underscorejs

Underscore.js is a JavaScript library that provides a collection of utility functions designed to simplify common programming tasks. It emphasizes a functional programming style, offering helpers for arrays, objects, and functions without mutating the original data.

Created by Jeremy Ashkenas and first released in 2009, Underscore became a widely used tool for JavaScript development before the rise of Lodash. It is open-source software released under the MIT License and can be integrated into web projects or Node.js applications.

Its API includes core functional helpers such as each (alias for forEach), map, reduce, filter, find, some, every, contains, pluck, and sortBy. It also provides collection utilities like groupBy, indexBy, countBy, size, and the chain feature for fluent chaining of operations. In addition, Underscore offers array and object helpers such as uniq, flatten, zip, extend, defaults, isArray, isObject, isFunction, and isEqual. The template utility allows simple string interpolation with embed tags, and the library includes function tools like bind, partial, memoize, throttle, debounce, once, and after.

Underscore is designed to work in both browsers and Node.js and can be loaded via a script tag or installed through package managers such as npm. It uses a small, dependency-free footprint and avoids extending native prototypes.

Although Lodash has since become more popular, Underscore remains in use, particularly for legacy codebases or developers who prefer its API design and simpler, smaller surface.