it('test', (done) => {
  expect.assertions(3);
  const events = new EventHandler();
  const p = new Promise((resolve, reject) => {
    // setup callback
    const callback = () => { expect(true).toBe(true); resolve(); };
    events.one(document, ‘onclick’, callback);
    expect(events.handlers.length).toBe(1);
    // trigger event
    document.click();
  });
  p.then(() => {
    // confirm that we've filtered out the handler
    expect(events.handlers.length).toBe(0);
    done();
  });
});