window.onload = function () {

  var el = document.getElementById('bar');

  el.addEventListener('foo', function (ev) {

    ev.preventDefault();

  }, {passive: true});

  var canceled = !(el.dispatchEvent(

    new CustomEvent('foo', {cancelable: true})

  ));

  el.textContent = canceled ? 'FAIL' : 'PASS';

};