Cross browser add and remove eventListener

gistfile1.txt

addEventListener = (el, evt, fn)->
    if el? && evt? && fn? && typeof(fn) == 'function'
        if el.addEventListener
            el.addEventListener(evt, fn, false)
        else if el.attachEvent
            el.attachEvent('on' + evt, fn)
        else
            el['on' + evt] = fn;

        return el
    else
        throw TypeError("Cant add eventListner!")

removeEventListener = (el, evt, fn)->
    if el? && evt? && fn? && typeof(fn) == 'function'
        if el.removeEventListener
            el.removeEventListener(evt, fn, false)
        else if el.detachEvent
            el.detachEvent('on' + evt, fn)
        else
            el['on' + evt] = '';

        return el
    else
        throw TypeError("Cant add eventListner!")