NodeJS - Deferred function inside a loop

Question

I have a conceptual problem with NodeJS... I need to call a deferred function within a loop and use the response for the next iteration. How can I do that without blocking? Here is what I did but of course, response.rev doesn't get updated for the next iteration:

first.function().then(function(response) {
    for (var int = 0; int < $scope.files.length; int++) {
        call.function(response.rev).then(function (res) {
            response.rev = res.rev;
        }, function (reason) {
            console.log(reason);
        });
    }
});

Edit, my real code with Benjamin's help (still not working):

pouchWrapper.updateClient(clientManager.clientCleaner($scope.client)).then(function(response) {
    if ($scope.files != null) {
        var p = $q.when();
        for (var int = 0; int < $scope.files.length; int++) {
            var doc = $scope.files[int].slice(0, $scope.files[int].size);
            p = p.then(function(formerRes){
                return pouchWrapper.uploadFiletoDoc($scope.client._id, $scope.contract.policy_number + '&' + $scope.damage.number + '-' + $scope.files[int].name, res.rev, doc, $scope.files[int].type).then(function (res) {
                    return res;
                }, function (reason) {
                    console.log(reason);
                });
            });
        }
        return p;
    }
});

It looks like angular though, I'm going to assume $q promises here, if you're using another server side promise library - let me know. — Benjamin Gruenbaum, Jun 01 '14 at 22:08

score 4 · Accepted Answer · answered Jun 01 '14 at 22:07

4

You use .then for that. .then is the asynchronous version of the semicolon:

first.function().then(function(response) {
   var p = $q.when(); // start with an empty promise
   for (var int = 0; int < $scope.files.length; int++) {
      p = p.then(function(formerValue){
              return call.function(formerValue).then(function (res) {
                  // use res here as the _current_ value
                  return res; // return it, so that the next iteration can use it;
              });
      });
   }
   return p; // this resolves with the _last_ value
});

answered Jun 01 '14 at 22:07

Benjamin Gruenbaum

270,886
87
504
504

Yes, important technique. Recursive promise completions. – djechlin Jun 01 '14 at 22:08
I'm a bit lost here... the call.function is the function I would like to call with the _current_ value. – Nate Jun 01 '14 at 22:22
1

Yes, this basically "queues" the work up. Instead of looping over the function calls, we are queuing them, `then` means `"perform that action after this one, and give it as a parameter whatever this returns"` – Benjamin Gruenbaum Jun 01 '14 at 22:24
I tried what you wrote but I still get an error... TypeError: Cannot read property 'name' of undefined (see edit) – Nate Jun 01 '14 at 22:33
@ncohen now you just have an unrelated closure/loop problem, use a `.forEach` loop instead of a `for` loop and read http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example – Benjamin Gruenbaum Jun 01 '14 at 22:35
Thanks but I'm still getting the same response.rev in my pouchWrapper.uploadFiletoDoc() function. I'm sorry if it's obvious but the concept of deferred functions is hard to catch for me. – Nate Jun 01 '14 at 22:54
Try reading http://stackoverflow.com/questions/14220321/how-to-return-the-response-from-an-ajax-call it may have some good pointers – Benjamin Gruenbaum Jun 01 '14 at 23:03
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/54937/discussion-between-ncohen-and-benjamin-gruenbaum). – Nate Jun 02 '14 at 15:29

NodeJS - Deferred function inside a loop

1 Answers1