At this point I stay their hand and point to Annotated ECMAScript, highlighting the superpowers baked into
Array.prototype. Granted it lacks some of the things Underscore has, but it often does what you want.
Mozilla's documentation is also good because it shows you how to duplicate the functionality with lengthy code samples, which is educational if you take the time to read it.
typeof [1, 2, 3] is
'object' you might want to pack your bags and give up altogether. I don't blame you. But hiding in that ES5 documentation is a beastie called
Array.isArray. You'll find it in
util in Node, which is puzzling -- you'll be OK using
Array.isArray([1, 2, 3]).