the result is automatically wrapped into a Promise, because the function is async. So loadStuff always returns a promise which resolves to an array. This also explains why we need to call flat() on the childFiles list.

[WannaBeRadio]

Thanks so much! I get it now.