After Ember setter

after.js

locations: function(key, value, oldValue) {
 if(value) {
   this.removeOldMarkers(oldValue);
   this.addMarkers(value);
 }
 return value;
}.property(),

removeOldMarkers: function(markers) {
 markers.forEach(function(marker) {
   // remove markers from Google map
 });
}

addMarkers: function(markers) {
 markers.forEach(function(marker) {
   // add markers to Google map
 });
}