Textarea start editing prevention

gistfile1.coffee

# demo: http://jsfiddle.net/q173wuoy/1/

KEY_DELETE = [8, 46]
KEY_NAV = [33..40]

$( ->
    inputPreventer = (e)-> 
        length = 10
        cursorPos = e.currentTarget?.selectionStart
        return if e.keyCode in KEY_NAV
        border = if e.keyCode in KEY_DELETE then length+1 else length
        e.preventDefault() if 0 < cursorPos < border
    ($ 'textarea').on 'keydown', inputPreventer
)

gistfile2.html

<textarea class='ta' name="" id="" cols="30" rows="10">Жуй тебе, дружок</textarea>