Underscore.js (http://underscorejs.org/) is a powerful utility library that provides a lot of functional programming support for your JavaScript code. In general, JavaScript comes up with a very low number of utility methods on its own, and most of the time we need to either develop our own functions or depend on another library for these methods. Underscore comes up with a bagful of highly efficient utility methods, which makes it an excellent tool for your JavaScript projects. The functions it provides can be grouped into the following sections:

These include functions for iterations, sorting, filtering, conversions, templating, comparisons, scope binding, and many more. The main benefits of using this small library are as follows:

Backbone.js has a hard dependency on Underscore.js, and you are bound to use it if you are developing your applications with Backbone.js. However, even when you are not using Backbone, we encourage you to use Underscore.js for your JavaScript projects. It adds no overhead, integrates easily, and makes your code more robust even when you are not aware of all the underlying engineering principles employed by this library.

There is another library named Lo-dash (http://lodash.com), which provides an Underscore built to perform drop-in replacement of the Underscore.js library. It is said to have a slightly better performance than Underscore.js. You can try either of them to achieve the same result.