Function.create

[Oliver Hunt]

I believe the goal is to eventually deprecate the magic of __proto__ as mutating the prototype of an object makes it difficult to reason about a program and supporting the mutation of an object's prototype also has a reasonably significant performance cost -- the exact time and place (and extent) of the cost is engine specific, but certainly it is not free.

That said I'm not yet convinced we'll ever be able to kill it off :-/

--Oliver


On Sep 24, 2011, at 10:30 AM, Xavier MONTILLET wrote:

> I don't get the difference between the proto operator and some
> setPrototype method that could be added (or the actual __proto__
> property)...
> 
> On Sat, Sep 24, 2011 at 6:03 PM, David Bruant <david.bruant at labri.fr> wrote:
>> Le 24/09/2011 15:33, Jake Verbaten a écrit :
>>> There is no standardized way to create a new function with a prototype
>>> which is not Function.prototype.
>>> 
>>> I propose Function.create
>>> 
>>>     /*
>>>       Creates a new function whose prototype is proto.
>>>       The function body is the same as the function fbody.
>>>       The hash of propertydescriptors props is passed to
>>> defineproperties just like
>>>       Object.create does.
>>>     */
>>>     Function.create = (function() {
>>>       var functionBody = function _getFunctionBody(f) {
>>>         return f.toString().replace(/.+\{/, "").replace(/\}$/, "");
>>>       };
>>>       var letters = "abcdefghijklmnopqrstuvwxyz".split("");
>>> 
>>>       return function _create(proto, fbody, props) {
>>>         var parameters = letters.slice(0, fbody.length);
>>>         parameters.push(functionBody(fbody));
>>>         var f = Function.apply(this, parameters);
>>>         f.__proto__ = proto;
>>>         Object.defineProperties(f, props);
>>>         return f;
>>>       };
>>>     })();
>>> 
>>> This is the same as Object.create except the second parameter is a
>>> function.
>>> 
>>> It will create a new function whose function body is the same as the
>>> function passed in.
>>> 
>>> I don't believe this is possible to do in ES5 without __proto__
>> It is not indeed.
>> This topic has been discussed a couple of times here. There is one
>> proposal that is more generic than your solution which is the proto
>> operator [1].
>> 
>> Basically, to create a function with a chosen prototype, you can do:
>> -----
>> var f = myProto <| function(a, b){return a+b;}
>> Object.getPrototypeOf(f); // myProto
>> f(1, 2); // 3
>> -----
>> 
>> I'm a big fan of the proto operator proposal, however, as raised
>> previously this operator relies on the object being created to have an
>> intialisation syntax. This prevents, for instance, Date objects to have
>> a custom prototype with this method. I am not very familiar with it yet,
>> but I think that if they ever came to life, ParallelArrays [2] would
>> suffer from the same problem.
>> By the way, could someone add this concern as a note ("open issue" or
>> "limitation" or something like this) in the proto operator page, please?
>> 
>> One even more generic way which would work even for objects that have no
>> initialization syntax would to standardize one of [3] or [4]. Is there a
>> wiki page mentionning these two functions somewhere? If not, may it be
>> added and linked in some way to the proto operator?
>> 
>> David
>> 
>> [1] http://wiki.ecmascript.org/doku.php?id=harmony:proto_operator
>> [2]
>> https://github.com/RiverTrail/RiverTrail/blob/15ae7f6f77d9d2842d9d75458017efd9fe0fbee7/jslib/ParallelArray.js#L29
>> [3] https://mail.mozilla.org/pipermail/es-discuss/2011-March/013141.html
>> [4] https://mail.mozilla.org/pipermail/es-discuss/2011-March/013154.html
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