Subj : Re: spidermonkey bug or feature? watch on array.length
To   : Jens Thiele
From : Brendan Eich
Date : Thu Aug 26 2004 03:28 pm

Jens Thiele wrote:

> a=[];
> a.watch("length",f);
> a.push(10);
> 
> will print:
> this[p]:0 p:length o:0 n:1
> this[p]:1 p:length o:1 n:1
> 
> => my function is called twice
> 
> bug or feature?
> 
> the nice feature of the "second version" is you are called again after
> the element is added which allows you to inspect the added element
> 


At this point, I'd say it's a feature.  Array.prototype.push sets each 
argument as an element starting at length, ending at length + argc - 1, 
and then (because it's generic and the object on which it is called may 
not be an Array), it sets length to length + argc.

So you'll get argc + 1 calls to the length setter, for an Array object. 
  If you use Array.prototype.push on a non-array, you'll get just the 
one length set at the end, after the argc set-element operations that 
(because the object is not an Array) do not magically set length to be 
one greater than the greatest index set so far.

/be

.