Calling .constructor on an instance with arguments seems to break.

[jwmerrill]

I'm trying to write a factory method that constructs instances of a subtype:

P(function (proto) {
  proto.factory = function(a1, a2) {return this.constructor(a1, a2)};
});

but using this.constructor with arguments doesn't succeed in passing the arguments to init.

Here's a jsfiddle with running code: http://jsfiddle.net/gKrK3/

[jneen]

Haha, I think I know what's going on. You're hitting the instanceof check because you're using the method calling convention, which sets this to the instance. As a workaround, you can stash it in a variable:

var constructor = this.constructor;
return constructor(a1, a2);

Alternatively, what if you were to put the factory on the class itself? The this context of the definition function is the class itself:

var MyClass = P(function(proto) {
  this.factory = function(a1, a2) { return this(a1, a2); };
});

Looks kinda weird calling this() as a function, but it totally works.

[jneen]

Another alternative is to use the new keyword with the varargs syntax:

return new (this.constructor)([a1, a2])

which also isn't great. Unfortunately I don't think there's anything that can be done about this, since foo.bar(...) always calls foo.bar with a context of foo. Unless you have any ideas?

[jwmerrill]

Your first suggestion works best in my context.

This seems like a pretty annoying sharp edge, though, since I'd guess 90% of the uses of proto.constructor would be to call it right away.

What about passing an extra flag when C is calling itself?

function C(args, fromself) {
  var self = this;
  if (!fromself || !(self instanceof C)) return new C(arguments, true);
  if (args && isFunction(self.init)) self.init.apply(self, args);
}

Haven't tested this--I can look at it again tomorrow.

[jwmerrill]

Looks like my suggestion wouldn't work in the case of calling

this.constructor(a1, false)

[jneen]

Yeah, that argument signature is misleading - hence the page-long comment. It's actually varargs when called without new.

[jneen]

Oh, forgot to mention - this is fixed in 3.x. We even added a test!

[jneen]

In v5.x, you'll need to add a new keyword to the call to this.constructor(...), since otherwise it will mutate the caller.