Lodash
JavaScript utility library. Fork of Underscore (2012, John-David Dalton). 200+ helper functions for arrays, objects, strings, collections. Most-installed npm package in the world.
Was indispensable 2012–2018. ES6+ + native browser APIs made much of lodash redundant: Array.flat, Object.entries, optional chaining (?.), nullish coalescing (??). Lodash-es gives tree-shakable imports. Competitors: Ramda (more functional, immutable), modern just use native. Despite redundancy, lodash has 50M+ weekly downloads. Kept for legacy + bundles where tree-shaking doesn't work.