Can be used inside client integration tests to wait for elements to change

_mutation_observer_helper.js

observeMutationsHelper = function(targetSelector, done, callback) {
  var target = document.querySelector(targetSelector);
  var observer = new MutationObserver(function(mutations) {
    callback(mutations);
    observer.disconnect();
    done();
  });
  var config = {
    attributes: true,
    childList: true,
    characterData: true
  };
  observer.observe(target, config);
};

test.js

describe('header', function() {
  it("should change after some event", function(done) {
      var btn = $("#change-header");
      expect($('h1').textContent).toEqual('old header');

      // Wait for header to change using MutationObserver, not part of meteor but a browser API
      observeMutationsHelper('h1', done, function(mutations) {
        expect(mutations[0].target.textContent).toEqual("new header");
      });

      btn.click();
    });
});

https://developer.mozilla.org/en/docs/Web/API/MutationObserver