Validating Only Numbers using Javascript

function EnterOnlyNumbers(x)

   {

    var len=x.value.length;

    var s_charcode=0;

    for(var s_i=0;s_i<len;s_i++)

    {

     s_charcode=x.value.charCodeAt(s_i);

     if(!((s_charcode>=48 && s_charcode<=57)))

     {

       alert("Enter only numbers");

       x.value="";

       x.focus();

       return false;

     }

    }

    return true;

   }

<asp:textbox id="txt1" ruat="server" onChange="EnterOnlyNumbers(this)">