VALIDATION ON TEXT BOX THROUGH JAVA SCRIPT in asp.net

ON FORM LOAD : - .CS FILE
--------------------------
protected void Page_Load(object sender, EventArgs e)
   {
        txtPhone.Attributes.Add("onkeydown", "return isNumeric(event.keyCode);");
       
        txtPhone.Attributes.Add("onkeyup", "keyUP(event.keyCode)");
    } 

  


CODE ON DESIRED TEXT BOX :-
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<asp:TextBox ID="txtPhone" runat="server" onkeyup = "keyUP(event.keyCode)" onkeydown = "return isNumeric(event.keyCode);"></asp:TextBox>




CODE ON MASTER PAGE :-
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    function isNumeric(keyCode)
{
      if(keyCode==16)
            isShift = true;

      return ((keyCode >= 48 && keyCode <= 57 || keyCode == 8 ||
            (keyCode >= 96 && keyCode <= 105)) && isShift == false);
}
</script>