Non-extensibility of Typed Arrays

Brendan Eich brendan at
Wed Sep 4 17:36:27 PDT 2013

> Filip Pizlo <mailto:fpizlo at>
> September 4, 2013 4:45 PM
> On Sep 4, 2013, at 2:41 PM, Brendan Eich <brendan at 
> <mailto:brendan at>> wrote:
>> Filip Pizlo wrote:
>>> I agree that both of these aspects of binary data are quirky.  My 
>>> observation is that prohibiting custom properties doesn't fix the 
>>> underlying issue.
>>> [snip]
>>>>    var MyArray = new ArrayType(...);
>>>>    var MyStruct = new ArrayType({f: MyArray});
>>>>    var struct = new MyStruct(...);
>>>>    var array1 = struct.f;
>>>>    var array2 = struct.f;
>>>> then `array1` and `array2` would have disjoint sets of extensible
>>>> properties.]
>>> Yes, they would.  But even if they didn't, then array1 != array2, 
>>> which is equally odd.
>> Both adds up to two, not one, so the counterargument is odd beats 
>> odder, and prohibiting expandos keeps the oddness down to == and 
>> nothing else.
> What about being consistently odd?  To me, 'struct.f' means allocating 
> a new [sic] array buffer view object.  That new object thus has all of 
> the features you'd expect from a new object: it won't have custom 
> properties, it will have some default prototype, and it will not be == 
> to any other object.  Hence if you say " = 42", then 
> "" will subsequently return undefined.  No big deal - it 
> was a new object.

In WebIDL, IIRC, this is considered bad style. You are supposed to use a 
method, not an IDL attribute (getter), when returning a fresh object 
each time. Anne can vouch, cc'ing him.

I'm not saying we must match WebIDL good style -- doing so would mean 
binary data does not support interior extractions via -- 
rather something awful such as, which won't fly. But it 
seems worth considering that the "odd" or "bad style" design decision is 
upstream of our petty difference over expandos.
>> But "lost expandos" due to loss of identity are an especially nasty 
>> kind of bug to find.
> I'm actually curious - are you aware of such bugs, and what do they 
> actually look like?

Search for "disappearing expando" on the web.


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