Function.prototype, [[Call]] and [[Construct]]

[Juriy Zaytsev]

On Nov 9, 2009, at 6:50 PM, Garrett Smith wrote:

> [[Construct]] of Function.prototype is not standardized.
>
> The definition of Function.prototype does not define the expected
> behavior for [[Call]] or [[Construct]]. Instead, there is a
> description of what happens when it is invoked:
>
> | 15.3.4 Properties of the Function Prototype Object
> |  The Function prototype object is itself a Function
> | object (its [[Class]] is "Function") that, when
> | invoked, accepts any arguments and returns undefined.
>
> Should Function.prototype implement [[Construct]]? Either way, the
> specification should state the outcome.

Yes, it's a slightly tricky one :) I was wading through Sputniktests  
suite couple of days ago (when finishing web runner — http://thinkweb2.com/projects/prototype/sputniktests-web-runner/ 
) and noticed that there were assertions for exactly this kind of  
compliance.

I even filed a bug with Sputniktests, thinking that they make  
incorrect assumptions — http://code.google.com/p/sputniktests/issues/detail?id=11

However, then I realized that specs do in fact mention this  
explicitly, although not directly under these specific sections.

If you look into Section 15 — Native ECMAScript Objects, you'll see  
this rather clear explanation:

"[...] None of the built-in functions described in this section shall  
implement the internal [[Construct]] method unless otherwise specified  
in the description of a particular function. None of the built-in  
functions described in this section shall initially have a prototype  
property unless otherwise specified in the description of a particular  
function. [...]"

In fact, I mentioned this exact phrase when writing about bugs in  
various browsers, revealed through Sputniktests web runner (http://thinkweb2.com/projects/prototype/sputniktests-web-runner/#construct-and-prototype-of-builtins 
).

And as I said in a post, Firefox and Chrome, for example, fail a whole  
bunch of tests due to most of built-in methods (like  
`Array.prototype.toString`, `parseInt` or `String.prototype.slice`)  
implementing [[Construct]] and having .prototype properties.

[...]

-- 
kangax