Must built-in prototypes also be valid instances? (Was: Why DataView.prototype object's [[Class]] is "Object"?)

Brendan Eich brendan at
Tue Oct 2 10:13:29 PDT 2012

Allen Wirfs-Brock wrote:
> On Oct 1, 2012, at 9:56 PM, Brendan Eich wrote:
>> Allen Wirfs-Brock wrote:
>>> We can try to tell ES implementors that they must do certain things in order to be in conformance but that really doesn't work for code written by users of the language.
>> You're right, we'd be letting usercode, not just some (benign or malign, but if malign then game over already) host object, spoof a core language built-in.
>> But if we have a solid branding mechanism (like Domado's ideal in latest browsers? ;-) then that should be used universally and this becomes a don't-care.
>> /be
> Great, but this is really an orthogonal issue from my extensible Obj.proto.toString design.

I know, but I'm trying to simplify the spec further than you are.

(Allen and I had a phone call -- thanks to that, better understanding of 
what's at stake.)

Unfortunately, toString is used as a brand test by old code and SES 
(targeting pre-ES6 browsers). But rather than enshrine this forever, I 
would like to discuss breaking one case slightly in order to have a 
simpler spec.

The case that the draft spec upholds is

* New ES6+ browser, old and new script mixing, old script uses 
O_p_toString_call to tag-test and new script uses @toStringTag to spoof.

I'd like to break this case, allowing old script in a new browser to be 
spoofed by new script using @toStringTag.

Mark, is this a worry? If so then perhaps we are stuck with the 
complicated Object.prototype.toString in the latest draft.

If not, and I would argue any SES subset must protect its "old script" 
from any spoofing new script, then we should try to unify 
[[NativeBrand]] and @@toStringTag by eliminating use of the former in 
Object.prototype.toString's spec, making the latter the sole extension 

Allen and I also discussed the plan I intend to try in SpiderMonkey: 
making Date.prototype, Number.prototype, etc. test as "Object" according 
to the O_p_toString_call tag test. We think this should not affect SES 
or any real code (as Oliver postulated).

Comments welcome,


More information about the es-discuss mailing list