Admittedly it makes code neat, but I'd be wary about adding to Array.prototype unless you are adding a function that has a spec in a later version of Javascript. What if later version defined an Array.prototype.multiply that was actually the dot product, say? Either your code would break, or other libraries that worked against the Javascript spec would break.

Either use Array.multiply (which isn't quite so evil), or just define as arrayMultiply() within the code.