Firefox、服务人员和后退按钮

我收到报告说，在最新版本的Firefox中，按Back会导致My ServiceWorker提供的"You Are Offline"页面出现。

以下是ServiceWorker的功能部分：

self.addEventListener('fetch',function(event) {
  // We only want to call event.respondWith() if this is a navigation request
  // for an HTML page.
  // request.mode of 'navigate' is unfortunately not supported in Chrome
  // versions older than 49, so we need to include a less precise fallback,
  // which checks for a GET request with an Accept: text/html header.
  if (event.request.mode === 'navigate' ||
      (event.request.method === 'GET' &&
       event.request.headers.get('accept').includes('text/html'))) {
    event.respondWith(
      fetch(event.request).catch(function(error) {
        // The catch is only triggered if fetch() throws an exception, which will most likely
        // happen due to the server being unreachable.
        // If fetch() returns a valid HTTP response with an response code in the 4xx or 5xx
        // range, the catch() will NOT be called. If you need custom handling for 4xx or 5xx
        // errors, see https://github.com/GoogleChrome/samples/tree/gh- pages/service-worker/fallback-response
        return caches.match(OFFLINE_URL);
      })
    );
  }

  // If our if() condition is false, then this fetch handler won't intercept the request.
  // If there are any other fetch handlers registered, they will get a chance to call
  // event.respondWith(). If no fetch handlers call event.respondWith(), the request will be
  // handled by the browser as if there were no service worker involvement.
});

因此，由于某种原因，在Firefox中，按Back返回的是OFFLINE_URL而不是目标页面。

这可能是什么原因造成的，我该如何进行调试？

---

解决方案

火狐在使用"后退"按钮时，显然有一个Chrome没有的额外步骤。

它执行一个"只有在缓存的情况下"的请求，这当然会失败，因为页面没有缓存(它们都是动态的)。由于失败，它会抛出一个错误，并调用catch。

添加此检查已修复：

&& event.request.cache !== 'only-if-cached'

这使Firefox能够意识到资源未缓存，并照常继续。