AngularJS: How should I update a property on a resolved promise inside of my watch listener?

All we need is an easy explanation of the problem, so here it is.

Consider this Plunkr.

Inside of my watch listener, I’d like to update a property on a resolved promise.

  • Is it correct to modify values on the $$v property?
  • If not, then what should I do?

Here is the HTML:

<!DOCTYPE html>
<html id="ng-app" ng-app="myAngularApp">

  <head>
    <script data-require="[email protected]*" data-semver="1.2.0-rc2" src="https://code.angularjs.org/1.2.0-rc.2/angular.js"></script>
    <script src="script.js"></script>
  </head>

  <body ng-controller="MyController">
    <input ng-model="myDto.Weight" />{{myDto.Status}}
  </body>

</html>

Here is the JavaScript:

var myDto = {Weight: 200, Status: 'Acceptable'};

var myAngularApp = angular.module('myAngularApp', []);

myAngularApp.factory('myService', function($q){
  return {
    getMyDto: function(){
      var deferred = $q.defer();
      deferred.resolve(myDto);
      return deferred.promise;
    }
  };
});

myAngularApp.controller('MyController', function MyController($scope, myService){
  $scope.myDto = myService.getMyDto();
  $scope.$watch('myDto.Weight', function(newVal, oldVal){
    if (newVal < 150) {
      $scope.myDto.$$v.Status = 'Too Light!'; // Is this the recommended way of doing it?
    }
    else if (500 < newVal) {
      $scope.myDto.$$v.Status = 'Too Heavy!';
    }
    else if (Number(newVal)) {
      $scope.myDto.$$v.Status = 'Acceptable';
    }
    else {
      $scope.myDto.$$v.Status = 'Not a number!';
    }
  });
});

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

I wouldn’t modify $$v directly, it’s an implementation detail. Instead, use then to get the result of the promise, then use it however you like. That requires minimal change to your code.

Demo link

myAngularApp.controller('MyController', function MyController($scope, myService){
  ($scope.myDto = myService.getMyDto()).then(function(myDto) {
    $scope.$watch('myDto.Weight', function(newVal, oldVal){
      if (newVal < 150) {
        myDto.Status = 'Too Light!'; // Is this the recommended way of doing it?
      }
      else if (500 < newVal) {
        myDto.Status = 'Too Heavy!';
      }
      else if (Number(newVal)) {
        myDto.Status = 'Acceptable';
      }
      else {
        myDto.Status = 'Not a number!';
      }
    });
  });
});

Method 2

It is a little bit weird to modify the promise since a promise represents the result of an action that is performed asynchronously. It is self-contained.

I think it is better to add another service function to update data the status. Try this

myAngularApp.factory('myService', function ($q) {
    var deferred = $q.defer();
    return {
        getMyDto: function () {
            deferred.resolve(myDto);
            return deferred.promise;
        },
        updateStatus: function (status) {
            myDto.Status = status;
            deferred.resolve(myDto);
            return deferred.promise;
        }
    };
});

myAngularApp.controller('MyController', function MyController($scope, myService) {
    $scope.myDto = myService.getMyDto();
    $scope.$watch('myDto.Weight', function (newVal, oldVal) {
        if (newVal < 150) {
            myService.updateStatus('Too Light!');
        } else if (500 < newVal) {
            myService.updateStatus('Too Heavy!');
        } else if (Number(newVal)) {
            myService.updateStatus('Acceptable');
        } else {
            myService.updateStatus('Not a number!');
        }
    });
});

Updated Plunker

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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