scenario.js

feature('My feature', function () {
  scenario('My scenario')
    .when(UserDoesSomething)
    .then(UserSeesSomething)
})

function scenario(name) {
  var fns = [];

  function when(fn) {
    fns.push(new Promise(fn))
  }

  function then(fn) {
    it(name, function (done) {
      fns.push(new Promise(fn));
      Promise.all(fns).then(done)
    });
  }

  return {
    when: when,
    then: then
  }
}