For an event that only happens once, then the navigator.serviceWorker.ready.then(function(){... version is preferable to document.addEventListener("s

For an event that only happens once, then the navigator.serviceWorker.ready.then(function(){... version is preferable to document.addEventListener("serviceready", function() { because A) you don't have to worry about removing your event listener after the fact, and B) if the event has already happened, then your callback will be called immediately.