How to add Characters left feature for Multi line Textbox in Asp.net using Javascript

Write the following design code in default.aspx source file

<asp:Label ID="lbdesc" runat="server" Text="Description"></asp:Label>

<asp:TextBox ID="txtDescription" runat="server" TextMode="MultiLine"  Width="220px" Onkeydown="checkTextAreaMaxLength(this,event,'2000')" ></asp:TextBox>

<asp:Label ID="lbdesccharleft" runat="server" Text="Characters Left "></asp:Label>

<asp:TextBox ID="txtdesccharleft"  runat="server" Width="50px" ReadOnly="True"

 Text=" 2000"> </asp:TextBox>

Write the following script functions in javascript <script> tag:

<script language="javascript" type="text/javascript">

  function chkcharleft(e,t1, t2,val) {

        e.returnValue = false;

       

        document.getElementById(t2).maxlength = val;

        var val1 = document.getElementById(t1).value.length;

       

        if (val - val1 <= 0)

            e.returnValue = false;

        document.getElementById(t2).value = val - val1;

      

    }

   

    function charsleft(t1, t2) {

        document.getElementById(t2).maxlength = val;

        var val1 = document.getElementById(t1).value.length;

        if (val - val1 <= 0)

            e.returnValue = false;

        document.getElementById(t2).value = val - val1;

    }

   function checkTextAreaMaxLength(textBox, e, length)

    {         

    var mLen = textBox["MaxLength"];        

    if(null==mLen)            

    mLen=length;         

    var maxLength = parseInt(mLen);        

    if(!checkSpecialKeys(e))        

    {         

    if(textBox.value.length > maxLength-1)         

    {            

    if(window.event)//IE              

    e.returnValue = false;

    else//Firefox                

        e.preventDefault();

}

}

}

function pageLoad(sender, args) {

        if (args.get_isPartialLoad()) {

document.getElementById('<%= txtdesccharleft.ClientID %>').value = document.getElementById('<%= txtdesccharleft.ClientID %>').value - document.getElementById('<%= txtDescription.ClientID %>').value.length;

}}

</script>

Write the following code in default.aspx.cs page load

protected void Page_Load(object sender, EventArgs e)

    {

      txtDescription.Attributes.Add("onkeyup", "chkcharleft(event, '" + txtDescription.ClientID + "','" + txtdesccharleft.ClientID + "', 2000)");

}