User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:23.0) Gecko/20100101 Firefox/23.0 (Beta/Release) Build ID: 20130730113002 Steps to reproduce: Get the reference of a <select> element and use the watch() method: <script> var mySelect = document.getElementById('myselect'); mySelect.watch('disabled', function(attribute, oldval, newval){ return newval; }); </script> Actual results: On FF <=22 works as expected, but with FF 23, raises an error: "can't watch non-native objects of class Proxy" With no external JavaScript libraries. Expected results: The watch() method should work as on previous versions of FF

Yep. That's because a <select> is in fact a proxy... Note that afaik trying to watch() the value of the "disabled" property never worked anyway, since it's an accessor property with a side-channel that can change it.

Thanks for the clarification. So, there is not a problem, just an error not triggered before FF 23.

Before FF23 <select> wasn't a proxy, but could have properties magically appear (but incorrectly not disappear) on it.

(In reply to Boris Zbarsky (:bz) (vacation Aug 10-19) from comment #1) > Yep. That's because a <select> is in fact a proxy... So what should we do? Can we now capture changes of select’s value property?

You couldn't before, and you can't now... If all you care about is when someone does "select.value = whatever;", then you can capture that by interposing your own setter that calls through to the canonical setter. But if the user actually selects a different option, the .value will change with no notifications to the JS engine.

Not a JS engine bug, at least -- more an incidental effect of a DOM change. I'd also note in passing that watchpoints installed/uninstalled by watch/unwatch have a bunch of bad effects in terms of performance and other metrics. (Not to mention their conceptually not working in some cases, like here before the FF23 change.) You really shouldn't use them if you can help it.

(In reply to Boris Zbarsky (:bz) (vacation Aug 10-19) from comment #5) > You couldn't before, and you can't now... Why? watch("value" …) DID work on HTMLSelectElement perfectly. > If all you care about is when someone does "select.value = whatever;", then > you can capture that by interposing your own setter that calls through to > the canonical setter. It is not so easy however… Have you meant I should get canonical setter by Object.getOwnPropertyDescriptor(Object.getPrototypeOf(combo), "value") and use it in Object.defineProperty(combo, "value", {…})?

> Why? watch("value" …) DID work on HTMLSelectElement perfectly. The call worked fine. The value would just change without your function being called. > have you meant I should get canonical setter by > Object.getOwnPropertyDescriptor(Object.getPrototypeOf(combo), "value") Yes, exactly.

Here is link for my solution. It seems to work properly… http://torbasow.livejournal.com/534771.html