change event of html hidden field

The change event doesn't fire when the value is programmatically changed, if it did it would cause infinite loops in many situations.

If you need the event to fire, then trigger it when changing the value yourself using .change(), like this:

$("#hiddenId").val("new value").change();

.change() is a shortcut for .trigger("change") or, if you don't want that change event to bubble for some reason, then use .triggerHandler("change").

in this case you know exactly, when the value changes (onchange of dropbox), because you change it by your code, so there is no need to catch an event.

simply call your reaction function in the onchange part of your dropbox event.