``event.prototype`` has 2 issues:

1. (most important one) ``prototype`` is a special property in JS. It will produce really weird behavior here
2. the name is a bit weird as the prototype of the form field is really the html. Your ``protoype`` variable contains the button triggering the addition. If you really want to pass it, I think the best solution would be to set it as ``event.relatedTarget`` which follows the DOM convention when a second element is actually involved in the event triggering