Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Obsolete since Gecko 2 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
The __count__ property used to store the count of enumerable properties on the object, but it has been removed.
Syntax
obj.__count__
Examples
{ 1: 1 }.__count__ // 1
[].__count__ // 0
[1].__count__ // 1
[1, /* hole */, 2, 3].__count__ // 3
Specifications
Not part of any standard.
Browser compatibility
Not supported anywhere. Historically supported until Firefox 4.