var needToConfirm = false;
var fieldsChanged = "";
var msgtext = "";

window.onbeforeunload = confirmExit;

function confirmExit() {
    if (needToConfirm)  {
      msgtext = "The following fields have changed and are not saved:\n" + fieldsChanged;
      fieldsChanged = "";
      needToConfirm = false;
      return msgtext;
    }
}

function fieldChanged(fieldName) {
    needToConfirm = true;
    fieldsChanged = fieldsChanged + " " + fieldName + "\n";
}