jashkenas/underscore — pull request #1675 by darkalemanbr (2014-06-11 05:01:03+00:00)
Title: Enhanced isArray and isObject speed

As you can see in these benchmarks, comparing the obj.constructor against the type constructor is at least twice as fast in both cases. It's not faster than typeof, but it's the only way since typeof doesn't know the difference between a "pure object" and an array.
http://jsperf.com/arrtc1337
http://jsperf.com/objtc1337