Rewire is a Node‑.js testing utility that lets developers monkey‑patch a module’s private variables, dependencies and globals without altering the source code. By loading a module through `rewire()` instead of `require()`, tests can inject mocks, read or change internal state via `__set__`, `__get__` and `__with__`, then cleanly revert those changes—making it ideal for unit‑testing CommonJS modules that need fine‑grained control over their internals.