Bug 3659 – Add Support for the watch() method of Object

• Status: RESOLVED
• Resolution: WONTFIX
• Component: JavaScriptCore

[David Wheeler]

I think that the watch() method of the Object class is a Netscape-specific add-on not included in the 
ECMAScript spec, but it's amazingly useful. I have a workaround for this using setTimeout(), but the watch
() method is more elegant and timely.

The watch() method (and the corresponding unwatch() method) is documented in the URL included in this 
bug report, but in short, it allows one to trigger a function whenever an object attribute is changed in 
order to trigger other actions, change the value to be assigned, or whatever. Think of it as a callback on an 
attribute assigment callback.

It's supported by FireFox. Not sure about IE/Windows.

[Mark Rowe (bdash)]

A quick check verifies that this method is not present in the ECMA specification, and is not supported by IE 
6.0.

[David Wheeler]

Yes, I found the same, Mark. Thanks.

Here's to hoping that it makes it into the ECMA 4.0 spec! Hrm, don't see it in the current draft...pity.

  http://www.mozilla.org/js/language/es4.html

[Joost de Valk (AlthA)]

sounds reasonable to me! confirming.

[Rob Moore]

Just FYI, IE does support a similar concept -- the propertychange event: 

http://msdn.microsoft.com/workshop/author/dhtml/reference/events/onpropertychange.asp

[Anders Carlsson]

Created attachment 19152 [details]
_Old_ work in progress patch

I found an old patch that implements this, maybe parts of it can be used.

[Gavin Barraclough]

A variant on this theme is now gaining traction with TC-39, with is Object.observe / Object.makeObservable.  Mozilla's documentation also warns against the use of watch.

In light of this, I don't think we want to implement watch.  This is a useful feature, and I think we should closely track progress of the Object.observe / Object.makeObservable proposals.