Ramda

Ramda is a practical, open-source functional programming library for JavaScript that provides a broad collection of pure, side-effect-free functions designed for immutable data transformations and declarative coding. The library emphasizes function composition and currying, with most functions automatically curried and designed to take their data as the last argument. This data-last style enables straightforward point-free composition using pipe and compose.

Core features include utilities for working with arrays and objects (map, filter, reduce, merge, assoc, prop, path), and function utilities (curry, compose, pipe). It also offers lenses (lensProp, view, set, over) to support immutable updates, and supports transduce for efficient data processing. Ramda provides a consistent API across data types and is designed to be used in both Node.js and in the browser, with modular imports to support tree-shaking.

Philosophy and usage: Ramda prioritizes declarative, readable code and referential transparency by avoiding mutation and favoring pure functions and composition. Compared with libraries like Lodash, Ramda's data-last, curried style is specifically aimed at enabling composition without manual currying. It is open-source and released under a permissive license, with an active community and ecosystem integrations.