Function != function in Cypress

[Falke-Design]

Hi,
I don't know if this is a bug of this library or of cypress. I didn't found an explanation why this is like them.

My onStart function never called in the fingerIsBusy() function because it checks if it is an instance of Function.

if (this._onStart && this._onStart instanceof Function) {
	this._onStart(this);
}

My code:

var hand = new Hand({
        timing: 'frame',
        onStart: function () {
          console.log('TEST')
        }
      });
      var toucher = hand.growFinger('mouse');
      toucher.moveTo(300, 280, 0)
        .down().moveBy(5, 0, 20).moveBy(256, 32, 1000).wait(100).up().wait(100);

I debugged the code:
_onStart is a function because I can call this._onStart() but this._onStart instanceof Function returns false.

Because of this bug the onStart and onStop function is never called.

Maybe it would be better if it changed to typeof this._onStart === "function"

Versions:
"prosthetic-hand": "^1.3.1"
"cypress": "^3.8.3"