libjs-underscore

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.