Parse query (callback) in for loop

Question

New to javascript here, so callbacks are still a little iffy in my brain.

What I'm trying to do is: given a "menu" which is an array of objectId's, query for each foodItem that corresponds to that objectId, get its votes, put it in a min-heap (to determine which are the top 5 items), and return those top 5 items.

My heap at the end is empty because I realize that JavaScript is asynchronous and that when I try to get the heap data, the callback might not have necessarily completed.

If it were just one call, I would just nest the callbacks, but since this is a loop I'm not really sure what to do.

function getTopFoods(menu, heap, callback) {
//go through each objectId, get its foodItem and then its votes, then heap it
    console.log("got to TopFoods");
        for (var i = 0; i < menu.length; i++) {
            var foodID = menu[i];
            var FoodItem = Parse.Object.extend("FoodItem");
            var foodQuery = new Parse.Query(FoodItem);
            foodQuery.equalTo("objectId", foodID);
            //get corresponding foodItem
            foodQuery.find({
                success: function(foodResult) {
                    //got specific food Item
                    var votes = foodResult.get("votes");
                    console.log("votes: " + votes);
                    if (heap.length < 5) {
                        heap.queue(foodResult);
                    } else {
                        if (votes > heap.peek().get("votes")) {
                            heap.dequeue();
                            heap.queue(foodResult);
                        }
                    }
                }, 
                error: function(error) {
                    console.log("Food error: " + error.code + " " + error.message);
                }
            }); 
        }
        var topFoods = [];
        for (var i = 0; i < 5; i++) {
            topFoods[i] = heap.dequeue();
        }
        callback(topFoods);
}

Amadan · Accepted Answer · 2014-12-01 05:39:48Z

1

The easiest way is to use promises; at this stage, this involves using a library (coming to JavaScript proper in ES6). If you want a low-tech solution, just count stuff:

var waitingCount = menu.length;
for (....) {
  ...
  success: function(foodResult) {
    ...
    if (!--waitingCount) {
      callback(topFive(heap));
    }
  },
  error: function(error) {
    --waitingCount;
    ...
  }
  ...
}

This is just the basic idea. It would be good if you also decremented the counter on failed responses, since this way a single fail will leave you hanging.

EDIT: Err, obviously, the check needs to go to the bottom of success, not to the top as my snippet indicated before, or you'll miss the last element. I also put in the error case.

EDIT2: As eth3lbert notes, parse.com API also supports promises (I don't work with parse.com, so... thanks for the tip). In that case, here's what you do:

var promises = [];
for (....) {
  var promise = foodQuery.find({
    ...
  });
  promises.push(promise);
});
Parse.Promise.when(promises).then(function()) {
  callback(topFive(heap));
}

edited Dec 1, 2014 at 5:39

answered Dec 1, 2014 at 5:08

Amadan

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3 Comments

eth3lbert Over a year ago

Agree, Parse javascript sdk support promises and you should give it a try.

sebster Over a year ago

Sort of related question: each of these queries in the loop require that response.success() or response.error() be called (but I obviously don't want it called prematurely). Are promises the best way to handle this?

Amadan Over a year ago

Promises are the cleanest way to do this: they are more legible than doing the bookkeeping yourself. They do not do anything hugely different under the hood, though. I put in a promise solution too...

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