How to validate Numeric Text Box using javascrit

Description : A text Box will not accept Character value in itself.

function validateInt(Mobile,msg)

{

      var o = Mobile.value;

      if(o.length!=0)

      {

     if(isNaN(o))

      {    

            alert(msg)  

            Mobile.value="";

            Mobile.focus();

            return false;

       }

      }

   }

OR

Best Code

<Script Type = "Javascript">

function numbersonly(e)

    {

//alert('hi');

var unicode=e.charCode? e.charCode : e.keyCode;

if (unicode!=8 && unicode!=46 && unicode!=9)

{

if (unicode<48||unicode>57)

return false

}

}

How to access

<asp:TextBox ID="TextBox1" runat="server" onkeypress="return numbersonly(event);"></asp:TextBox>