Textbox 自动调节高度

var autoTextarea = function (elem, extra, maxHeight) {
            extra = extra || 20;
            var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
                isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
                addEvent = function (type, callback) {
                    elem.addEventListener ?
                        elem.addEventListener(type, callback, false) :
                        elem.attachEvent('on' + type, callback);
                },
                getStyle = elem.currentStyle ? function (name) {
                    var val = elem.currentStyle[name];
                    if (name === 'height' && val.search(/px/i) !== 1) {
                        var rect = elem.getBoundingClientRect();
                        return rect.bottom - rect.top -
                            parseFloat(getStyle('paddingTop')) -
                            parseFloat(getStyle('paddingBottom')) + 'px';
                    };

                    return val;
                } : function (name) {
                    return getComputedStyle(elem, null)[name];
                },
                minHeight = parseFloat(getStyle('height'));
            elem.style.maxHeight = elem.style.resize = 'none';
            var change = function () {
                var scrollTop, height,
                    padding = 0,
                    style = elem.style;
                if (elem._length === elem.value.length) return;
                elem._length = elem.value.length;

                if (!isFirefox && !isOpera) {
                    padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
                };
                scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
                elem.style.height = minHeight + 'px';
                if (elem.scrollHeight > minHeight) {
                    if (maxHeight && elem.scrollHeight > maxHeight) {
                        height = maxHeight - padding;
                        style.overflowY = 'auto';
                    } else {
                        height = elem.scrollHeight - padding;
                        style.overflowY = 'hidden';
                    };

                    style.height = height + extra + 'px';
                    scrollTop += parseInt(style.height) - elem.currHeight;
                    document.body.scrollTop = scrollTop;
                    document.documentElement.scrollTop = scrollTop;
                    elem.currHeight = parseInt(style.height);
                };
            };
            addEvent('propertychange', change);
            addEvent('input', change);
            addEvent('focus', change);
            change();
        };

TextBox tbx_2 = new TextBox();
tbx_2.TextMode = TextBoxMode.MultiLine;
   tbx_2.ID = "tbx_2_" + tablerow_id;
        tbx_2.ClientIDMode = ClientIDMode.Static;        
 Page.ClientScript.RegisterStartupScript(this.GetType(), tbx_2.ID, "autoTextarea(document.getElementById('" + tbx_2.ID + "'));", true);