allow operation with dense array for Backbone collection

[jdkanani]

It throws TypeError for collection operations with Backbone v1.1.0.

//  TypeError: Cannot read property 'id' of undefined
new Backbone.Collection(new Array(5)); 
col.add([undefined, {label: 'a'}]);
col.reset([{label: 'a'}, undefined, new Backbone.Model()]);

[akre54]

I'm not sure we should encourage passing undefined models to a collection since it will screw up most of the collection's methods. What's your use case?

[jdkanani]

I agree. But, it was supported till Backbone v1.0.0. You can try that out same example with Backbone v1.0.0 and v1.1.0. And TypeError should not be thrown in any case.

[akre54]

In that case I'd consider it a fixed bug in Backbone 1.0.0. Absent a good use case, I think this should stay fixed.

[tgriesser]

This was introduced by the changes in #2755... Although this behavior wasn't documented, it might not be bad to either discard the empty values (as is done elsewhere, like events), or assume that an empty value is an empty object, as would be the case in this pull request.

[tgriesser]

Since you can do new Model(null) and have it create an empty model, I think that just letting the falsy values pass along to _prepareModel makes sense as the behavior here, and the use case of new Collection(new Array(n)) is valid for a simple way of creating a collection with a particular length.

@jdkanani - can you add a few more tests to cover the cases you provided above?

[jashkenas]

We'll need a better implementation, though.

[tgriesser]

} else if (_.isObject(attrs)) { ?

[jashkenas]

No, something clearer and less incidental.

attrs = models[i] || {};

Should do it.

[tgriesser]

Oh right, of course :)

[jdkanani]

@tgriesser @jashkenas Added one more commit for test cases and better fix.