Question

I have some synchronous code that looks like this:

function bulk_upload(files, callback) { 
  for (file in files) {
   sync_upload(file); // blocks till file uploads
  }
  print('Done uploading files');
  callback();
}

I now have to use an asynchronous API async_upload(file, callback) instead of sync_upload(file) to do the same. I have various options but not sure what is the best:

1) Use a sleep after the for-loop – that’s a hack as I have to hope my timing is correct

2) Recursively chain my array:

function bulk_upload(files, callback) {
  if (files.length == 0) {
    print('Done uploading files');
    callback();
  } else {
    async_upload(files.removeLast(), function() { bulk_upload(files, callback); });
  }
}

This is not only hacky but sub-optimal as I could have uploaded my files in parallel using the new async_upload API but I ended up uploading sequentially.

3) Use a global counter:

function bulk_upload(files, callback) {
  uploads = 0
  for(file in files) {
    async_upload(file, function() { uploads++; }); 
  }
  while(uploads < files.length) ; // Empty spin - this is stupid
  print('Done uploading files');
  callback();
}

4) Slightly better counter (but still awful):

function bulk_upload(files, callback) {
  uploads = 0
  for(file in files) {
    async_upload(file, function() { 
      if (++uploads == files.length) { // this becomes uglier as I want to await on more stuff
        print('Done uploading files');
        callback();
      }; 
    }); 
  }
}

Answer 1

You can use the async module’s forEach method to do this:

function bulk_upload(files, callback) {
    async.forEach(files, async_upload(file, callback), function (err) {
        if (err) {
            console.error('Failed: %s', err);
        } else {
            console.log('Done uploading files');
        }
        callback(err);
    });
}