Reflection of global bindings

Brendan Eich brendan at
Sat Dec 15 11:25:39 PST 2012

Allen Wirfs-Brock wrote:
> On Dec 15, 2012, at 9:21 AM, David Bruant wrote:
>> Hi,
>> On public-script-coord, Boris Zbarsky showed an example [1] where a global variable is var-defined and then observed to be absent from the global object it was attached to (because the WindowProxy has changed of underlying window object).
>> The careful ECMAScript 5.1 reader knows that global var-defined variables are supposed to be reflected as own non-configurable properties of the global object. Reflecting the property as such commits the object to keep the property forever which, as we've seen in Boris example, is not compatible with how the web works.
>> I don't think it'll be possible to change how global objects in same-origin iframes work nor relax ES5 invariants; so my proposal is to have var/let/const/function global declarations *reflected* as configurable, but acting as if they were not.
>> This would also require a couple of changes in the spec and consequently JS engines and certainly test262.
>> Other proposals welcome, of course.
> Note that in ES6 global bindings introduced by  let/const/class/import bound using a declarative environment record  and will not be reflected at all on the global object.
> That still leaves any issues relating to var and function declarations.

Reflecting var and function bindings on window (or |this| or |self| or 
other aliases) as configurable properties, but refusing to allow 
reconfiguration, is a solution, however distasteful. Allowing 
configuration is no good, of course.


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