Typesafe event emitters

event_emitter.ts

class EventEmitter<T extends object> {
  private readonly listeners: {
    [key in keyof T]?: ((value: T[key]) => void)[];
  } = {};

  emit<Event extends keyof T>(event: Event, value: T[Event]) {
    const listeners = this.listeners[event];

    if (listeners) {
      listeners.forEach(listener => listener(value));
    }
  }

  listen<Event extends keyof T>(event: Event, onChange: (e: T[Event]) => void) {
    let listeners = this.listeners[event] || [];
    if (!listeners) {
      listeners = [];
      this.listeners[event] = listeners;
    }

    const index = listeners.push(onChange) - 1;

    return () => {
      if (listeners) {
        listeners.splice(index, 1);
      }
    };
  }
}