Using IsCallable Operation?

[Garrett]

There seems to be a lot of controversy on how to properly check if
something should be callable. There isn't any way to determine that in
ES3.

The internal IsCallable determines if an object has a [[Call]] property.

Will that be available as a language feature, not internal?

For example:-
  Object.isCallable( document.images );

- would provide a check to internal IsCallable and return the result.

Garrett
_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

[Allen Wirfs-Brock-2]

Not in ES5.  Generally checking that typeof x === 'function' is a close approximation. ES5 (11.4.3) requires that  both native (ie, pure actual ECMAScript objects) and host objects that implement [[Call]] produce "function" when typeof is applied to them.  This is a change from ES3 that did not impose this requirement upon host object objects.

Of course it may take a while for browser implementations to respond to this change.

Allen

>-----Original Message-----
>From: es-discuss-bounces@... [mailto:es-discuss-
>bounces@...] On Behalf Of Garrett Smith
>Sent: Friday, June 05, 2009 1:31 PM
>To: es-discuss
>Subject: Using IsCallable Operation?
>
>There seems to be a lot of controversy on how to properly check if
>something should be callable. There isn't any way to determine that in
>ES3.
>
>The internal IsCallable determines if an object has a [[Call]] property.
>
>Will that be available as a language feature, not internal?
>
>For example:-
>  Object.isCallable( document.images );
>
>- would provide a check to internal IsCallable and return the result.
>
>Garrett
>_______________________________________________
>es-discuss mailing list
>es-discuss@...
>https://mail.mozilla.org/listinfo/es-discuss

_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

[Mark S. Miller-2]

On Fri, Jun 5, 2009 at 1:39 PM, Allen Wirfs-Brock
<Allen.Wirfs-Brock@...> wrote:
> Not in ES5.  Generally checking that typeof x === 'function' is a close approximation. ES5 (11.4.3) requires that  both native (ie, pure actual ECMAScript objects) and host objects that implement [[Call]] produce "function" when typeof is applied to them.  This is a change from ES3 that did not impose this requirement upon host object objects.

Why is this an approximation? In ES5, how can
    typeof x === 'function'
differ from testing whether something is callable?

--
    Cheers,
    --MarkM
_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

[Allen Wirfs-Brock-2]

Well, it depends upon implementations actually adhering to the typeof specification (which may take some time to come about).  In ES5 IsCallable is a specification device that essentially means you really truly can call it (which is what it means to have a [[Call]] specification method).

Allen

>-----Original Message-----
>From: Mark S. Miller [mailto:erights@...]
>Sent: Friday, June 05, 2009 2:19 PM
>To: Allen Wirfs-Brock
>Cc: Garrett Smith; es-discuss
>Subject: Re: Using IsCallable Operation?
>
>On Fri, Jun 5, 2009 at 1:39 PM, Allen Wirfs-Brock
><Allen.Wirfs-Brock@...> wrote:
>> Not in ES5.  Generally checking that typeof x === 'function' is a
>close approximation. ES5 (11.4.3) requires that  both native (ie, pure
>actual ECMAScript objects) and host objects that implement [[Call]]
>produce "function" when typeof is applied to them.  This is a change
>from ES3 that did not impose this requirement upon host object objects.
>
>Why is this an approximation? In ES5, how can
>    typeof x === 'function'
>differ from testing whether something is callable?
>
>--
>    Cheers,
>    --MarkM

_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

[Garrett]

On Fri, Jun 5, 2009 at 2:18 PM, Mark S. Miller<erights@...> wrote:
> On Fri, Jun 5, 2009 at 1:39 PM, Allen Wirfs-Brock
> <Allen.Wirfs-Brock@...> wrote:
>> Not in ES5.  Generally checking that typeof x === 'function' is a close approximation. ES5 (11.4.3) requires that  both native (ie, pure actual ECMAScript objects) and host objects that implement [[Call]] produce "function" when typeof is applied to them.  This is a change from ES3 that did not impose this requirement upon host object objects.
>
> Why is this an approximation? In ES5, how can
>    typeof x === 'function'
> differ from testing whether something is callable?
>
ES5 typeof table:
+--------------------------+------------+
| Object (native or host   | "function" |
| and implements [[Call]]) |            |
+--------------------------+------------+

That is an idealistic change. IMO ES3 should have had that.

Microsoft could have taken a cue from the ES3 spec behavior for typeof
with "object that implements call" and had the typeof operator return
"function" instead of one of "function", "object", or "unknown".

One could theoretically perform a capability test on the typeof
operator, but that would not be practical because implementations
today have varied results with the typeof operator on host objects.

The classic document.images problem originates from IE.
The document.images collection is callable in Safari, Opera, Chrome,
and IE and most likely several other browsers.

typeof document.images

- "function" in Safari, "object" in IE Chrome and Opera 9+. Also
"object" in Gecko (where it is not callable).

Testing the document.images collection to see if is callable can't
work with a typeof operator that is inconsistent across versions of
the language.

A hand-rolled isCallable method that has a (poor) fallback (the
current "best practices") could expect the new behavior specified for
typeof operator if and only if it was known to the program be the
newer typeof.

function isCallable(obj, prop) {
  var type = typeof obj[prop];
  if(isNewTypeof) {
    return type === "function";
  } else {
    // fallback "best practice".
  }
}

That code requires an "isNewTypeof" variable. A strategy for
determining the correct value of isNewTypeof would be an
impossibility.

I think it would be worth considering a native isCallable method.

Garrett
_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss

[Mark S. Miller-2]

On Fri, Jun 5, 2009 at 7:22 PM, Garrett Smith <dhtmlkitchen@...> wrote:

function isCallable(obj, prop) {
 var type = typeof obj[prop];
 if(isNewTypeof) {
   return type === "function";
 } else {
   // fallback "best practice".
 }
}

 function isCallable(obj, prop) {
   var type = typeof obj[prop];
   if (function(){"use strict";return this;}()) {
     // fallback "best practice".
   } else {
     return type === "function";
   }
 }

--
   Cheers,
   --MarkM

_______________________________________________
es-discuss mailing list
es-discuss@...
https://mail.mozilla.org/listinfo/es-discuss