docs(ngModel): getterSetter and arguments.length vs. angular.isDefined

src/ng/directive/ngModel.js

@@ -1205,7 +1205,11 @@ var DEFAULT_REGEXP = /(\s+|^)default(\s+|$)/;
           var _name = 'Brian';
           $scope.user = {
             name: function(newName) {
-              return angular.isDefined(newName) ? (_name = newName) : _name;
+              // Note that newName can be undefined for two reasons:
+              // 1. Because it is called as a getter and thus called with no arguments
+              // 2. Because the property should actually be set to undefined. This happens e.g. if the
+              //    input is invalid
+              return arguments.length ? (_name = newName) : _name;
             }
           };
         }]);