The ExtendableEvent.waitUntil() method tells the event dispatcher that work is ongoing. It can also be used to detect whether that work was successful. In service workers, waitUntil() tells the browser that work is ongoing until the promise settles, and it shouldn't terminate the service worker if it wants that work to complete.

The install events in service workers use waitUntil() to hold the service worker in the installing phase until tasks complete. If the promise passed to waitUntil() rejects, the install is considered a failure, and the installing service worker is discarded. This is primarily used to ensure that a service worker is not considered installed until all of the core caches it depends on are successfully populated.

The activate events in service workers use waitUntil() to buffer functional events such as fetch and push until the promise passed to waitUntil() settles. This gives the service worker time to update database schemas and delete outdated caches, so other events can rely on a completely upgraded state.

The waitUntil() method must be initially called within the event callback, but after that it can be called multiple times, until all the promises passed to it settle.

A Promise.

None (undefined).

Using waitUntil() within a service worker's install event:

addEventListener("install", (event) => {
  const preCache = async () => {
    const cache = await caches.open("static-v1");
    return cache.addAll(["/", "/about/", "/static/styles.css"]);
  };
  event.waitUntil(preCache());
});

• Using Service Workers
• Is ServiceWorker ready?
• Promise