Don't discard return value of default constructors. (with tests)

[valueof]

If you try to transform a Backbone class into an implicit factory
(i.e. a class that acts as a factory even when used with new) you
must have a reference to an object created by the default constructor.

This patch makes Backbone to not to discard return value of default
constructors. In addition to the scenario described above it also
makes both code paths consistent with each other.

This is a version of GH-1435 and GH-1429 (but with tests; not sure why
all other PRs didn't have tests) and an inverse of GH-1382.

[wookiehangover]

seems like a reasonable addition to me, especially since multiple issues have been opened up about it in the past few months...

@braddunbar @jashkenas curious to hear your take on getting this landed.

[jashkenas]

Not allowing a constructor to change the type of the returned object is more of a feature than a bug. Instead of "transforming a Backbone class into an implicit factory" ... just do this:

var factory = function(attrs) {
  return new MyModel(attrs);
};

[valueof]

@jashkenas I'd love to but then you can't use collections and such because they often automatically create model instances. In case of Disqus this results in multiple memory leaks that we're now trying to fix. It'd be awesome to have a simpler solution though—so if you know something from your experience I'd love to hear that.

[joesteele]

In a few cases, I just override the model attribute of a few of our collections to be a factory.

So in CoffeeScript...

class SomeCollection extends Backbone.Collection
  model: (attrs, options) ->
    if model = KnownModelTypes[attrs.type]
      return new model(attrs, options)
    else
      return new GenericModel(attrs, options)

Is something like that what you meant?

[jashkenas]

Yep, exactly.

Backbone.Collection.extend({
  model: modelFactoryFunction
});

[valueof]

That's way more elegant. Thanks @joesteele and @jashkenas!

Closing this PR. But the consistency question is still up. GH-1382 to the rescue?

[jashkenas]

I'm a little iffier on that one. It's one thing for our default implementation to (IMO) do the right thing re: constructors ... but it's another thing for us to wrap your chosen constructor function, just to prevent you from returning from it.