What is the difference between Underscore.js and Backbone.js?

The difference between Underscore.js and Backbone.js lies in their purpose and functionality. Underscore.js is a utility library for JavaScript, it provides a lot of functional programming helpers for working with data: mapping, filtering, sorting, and reducing of arrays and objects. The main purpose of Underscore.js is to make coding in JavaScript easier by providing utility functions that enable more readable data handling.
Backbone.js is a light-weight JS library developed to build a single-page web app (SPA) in a structured manner. It offers models with key-value binding, collection, view with event handling, and routers that handle navigation. Backbone.js depends on Underscore.js for utility functions but gives structure to the applications by managing interaction between data and user interface elements.
Underscore.js is a functional utility, while Backbone.js is a framework to build up well-structured and data-driven web applications.