js 限制文本框输入字数

JS部分： 

<script type="text/javascript">
         function setMaxLength(object,length) 
        { 
            var result = true; 
            var controlid = document.selection.createRange().parentElement().id; 
            var controlValue = document.selection.createRange().text; 
            if (controlid == object.id && controlValue != "") 
            { 
              result = true; 
            } 
            else if (object.value.length >= length) 
            { 
               result = false; 
            } 
            if (window.event) 
            { 
                window.event.returnValue = result; 
                return result; 
            } 
        }


        function limitPaste(object,length) 
        { 
        var tempLength = 0; 
        if(document.selection) 
        { 
            if(document.selection.createRange().parentElement().id == object.id) 
            { 
            tempLength = document.selection.createRange().text.length; 
            } 
        } 
        var tempValue = window.clipboardData.getData("Text"); 
        tempLength = object.value.length + tempValue.length - tempLength; 
        if (tempLength > length) 
        { 
            tempLength -= length; 
            tempValue = tempValue.substr(0,tempValue.length - tempLength); 
            window.clipboardData.setData("Text", tempValue); 
            }

        window.event.returnValue = true; 
        }
</script>

aspx部分

<asp:TextBox ID="txtEventDesc" runat="server" Height="77px"  Width="685px"
        TextMode="MultiLine" onkeypress="javascript:setMaxLength(this,300);" onpaste="javascript:limitPaste(this, 300)">

</asp:TextBox>