Immutable JS is a library that provides efficient, persistent immutable collections (e.g., List, Map, Set, Record) and a lazy Seq API for JavaScript. It lets developers write functional‑style code—especially in React/Flux apps—without worrying about accidental data mutation, while offering type definitions for Flow and TypeScript. The target audience is front‑end engineers and any JavaScript developers who need predictable state handling and performant immutable data structures.