Mutex‑Promise is a tiny JavaScript library that provides a cross‑environment (browser + Node.js) mutex built on top of Promises and localStorage (or a pluggable storage). It lets developers guarantee that only one asynchronous operation—e.g., a server request—runs at a time across tabs or processes, with optional timeout and polling settings. Ideal for web apps or Node scripts that need simple, reliable mutual‑exclusion without external dependencies.