function testPostCode (postcode, country) {
  var myPostCode = document.getElementById(postcode).value;
  var myCountryCode = document.getElementById(country).value;
  var result = checkPostCode(myPostCode, myCountryCode);
  if (result) {
    document.getElementById(postcode).value = result; 
  } else {
    document.getElementById(postcode).value = ''; 
    alert ("Postcode has invalid format")
  };
}