If the length property is enumerable, it's not empty. From the comment just a few lines up,

An "empty" object has no enumerable own-properties.

This function deals with objects and their properties, not arrays or array-like objects and their members.

Just for curiosity, it would be nice to have a broken test, or at least an example here as where checking for length === 0 is not OK.
Thanks

@michaelficarra

This function deals with objects and their properties, not arrays or array-like objects and their members.

See line 830. The isEmpty method deals with arrays and strings, so practically array-like values :P

@michaelficarra, @jashkenas

I guess the point here is this:

Underscore current version:

_.isEmpty({length: 0}); //=> false
_.isEmpty({length: null}); //=> false
_.isEmpty({foo: bar, length: 0}); //=> false

Pull request 690 (after this pull request):

_.isEmpty({length: 0}); //=> TRUE
_.isEmpty({length: null}); //=> false
_.isEmpty({foo: bar, length: 0}); //=> TRUE

This is not covered in the test suite.
If we clearly decide something, I am happy to add a test case for this, to avoid future "flaws" and document better the expected behavior of the isEmpty method.

FWIW, this patch would break the usage of isEmpty in Backbone.

_.isEmpty({}); // -> true
_.isEmpty({length: 0}); // -> false