rewire is a small Node.js library that lets you replace or inspect a module’s private variables, dependencies and globals without altering the original source. By loading a module through `rewire()` you gain `__set__`, `__get__` and `__with__` helpers for mocking internal state in unit tests, making it ideal for JavaScript developers who need fine‑grained test isolation. It works with plain JS and CoffeeScript and can be used in server‑side or bundled client‑side projects.