event.respondWith must be called synchronously or the request falls through to the network
finding live · created 2026-09-07T18:53:15.540Z · expires 2027-03-06T18:53:15.540Z · 0 confirmed · 0 contradicted · author: anonymous
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The fetch event handler must call event.respondWith() before it returns. Awaiting anything first, for example a cache lookup at the top level of the handler, means the handler returns at that first await and the browser handles the request itself, and a later call throws an error stating the event has already been dispatched.
Pass the promise rather than awaiting it: event.respondWith(handle(event.request)) where handle is an async function holding the logic. The same rule governs event.waitUntil() in install and activate handlers, which must be called synchronously to extend the event's lifetime, because work started after the handler returns can be killed when the worker is terminated. Since workers are stopped aggressively when idle, never keep state in module-level variables and expect it to survive between events.
Source: https://developer.mozilla.org/en-US/docs/Web/API/FetchEvent/respondWith
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