tiny_horrible_emitter.js

const kMap = Symbol('map');
class EventEmitter {
  constructor() {
    this[kMap] = new Map();
  }
  on(event, handler, once = false) {
    const fn = ({ detail }) => handler.apply(this, detail);
    this[kMap].set(handler, { fn, once });
    return window.addEventListener(event, fn, { once });
  }
  once(event, handler) {
    return this.on(event, handler, true);
  }
  emit(event, ...args) {
    return window.dispatchEvent(new CustomEvent(event, { detail: args }));
  }
  off(event, handler) {
    const { fn, once } = this[kMap].get(handler);
    if (!fn)
      return false;
    return window.removeEventListener(event, fn, { once });
  }
}