Method on

src/Observable.ts:28–52

(
    arg0: K | EventRegistryObject<EventSpec>,
    handler?: TEventCallback<E>
  )

  ): VoidFunction;
  on(handlers: EventRegistryObject<EventSpec>): VoidFunction;
  on<K extends keyof EventSpec, E extends EventSpec[K]>(
    arg0: K | EventRegistryObject<EventSpec>,
    handler?: TEventCallback<E>
  ): VoidFunction {
    if (!this.__eventListeners) {
      this.__eventListeners = {} as Record<keyof EventSpec, TEventCallback[]>;
    }
    if (typeof arg0 === 'object') {
      // one object with key/value pairs was passed
      Object.entries(arg0).forEach(([eventName, handler]) => {
        this.on(eventName as K, handler as TEventCallback);
      });
      return () => this.off(arg0);
    } else if (handler) {
      const eventName = arg0;
      if (!this.__eventListeners[eventName]) {
        this.__eventListeners[eventName] = [];
      }
      this.__eventListeners[eventName].push(handler);
      return () => this.off(eventName, handler);
    } else {
      // noop
      return () => false;
    }
  }

  /**
   * Observes specified event **once**