[[Call]] and [[Construct]]

Maciej Stachowiak mjs at apple.com
Tue Sep 29 20:13:06 PDT 2009


On Sep 27, 2009, at 4:19 PM, David-Sarah Hopwood wrote:

> Maciej Stachowiak wrote:
>> On Sep 27, 2009, at 11:14 AM, Brendan Eich wrote:
>>> On Sep 27, 2009, at 10:41 AM, David-Sarah Hopwood wrote:
>>>> That can already be done in ES5. As I've previously suggested:
>>>> function Date(yearOrValue, month, date, hours, minutes, seconds,  
>>>> ms) {
>>>>  "use strict";
>>>>  if (this === undefined) {
>>>>    return TimeToString(CurrentTime());
>>>>  }
>>>>  // constructor behaviour
>>>>  ...
>>>> }
>>> Of course, a variation on "the idiom".
>>> This is similar to what many implementations do too, rather than the
>>> implementation providing analogues of [[Call]] and [[Construct]]
>>> internal method on a non-function Date object. It works for Boolean,
>>> Number, String, and RegExp too.
>>> But it is just a bit unsightly!
>> Will this do the right thing if you explicitly bind Date to a "this"
>> value, for example, by calling it as window.Date(), or using call,
>> apply, or function.bind, or by storing Date as the property of  
>> another
>> random object?
> Now that I think about it, probably not (it will attempt to set the
> [[Class]] of 'this' to "Date", which is unsafe). But the problem here
> isn't introduced by the fact that [[Call]] and [[Construct]] have
> different behaviour: none of the other built-in constructors can be
> safely expressed in ES5 for the same reason, regardless of their
> [[Call]] behaviour.

I believe the built-in constructors could be implemented correctly by  
ignoring their "this" parameter and making a fresh object of the  
appropriate type, rather than by modifying the [[Class]] of a presumed  
"this" passed as part of construction. For any that have identical  
[[Call]] and [[Construct]] behavior, it would then be fine to write it  
as a function - either way it produces a new object and ignores  
whatever may have been passed for "this". This is true for Array for  
instance. (Object call and construct also look like they have the same  
behavior, though written in two different ways for some reason.)


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