function isFunction(functionToCheck: any) {
  const getType = {};
  return functionToCheck && getType.toString.call(functionToCheck) === '[object Function]';
}

export class Event<T> {
  private handlers: ((data?: T) => void)[] = [];

  public on(handler: (data?: T) => void) {
    if (!isFunction(handler)) {
      throw new Error('Handler is not a function');
    }
    this.handlers.push(handler);
  }

  public off(handler: (data?: T) => void) {
    this.handlers = this.handlers.filter(h => h !== handler);
  }

  public allOff() {
    this.handlers = [];
  }

  public trigger(data?: T) {
    if (this.handlers) {
      this.handlers.slice(0).forEach(h => h(data));
    }
  }
}