javascript - What happens when $scope.$watch expression depends on multiple variables? -

if in controller, have like:

$scope.service = service;  $scope.$watch('service.properties[service.selectedproperty]', function(newval,oldval){}); 

what happens here? both service.properties , service.selectedproperty can change. watch end watching both service.properties , service.selectedproperty?

i have tested , watch seems execute when either variable changes, want. wondering if there unintended consequences when using .$watch way, , how works.

does watch end watching both service.properties , service.selectedproperty?

in fact, watching neither, result of evaluating expression whole. explain:

angular watches use dirty checking principle: if expression whole evaluated else when last evaluated, change has happened, , associated handlers run. (this behaviour more apparent when use form of $watch takes function first argument - returning value watched.)

that means technically there exist situation service.properties[service.selectedproperty] evaluated same before, service.properties, associative object, and service.selectedproperty, property key have both (or either of them) changed in meantime - yet, expression evaluated same. in such cases, change event not triggered.

for example, given:

var myobj = {     a: 123,     b: 123 }; var key = 'a'; 

the value of expression myobj[key] stay same if change key 'b' instead of 'a'.

as long expression evaluates same, viewpoint of angular, nothing has changed. , intents , purposes, nor should matter - if does, watch expression set improperly, , want watch more generic expression.