On Sep 11, 2013, at 5:13 PM, Brendan Eich wrote:
>> Jason Orendorff <mailto:[email protected]>
>> September 11, 2013 3:38 PM
>>
>> But as Allen said, [[Invoke]] is not a performance hack. It's a
>> correctness hack.
>>
>> It permits proxies to customize their behavior around `this`, and even
>> naive .invoke trap users would definitely want those customizations to
>> apply for implicit .toString() and .then().
>
> Sorry, how can this be correctness when ECMA-262 and implementations have
> used [[Get]] + [[Call]] (after some IsCallable conditioning) forever?
It has nothing to do with what has worked forever in the absence of Proxy. The
issue is consistent behavior in the presence of Proxies.
Consider an Proxy-based object the exposes a toJSON method. The proxy might be
implementing a virtual object or it might be implementing a caretaker or some
hybrid of the two styles. In does matter, other than it means that the proxy
might be doing a translation of the this value (or even argument values) when
its methods are invoked.
Let's assume p is such an object.
Presumably, p.toJSON() and JSON.stringify(p) should produce the same result
because stringify internally (and conditionally) calls toJSON. But if
p.toJSON() is implements as:
result = p.[[Invoke]]("toJSON",(), p)
and the internal call of toJSON within stringify is implemented as:
func = p.[[Get]]("toJSON");
if (func && isCallable(func) result = func.[[call]](p,());
else result = undefined;
then the results may not be identical because in the second case P's handler
has not had a change to intercede of the passing of the this value and
arguments to the function.
Allen
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