FetchEvent.respondWith()

FetchEvent의 respondWith() 메소드는 브라우저의 기본 fetch 핸들링을 막고, 당신 스스로 Response에 대한 promise를 제공할 수 있게 허락합니다.

대부분의 상황에서 당신은 수신자가 이해하는 어떠한 응답이라도 제공할 수 있습니다. 예를 들어, <img> 엘리먼트가 해당 요청을 시작했다면, 응답 객체는 이미지 데이터를 필요로 합니다. 보안적인 이유들로, 여기엔 몇가지 전역 규칙이 존재합니다.

• You can only return Response objects of type "opaque" if the fetchEvent.request object's mode is "no-cors".  This prevents the leaking of private data.
• You can only return Response objects of type "opaqueredirect" if the fetchEvent.request object's mode is "manual".
• You cannot return Response objects of type "cors" if the fetchEvent.request object's mode is "same-origin".

From Firefox 59 onwards, when a service worker provides a Response to FetchEvent.respondWith(), the Response.url value will be propagated to the intercepted network request as the final resolved URL.  If the Response.url value is the empty string, then the FetchEvent.request.url is used as the final URL.

In the past the FetchEvent.request.url was used as the final URL in all cases.  The provided Response.url was effectively ignored.

This means, for example, if a service worker intercepts a stylesheet or worker script, then the provided Response.url will be used to resolve any relative @import or importScripts() subresource loads (bug 1222008).

For most types of network request this change has no impact because you can't observe the final URL.  There are a few, though, where it does matter:

• If a fetch() is intercepted, then you can observe the final URL on the result's Response.url.
• If a worker script is intercepted, then the final URL is used to set self.location and used as the base URL for relative URLs in the worker script.
• If a stylesheet is intercepted, then the final URL is used as the base URL for resolving relative @import loads.

Note that navigation requests for Windows and iframes do NOT use the final URL.  The way the HTML specification handles redirects for navigations ends up using the request URL for the resulting Window.location.  This means sites can still provide an "alternate" view of a web page when offline without changing the user-visible URL.

fetchEvent.respondWith(
  // Promise that resolves to a Response.
​)

A Promise for a Response.

Void.

This fetch event tries to return a response from the cache API, falling back to the network otherwise.

addEventListener('fetch', event => {
  // Prevent the default, and handle the request ourselves.
  event.respondWith(async function() {
    // Try to get the response from a cache.
    const cachedResponse = await caches.match(event.request);
    // Return it if we found one.
    if (cachedResponse) return cachedResponse;
    // If we didn't find a match in the cache, use the network.
    return fetch(event.request);
  }());
});

• Using Service Workers
• Is ServiceWorker ready?
• Promise
• Fetch API