EventEmitter

A lightweight, TypeScript-typed pub/sub event emitter. Used internally by the reactive system, but also exported as a standalone utility.

import { EventEmitter } from '@hx/data';

API

on(eventName, listener)

Subscribe to an event. Returns this for chaining.

const emitter = new EventEmitter();

emitter.on('click', (x, y) => {
  console.log(`click at ${x}, ${y}`);
});

emit(eventName, ...args)

Fire an event. Returns true if there were listeners, false otherwise.

emitter.emit('click', 100, 200); // "click at 100, 200" → returns true
emitter.emit('unknown');         // no listeners → returns false

off(eventName, listener)

Remove a specific listener.

const handler = (msg: string) => console.log(msg);

emitter.on('log', handler);
emitter.emit('log', 'hello');     // fires

emitter.off('log', handler);
emitter.emit('log', 'world');     // does NOT fire

once(eventName, listener)

Register a one-shot listener. Fires once then auto-removes.

emitter.once('init', () => {
  console.log('runs only once');
});

emitter.emit('init'); // fires
emitter.emit('init'); // no listener → returns false

offAll(eventName?)

Remove all listeners. If no event name, clear everything.

emitter.on('a', () => {});
emitter.on('a', () => {});
emitter.on('b', () => {});

emitter.offAll('a');  // removes 'a' listeners only
emitter.offAll();     // removes everything

Type

type Listener = (...args: any[]) => void;

Pattern: Chaining

All mutation methods return this:

emitter
  .on('start', () => console.log('started'))
  .on('stop', () => console.log('stopped'))
  .emit('start');

Pattern: Use with Reactive

The reactive system uses EventEmitter internally for path-based event routing. You can use it independently for any pub/sub needs:

class Store {
  private events = new EventEmitter();

  subscribe(listener: () => void) {
    return this.events.on('change', listener);
  }

  unsubscribe(listener: () => void) {
    this.events.off('change', listener);
  }

  protected notify() {
    this.events.emit('change');
  }
}