When called, requests that the Node.js event loop _not_ exit so long as the `fs.StatWatcher` is active. Calling `watcher.ref()` multiple times will have no effect. By default, all `fs.StatWatcher` objects are "ref'ed", making it normally unnecessary to call `watcher.ref()` unless `watcher.unref()` had been called previously.

When called, the active `fs.StatWatcher` object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the `fs.StatWatcher` object's callback is invoked. Calling `watcher.unref()` multiple times will have no effect.