Underscore is a utility-belt library for JavaScript that provides a lot of the functional programming support that you would expect in Prototype.js, but without extending any of the built-in JavaScript objects.

Underscore provides 80-odd functions that support both the usual functional suspects 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.

Source: Underscore.js