angularjs directive to monitor class attribute

class.directive.js

      .directive('class', function() {
        return {
          restrict: 'A',
          link: function(scope, elem) {
            var observer = new MutationObserver(function(mutations) {
              try {
                observer.disconnect();
                let e = mutations[0].target;
                e.classList.forEach((c) => {
                  if (c.startsWith('ng-') || c.startsWith('text') || c.startsWith('pre-ep7-')) {
                    // skip
                  } else {
                    e.classList.remove(c);
                    e.classList.add('pre-ep7-' + c);
                  }
                });
                console.log(e.classList);
              } finally {
                observer.observe(elem[0], {
                  attributes: true,
                  attributeFilter: ['class']
                });
              }
            });
            observer.observe(elem[0], {
                attributes: true,
                attributeFilter: ['class']
            });
            elem.on('$destroy', function() {
              observer.disconnect();
            });
          }
        };