Strongloop promise inside loop

Question

I am trying to call a loopback find function inside of a for loop, passing in a value from the iteration into the loopback function. The main issue of the code can be represented by the following:

for (var a = 0; a < $scope.countries.length; a++) {
  $scope.getEmFacPurElec($scope.countries[a], 'ton/kWh', 'CO2e').then(function(result) {
    emFacPurElecToUse = $scope.emFacPurElecs;
}

And here is the function being called:

$scope.getEmFacPurElec = function (country, unit, ghgType) {
   var defer = $q.defer();
   $scope.emFacPurElecs = [];

   $scope.emFacPurElecs = Country.emFacPurElecs({
      id: country.id,
      filter: {
               where: {
                       and: [
                             {unit: unit},
                             {ghgType: ghgType}
                            ]
                      }
              }
   });   

   defer.resolve('Success getEmFacPurElec');
   return defer.promise;
};             

The problem is that the loopback promise function is called and then returned undefined which means that it moves to the next iteration of the for loop before getting the value to assign to emFacPurElecToUse. I need to do some more calculations with that variable for that country before moving to the next country.

I have looked at using $q.all as a possible solution and also using array.map as per http://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html (Rookie mistake #2: WTF, how do I use forEach() with promises?), but I just cannot figure out how to pull it all together to make it work. Should I be using a forEach instead?

I also saw this link angular $q, How to chain multiple promises within and after a for-loop (along with other similar ones) but I do not have multiple promises that I need to process inside the for loop. I need to retrieve the value of one emFacPurElecs for that country, do some work with it, then move to the next country. I feel I am close but I just cannot get my head around how I would code this particular functionality. Any help is greatly appreciated.

Answer 1

It seems to me that you do have multiple promises to process inside your for loop, as you say "I need to do some more calculations with that variable for that country before moving to the next country." This should all be done with in the promise chain that I've suggested - calcEmFacPurElec.

$scope.calcEmFacPurElec = function (country, unit, ghgType) {
   $scope.getEmFacPurElec(country, unit, ghgType).then(function(countryEmFacPurElecs) {
    // do something with countryEmFacPurElecs

    return countryEmFacPurElecs;
}

$scope.getEmFacPurElec = function (country, unit, ghgType) {
   var defer = $q.defer();

   defer.resolve(Country.emFacPurElecs({
      id: country.id,
      filter: {
               where: {
                       and: [
                             {unit: unit},
                             {ghgType: ghgType}
                            ]
                      }
              }
   });   );
   return defer.promise;
};      

Hopefully the above is a pointer in the right direction!

When you want to carry out a promise chain on an array of items, then as you have identified, Promise.all (using whatever promises implementation you require) is what you want. .all takes in an array of Promises, so in your for loop you can do:

var promises = []; 
for (var a = 0; a < $scope.countries.length; a++) {
  promises.push($scope.calcEmFacPurElec($scope.countries[a], 'ton/kWh', 'CO2e')); // new promise chain that does all of the work for that country
}

$q.all(promises).then(function(arrayofCountryEmFacPurElecs) {console.log('all countries completed')});