js 取得文本框内，鼠标索引

 var cursurPosition = -1;        if (obj.selectionStart) {//非IE浏览器            cursurPosition = obj.selectionStart;        }        else {//IE            var range = document.selection.createRange();            range.moveStart("character", -obj.value.length);            cursurPosition = range.text.length;        }