FetchEvent: handled property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨September 2022⁩.

Note: This feature is only available in Service Workers.

The handled property of the FetchEvent interface returns a promise indicating if the event has been handled by the fetch algorithm or not. This property allows executing code after the browser has consumed a response, and is usually used together with the waitUntil() method.

Value

A Promise that is pending while the event has not been handled, and fulfilled once it has.

Examples

addEventListener("fetch", (event) => {
  event.respondWith(
    (async function () {
      const response = await doCalculateAResponse(event.request);

      event.waitUntil(
        (async function () {
          await doSomeAsyncStuff(); // optional

          // Wait for the event to be consumed by the browser
          await event.handled;

          return doFinalStuff(); // Finalize AFTER the event has been consumed
        })(),
      );

      return response;
    })(),
  );
});