794990 - An object that extends NodeList does not satisfy instanceof

Status: RESOLVED FIXED

(Core :: DOM: Core & HTML, defect)

Description

[Matthew Prestifilippo]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.89 Safari/537.1

Steps to reproduce:

I extended NodeList (my subclass is called ElementList).  But an instance of ElementList does not satisfy instanceof NodeList.  The following code will illustrate this.

var ElementList = function() {}
ElementList.prototype = document.querySelectorAll('doesntexist')
var a = new ElementList
console.log(a instanceof NodeList)


Actual results:

a instanceof NodeList should return true


Expected results:

a instanceof NodeList should returned false

Comment 1

[Matthew Prestifilippo]

Got it backwards...

Actual results:

a instanceof NodeList returns false


Expected results:

a instanceof NodeList should be true

Comment 2

[Trevor Norris [:tnorris]]

Just ran the following code in Nightly, still returns false.

Comment 3

[Loic]

Mozregression range:

m-c
good=2011-10-10
bad=2011-10-11
http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=e9c620a5c85f&tochange=ccea01542d0b

m-i
good=2011-10-10
bad=2011-10-11
http://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=e1927273c6c9&tochange=568821b5cdca

Suspected bug:
Bug 648801 - Prototype a proxy-based NodeList implementation

Comment 4

[Boris Zbarsky [:bzbarsky]]

Yep, NodeList has a somewhat magic [HasInstance] hook...

Peter, is this changing with the WebIDL bindings?

Mathhew, note that there isn't all that much you can do with the resulting object, per spec... (e.g. getting .length on it should throw).

Comment 5

[Matthew Prestifilippo]

I agree it's hard to work with.   If I do the following, I can see what I added to the ElementList

l = new ElementList;
l[0] = document.createElement('div');
l[1] = document.createElement('a');
for(var i in l) console.log(l[i]);

I don't have to subclass to get this functionality.  I'm not even sure this is supposed to be legal.  Please don't remove the functionality though!  haha

Comment 6

[Boris Zbarsky [:bzbarsky]]

> for(var i in l) console.log(l[i]);

Per spec this should work for 0 and 1, then throw for i == "length".

Comment 7

[Matthew Prestifilippo]

The spec says the length property is readonly.  Why would it throw an error if I read the length property?

Comment 8

[Boris Zbarsky [:bzbarsky]]

Because the "this" object for the getter is not an actual NodeList object.  Please see http://dev.w3.org/2006/webapi/WebIDL/#es-operations third bullet point step 2.

Comment 9

[Boris Zbarsky [:bzbarsky]]

Fixed in WebIDL bindings.