Weird spec hole in ES3 and ES5
Mark S. Miller
erights at google.com
Thu Jul 2 10:29:52 PDT 2009
On Thu, Jul 2, 2009 at 9:26 AM, Allen Wirfs-Brock <
Allen.Wirfs-Brock at microsoft.com> wrote:
> Because built-in constructors are not (necessarily) implemented in
> ECMAScript code they are not required to use the definitions of [[Call]] and
> [[Construct]] given in 13.2.2 and 13.2.3. Arguably the "Constructor Called
> as a Function" and "called as part of a new expression" sections of chapter
> 15 are specifying the behavior of custom [[Call]] and [[Construct]] internal
> methods for each built-in constructor.
The problem is that .call(), .apply(), and .bind() are specified in terms of
[[Call]], so unless the connection is somehow made, it technically becomes
unspecified how native constructors respond to these reflective operations.
AFAIK, the observed behavior across browsers is consistent with the theory
that "called as a function" actually documents the constructor's [[Call]]
> I'll see if can find a way to clarify this in the introduction to chapter
Some introductory text there would be adequate. No need to propagate local
In my initial message, I was also confused about whether "called as a
function" was meant to cover cases such as
which I normally distinguish by saying "called as a method". If this is not
unclear in the context of the rest of the ES5 spec language, no further
explanatory note is needed, but may be helpful to readers as confused as I.
> >always make a brand-new object of a specific [[Class]], they ignore
> >the object that would be passed in by "new" for a JS-implemented
> >constructor function. The spec for Error seems unclear on this since
> >it refers to "the newly constructed object" without clearly explaining
> >how it is created.
> >I think this should be the behavior. Built-in or host constructors
> Actually, I think the spec. language is pretty clear on this for Error in
> both ES5 and ES5 (same basic language). 15.11.1 says "When Error is called
> as a function rather than as a constructor, it creates and initialises a new
> Error object. " The phrase "the newly constructed object" in 188.8.131.52
> seems to clearly be a reference to that new Error object.
Actually, this reasoning helps clarify why there is currently a potential
confusion. In a normal function call "Error(...)", there is no object
explicitly passed in as the thisArg, so the spec language above does seem
clear for that case. For the reflective or method cases there is an
explicitly provided thisArg, so it is not unreasonable to guess that "the
newly constructed object" may refer to that. Fortunately, the same simple
clarification at the beginning of Ch15 should take care of this too.
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