Calling toString on function proxy throws TypeError exception

[Jordan Harband]

Mark: considering that explicitly invoking a builtin prototype method,
expecting a throw, to test for the [[Call]] internal slot, was the branding
approach the committee agreed to preserve when it reaffirmed
Symbol.toStringTag, as an alternative to `Object#toString.call`, I think
Claude's point ("The expectation is not that `F.p.toString` will always
return something useful; it is that, for any callable object, it will
return a string and not throw, because it was so since the dawn of JS.") is
the one that convinces me.

Shouldn't tests in existing code that rely on `Function#toString` not
throwing to indicate that something is callable, or throwing to indicate
that it is not, remain true for a function proxy?

On Tue, Oct 27, 2015 at 7:52 AM, Mark S. Miller <erights at google.com> wrote:

> Notice that whatever we decide on the issue, functionProxy.toString() will
> work regardless, since you'd be getting the toString method itself through
> the membrane. functionProxy.toString will be a function proxy for the
> target.toString method. The invocation on the toString proxy with
> functionProxy as this will be translated by the membrane back into an
> invocation of target.toString with target as this.
>
> The issue we're debating is only relevant on an edge case -- when
> explicitly invoking F.p.toString.call(functionProxy).
>
>
>
> On Tue, Oct 27, 2015 at 10:04 AM, Claude Pache <claude.pache at gmail.com>
> wrote:
>
>>
>> > Le 27 oct. 2015 à 14:14, Boris Zbarsky <bzbarsky at mit.edu> a écrit :
>> >
>> > On 10/27/15 4:35 AM, Claude Pache wrote:
>> >> it is that, for any callable object, it will return a string and not
>> throw, because it was so since the dawn of JS.
>> >
>> > It's totally false for random "host objects" with a [[Call]] in ES5,
>> per spec and in at least some implementations.  As you can tell in Firefox
>> for example:
>> >
>> >  Function.prototype.toString.call(document.createElement("object"))
>> >
>> > (though it does not throw for document.all in Firefox, for interesting
>> implementation reasons).
>> >
>> >> That function will work (in the sense of: will return an answer; I'm
>> not judging the quality of that answer) with anything reasonable fed to it
>> (where "reasonable" excludes things like `(class { static toString() {
>> throw "pwnd!" }})`).
>> >
>> > Won't work with an HTMLObjectElement in at least some browsers.  How
>> "reasonable" that is, who knows.
>> >
>> > -Boris
>>
>> You're right. But since `document.createElement("object")` does not
>> inherit from `Function.prototype`, the code (`f.toString()`) accidentally
>> works after all.
>>
>> (I've tried not to be too smart in my example by writing `f.toString()`
>> instead of `Function.prototype.toString.call(f)`. Maybe I should have been
>> even less smart by defining an instance method on `Function.prototype`
>> instead of a static method on `Function`...)
>>
>> —Claude
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>>
>
>
>
> --
>     Cheers,
>     --MarkM
>
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