Underscore.js

Underscore.js is a JavaScript utility-library created by Jeremy Ashkenas that provides a broad set of functions for working with arrays, objects, functions, and other data types — essentially a “utility belt” for functional programming in JS. Instead of extending built-in objects or modifying prototypes, Underscore provides its helpers in a single _ namespace, enabling cross-browser support and consistent behaviour across environments. It offers map/filter/reduce, deep-cloning, templating, object traversal, function binding, and more, which makes it easier to write concise, expressive code rather than verbose loops and conditional logic. Many other libraries and frameworks (including Backbone) historically depended on or partnered with Underscore for their utility functions. Even though ES6+ and Lodash have overshadowed some of its role, Underscore remains influential and educational for how to design utility APIs.