JavaScript's functional programming helper library

 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.