Underscore is a utility-belt library for javascript that provides a lot of the functional programming support that you would expect in prototype.js (or ruby), but without extending any of the built-in javascript objects. it's the tie to go along with jquery's tux.

underscore provides 60-odd functions that support both the usual functional suspects: map, select, invoke - as well as more specialized helpers: function binding, javascript templating, deep equality testing, and so on. it delegates to built-in functions, if present, so modern browsers will use the native implementations of foreach, map, reduce, filter, every, some and indexof.