The class operator: a bridge between object and function exemplers

Allen Wirfs-Brock allen at
Mon Nov 14 15:47:43 PST 2011

On Nov 14, 2011, at 3:09 PM, Axel Rauschmayer wrote:

>> The concept of a constructor function being the "class" of an object is one of those things that has been around JS apparently from the beginning. It just didn't have a suggestively named operator.  However an object is instantiated, this operator is simply reporting that the "class"of the object  whatever is accessible as the object's "constructor".
> So you don’t think the two roles clash semantically? (1) “Turn an object exemplar into a class” versus (1) “get the class of an object”. Both are definitely useful! But that they can be performed by the same operator seems like a happy coincidence. If (1) was to be further refined, e.g., as Brendan suggested, by throwing an exception if an object exemplar does not have a "constructor" method then it seems both roles would drift further apart. I’m mainly arguing for a clear separation of concerns.

The two roles are the same!  The value of the constructor property of an object exemplar is a function exemplar (AKA class exemplar)  whose prototype property's value is the object exemplar. When you say:
     let P = class {constructor() {} };
You aren't turning an object exemplar into a class exemplar your are simply accessing the class exemplar that is explicitly defined  as part of  the object exemplar.

The above let is also equivalent to:

   let PrototypalP = {constructor(){} };
   let P = class PrototypalP;

which is equivalent to:

   let P = class  {constructor(){} };
   let PrototypalP = P.prototype;

Finally, when I talk about an object's constructor I include the possibility that 'constructor' is inherited, so the above identifies only hold when an own constructor property is explicitly provided.  In fact, that is really what I mean by an object exemplar

   var foo = { };
   class foo is Object;   //true
   Object.prototype is foo;  //also true

   bar = new foo;  //same as new foo.constructor which is the same as new Object
   class bar is class foo;   //and both === Object.
It all falls out of the plumbing.


> -- 
> Dr. Axel Rauschmayer
> axel at
> home:
> twitter:
> blog:

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