fix(gatsby-plugin-offline): Improve reliability of JS detection (#18760)

* Combine offline API with redirection to improve reliability over preload

* fix API route not working for navigation requests + make code order more logical

* check if API method exists before calling function

* add comments to relevant sections

packages/gatsby-plugin-offline/src/gatsby-ssr.js

@@ -30,12 +30,10 @@ export const onRenderBody = ({ pathname, setHeadComponents }) => {
 
   setHeadComponents([
     <noscript key="disable-offline-shell">
-      <link
-        rel="preload"
-        as="fetch"
-        href="/.gatsby-plugin-offline:disableOfflineShell"
+      <meta
+        httpEquiv="refresh"
+        content="0;url=/.gatsby-plugin-offline:api=disableOfflineShell&redirect=true"
       />
-      <meta httpEquiv="refresh" content="0;url=" />
     </noscript>,
   ])
 }

packages/gatsby-plugin-offline/src/sw-append.js

@@ -3,13 +3,78 @@
 importScripts(`idb-keyval-iife.min.js`)
 
 const { NavigationRoute } = workbox.routing
+
+let lastNavigationRequest = null
 let offlineShellEnabled = true
 
+// prefer standard object syntax to support more browsers
+const MessageAPI = {
+  setPathResources: (event, { path, resources }) => {
+    event.waitUntil(idbKeyval.set(`resources:${path}`, resources))
+  },
+
+  clearPathResources: event => {
+    event.waitUntil(idbKeyval.clear())
+  },
+
+  enableOfflineShell: () => {
+    offlineShellEnabled = true
+  },
+
+  disableOfflineShell: () => {
+    offlineShellEnabled = false
+  },
+}
+
+self.addEventListener(`message`, event => {
+  const { gatsbyApi: api } = event.data
+  if (api) MessageAPI[api](event, event.data)
+})
+
+function handleAPIRequest({ event }) {
+  const { pathname } = new URL(event.request.url)
+
+  const params = pathname.match(/:(.+)/)[1]
+  const data = {}
+
+  if (params.indexOf(`=`) !== -1) {
+    params.split(`&`).forEach(param => {
+      const [key, val] = param.split(`=`)
+      data[key] = val
+    })
+  } else {
+    data.api = params
+  }
+
+  if (MessageAPI[data.api] !== undefined) {
+    MessageAPI[data.api]()
+  }
+
+  if (!data.redirect) {
+    return new Response()
+  }
+
+  return new Response(null, {
+    status: 302,
+    headers: {
+      Location: lastNavigationRequest,
+    },
+  })
+}
+
 const navigationRoute = new NavigationRoute(async ({ event }) => {
+  // handle API requests separately to normal navigation requests, so do this
+  // check first
+  if (event.request.url.match(/\/.gatsby-plugin-offline:.+/)) {
+    return handleAPIRequest({ event })
+  }
+
   if (!offlineShellEnabled) {
     return await fetch(event.request)
   }
 
+  lastNavigationRequest = event.request.url
+
   let { pathname } = new URL(event.request.url)
   pathname = pathname.replace(new RegExp(`^%pathPrefix%`), ``)
 
@@ -36,35 +101,5 @@ const navigationRoute = new NavigationRoute(async ({ event }) => {
 
 workbox.routing.registerRoute(navigationRoute)
 
-// prefer standard object syntax to support more browsers
-const MessageAPI = {
-  setPathResources: (event, { path, resources }) => {
-    event.waitUntil(idbKeyval.set(`resources:${path}`, resources))
-  },
-
-  clearPathResources: event => {
-    event.waitUntil(idbKeyval.clear())
-  },
-
-  enableOfflineShell: () => {
-    offlineShellEnabled = true
-  },
-
-  disableOfflineShell: () => {
-    offlineShellEnabled = false
-  },
-}
-
-self.addEventListener(`message`, event => {
-  const { gatsbyApi: api } = event.data
-  if (api) MessageAPI[api](event, event.data)
-})
-
-workbox.routing.registerRoute(/\/.gatsby-plugin-offline:.+/, ({ event }) => {
-  const { pathname } = new URL(event.request.url)
-
-  const api = pathname.match(/:(.+)/)[1]
-  MessageAPI[api]()
-
-  return new Response()
-})
+// this route is used when performing a non-navigation request (e.g. fetch)
+workbox.routing.registerRoute(/\/.gatsby-plugin-offline:.+/, handleAPIRequest)