MooTools allows you to easily add event listeners to elements using the addEvent and removeEvent methods. One thing MooTools doesn’t have is a hasEvent method which will check an element’s list of listeners to see if the element has been assigned a specific function per event. I experimented with implementing this functionality and believe I’ve found the best solution.

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Element.implement({
	hasEvent: function(eventType,fn) {
		//get the element's events
		var myEvents = this.retrieve('events');
		//can we shoot this down?
		return myEvents && myEvents[eventType] && (fn == undefined || myEvents[eventType].keys.contains(fn));
	}
});