Promises and Events! and Pseudo-Code!

main.js

function tellMeWhenThatThingHappens() {
  var promise = new Promise(function (resolve, reject) {
    window.addEventListener('thatAwesomeThingHappens', function () {
      resolve(/* maybe with some data here */);
    });

    // if it's possible for that awesome thing to fail...
    window.addEventListener('refusingToBeAwesome', function () {
      reject(/* maybe with an error */);
    });
  });
  
  // do something that might trigger that event
  window.maybeMakeAwesomeThingsHappen();
  
  return promise;
}

tellMeWhenThatThingHappens().then(function (data) {
  // do something!
});