Ugh, that formatted poorly, at least in my mail client. Here's the example
again:
js> var obj = Proxy.create(myEmptyHandler, proto);
js> String(obj)
trying: toString
trying: valueOf
TypeError: can't convert obj to string
js> obj + ""
trying: valueOf
trying: toString
TypeError: can't convert obj to primitive type
Dave
On Oct 16, 2011, at 1:46 PM, David Herman wrote:
> If you want to create a clean-slate proxy object -- for example, a dictionary
> -- then you can't predefine toString or valueOf. But this means your object
> will always fail at the semantic operations [[ToString]] and [[ToPrimitive]].
> For example:
>
>> var obj = Proxy.create(myEmptyHandler, proto);
>> String(obj)
> TypeError: can't convert obj to string
>> obj + ""
> TypeError: can't convert obj to primitive type
>
> If you actually instrument the proxy to watch which operations it's trying,
> you'll see:
>
>> var obj = Proxy.create(myEmptyHandler, proto);
>> String(obj)
> trying: toString
> trying: valueOf
> TypeError: can't convert obj to string
>> obj + ""
> trying: valueOf
> trying: toString
> TypeError: can't convert obj to primitive type
>
> Should we not offer derived traps for toString and valueOf (which, if not
> defined, default to looking up the "toString" and "valueOf" properties,
> respectively, on the receiver), to allow for stratified implementation of
> this reflective behavior, i.e., without polluting the object's properties?
>
> Dave
>
> PS SpiderMonkey also has the unstratified Object.prototype.toSource, which is
> used for the non-standard uneval function, for printing out values at the
> console. This is kind of unfortunate since it suggests a need for another
> proxy trap, but this time it's not for a standard functionality.
>
> _______________________________________________
> es-discuss mailing list
> es-discuss@mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
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