Eventemitting

Eventemitting refers to the practice of an object or module issuing notifications about occurrences by emitting named events. Emitters allow other parts of a system to respond by registering listeners or callbacks. This enables loose coupling, as the emitter does not need to know which components will handle the event.

In many environments, the most common implementation is an event emitter, which maintains a registry of listeners for each event name. When emit is called with an event name and optional payload, all registered listeners for that event are invoked, typically in the order of registration. Listeners may perform synchronous work or schedule asynchronous tasks. Some environments have special handling for error events; for example, emitting an error event without any listeners can lead to an exception or process termination.

Node.js provides a standard event emitter interface via the EventEmitter class. Typical methods include on or addListener to register a callback, once to register a one-time listener, removeListener or off to unregister, and emit to fire the event. The browser provides a related model through the EventTarget interface, with addEventListener, removeEventListener, and dispatchEvent; custom events can be created using CustomEvent.

Key design considerations include event naming, payload structure, and lifecycle management. Event-driven designs benefit from decoupling and responsiveness but can be harder to trace and debug. Proper listener management is essential to avoid memory leaks, and developers often balance synchronous and asynchronous handling and consider event propagation semantics in environments like the DOM.

See also: event-driven architecture, observer pattern, publish-subscribe.