638044 - JavaScript needs whole-object watchpoints

Status: NEW

(Core :: JavaScript Engine, defect)

Description

[Jim Blandy :jimb]

It would be neat to have watchpoints that watch all properties on a particular object, watchpoints on property additions, deletions, reconfigurations, and so on.

Comment 1

[John J. Barton]

See also Andreas Gal's comment on 
See also https://bugzilla.mozilla.org/show_bug.cgi?id=579342

The 'recursion' problem, or the extension of watchpoint to variables is critical:
   var foo ={};
   foo.bar ="baz";
We need to know both that foo.bar was created *and* that foo was created. If the developer is concerned with 'foo' they don't want to have two different approaches if 'foo' is a property or a variable.

Comment 2

[Sebastian Zartner [:sebo]]

As bug 637913 indicates this issue is having a high priority for Firebug. Our use case is to stop the script execution on property addition and removal[1].

Sebastian

[1] http://code.google.com/p/fbug/issues/detail?id=4162

Comment 3

[David Bruant]

I believe implementing Object.observe (bug 800355) would work here.

Comment 4

[Sebastian Zartner [:sebo]]

So maybe this bug should be merged into bug 800355?
Or do "watchpoints" cover something additional or different?

Sebastian

Comment 5

[Jim Blandy :jimb]

Perhaps not additional, but different. Object.observe reports activity on some subsequent event loop tick, whereas debugger watchpoints need to trigger immediately when the modification takes place.

Comment 6

[Sebastian Zartner [:sebo]]

I'm not quite sure what you mean with "some subsequent event loop tick" but it sounds like we really need watchpoints as you describe them for our use case.

Sebastian