The Node.js EventEmitter method that registers a listener for one firing is `once()`. When the named event is emitted, Node removes that listener and then invokes it, so subsequent emissions do not call the same registration again.
This is useful for one-time setup and lifecycle events: waiting for a connection, receiving an initial response, completing a startup step, or handling a resource that should be opened only once. A typical call is `emitter.once("ready", callback)`. The method returns the EventEmitter, allowing calls to be chained.
The closest mix-up is `on()`, which registers a persistent listener and runs it every time the event is emitted. `addListener()` is an alias for `on()`, so it also remains active until removed. `emit()` does not register anything; it triggers an event and passes arguments to the listeners currently attached to it.
Node’s event system is built around the EventEmitter class in the `node:events` module. Listener execution is synchronous by default and follows registration order. A one-time listener is still called at most once if the event never occurs, of course, it is never invoked.