EventEmitter

EventEmitter is a class in Node.js' events module that provides a lightweight publish/subscribe pattern for event-driven programming. It enables objects to emit named events and allows other code to register listeners that respond when those events occur. This pattern is foundational to many Node.js core modules and user-defined components.

The core API centers on methods for managing listeners and emitting events. The on or addListener methods register a listener function for a given event name. The once method registers a listener that runs only the first time the event is emitted. The emit method triggers all listeners registered for a given event name, passing along any arguments to the listeners. Listeners can be removed with removeListener or removeAllListeners, and the current set of listeners for an event can be retrieved with listeners. The setMaxListeners method can be used to adjust the reported limit on how many listeners can be attached to a single event, helping to detect potential memory leaks.

Behavior and usage patterns: listeners are invoked in the order they were added. If a listener throws an error, it may affect subsequent listeners unless handled. EventEmitter also supports querying the number of listeners indirectly, and it provides a mechanism to warn about possible memory leaks when many listeners are attached to the same event.

Subclassing and usage: developers often subclass EventEmitter to create custom components that expose an event-based API. It is a core module in Node.js and is not part of standard browser APIs, where EventTarget or other libraries provide similar functionality. EventEmitter remains a central tool for coordinating asynchronous activities in server-side JavaScript environments.