Validate TextBox

//Take one text box and call the function obblur method........

asp:TextBox ID="txt" runat="server" onblur="ValidateCCNum(this);">

//.js File
function ValidateCCNum(ccNum)
{
var ccno = ccNum.value;
if(ccno == "")
{

return true;
}

if(isNaN(ccno))
{

alert("Credit card number should be numeric");
ccNum.value="";
ccNum.focus();

return false;
}
if(ccno.length < 14 || ccno.length >18)
{

alert("Credit card number should have 14-18 digits length");
ccNum.value="";
ccNum.focus();

return false;
}
return true;
}