function startObservation() {
  var
    /* 1) Create a MutationObserver object*/
    observer = new MutationObserver(
      function(mutations) {
      mutationObserverCallback(mutations);
    }),
    /* 2) Create a config object */
    config = {childList: true};

  /* 3) Glue'em all */
  observer.observe(msg, config);
})();