js 定位光标



    function getSelectionStart(o)
        {
            if (o.createTextRange)
            {
                var r = document.selection.createRange().duplicate()
                r.moveEnd('character', o.value.length)
                if (r.text == '') return o.value.length
                return o.value.lastIndexOf(r.text)
            } else return o.selectionStart
        }