fix: improve logging for _devPagesManifest.json loading failures (#38046

)

## Bug

- [x] Fixes #38047
- ~[ ] Integration tests added: do you want them added for such an unusual edge case?~ [Comment below: skipping](#38046 (comment))
- [x] Errors have helpful link attached, see `contributing.md`: ~I'll wait until this approach & error message are confirmed before thinking more deeply on how to explain the error~

Sending a draft PR as reference ahead of time. 🙂 

Doesn't resolve the root issue of why `_devPagesManifest.json` might fail to load. But does improve the log for when it happens. I'd suggest applying this same fix to `_devMiddlewareManifest.json` too.

<table>
<thead>
<tr>
<th></th>
<th>Before</th>
<th>After</th>
</tr>
</thead>
<tbody>
<tr>
<th>Console</th>
<td>
<pre>
<code>
Failed to fetch devPagesManifest TypeError: Failed to fetch
    at PageLoader.getPageList (page-loader.js?e87a:30:53)
    at _callee$ (router.js?8684:955:45)
    at ...
</code>
</pre>
<pre>
<code>
router.js?8684:1319 Uncaught (in promise) TypeError: Cannot \
read properties of undefined (reading 'includes')
    at resolveDynamicRoute (router.js?8684:1319:16)
    at _callee$
    at ...
</code>
</pre>
</td>
<td>
<pre>
<code>
Failed to fetch devPagesManifest: TypeError: Failed to fetch
    at PageLoader.getPageList (page-loader.js?e87a:30:53)
    at _callee$ (router.js?8684:955:45)
    at ...
</code>
</pre>
</td>
</tr>
<tr>
<th>Visual</th>
<td><img alt="Screenshot of a Next.js runtime error: TypeError: Cannot read properties of undefined (reading 'includes')" src="https://user-images.githubusercontent.com/3335181/175854728-8d5c2051-1229-4da7-8af1-fc95236befae.png" /></td>
<td><img alt="Screenshot of a Next.js runtime error: Error: Failed to fetch _devPagesManifest.json. Is something blocking that network request?" src="https://user-images.githubusercontent.com/3335181/175854774-07895846-1a1c-4bb8-bf57-fa696b8c6ba4.png" /></td>
</tr>
</tbody>
</table>

errors/failed-to-fetch-devpagesmanifest.md

@@ -0,0 +1,18 @@
+# Failed to fetch devPagesManifest
+
+#### Why This Error Occurred
+
+The network request to load `_devPagesManifest.json` did not succeed.
+
+The dev pages manifest file is used by `next/link` to retrieve the list of pages to be (pre-)loaded by Next.js.
+If it fails, Next.js cannot properly navigate and link between pages.
+
+#### Possible Ways to Fix It
+
+- Make sure your browser developer tools, extensions, and any other network tools aren't blocking that request.
+- If you're running your Next.js application through a proxy, nginx, or other network layer, make sure links like `/_next/*` are configured to be allowed.
+
+### Useful Links
+
+- [Original GitHub Issue Thread](https://github.com/vercel/next.js/issues/16874)
+- [GitHub Issue Thread With Reproduction](https://github.com/vercel/next.js/issues/38047)

errors/manifest.json

@@ -715,6 +715,10 @@
         {
           "title": "invalid-next-config",
           "path": "/errors/invalid-next-config.md"
+        },
+        {
+          "title": "failed-to-fetch-devpagesmanifest",
+          "path": "/errors/failed-to-fetch-devpagesmanifest.md"
         }
       ]
     }

packages/next/client/page-loader.ts

@@ -58,23 +58,22 @@ export default class PageLoader {
       if (window.__DEV_PAGES_MANIFEST) {
         return window.__DEV_PAGES_MANIFEST.pages
       } else {
-        if (!this.promisedDevPagesManifest) {
-          // TODO: Decide what should happen when fetching fails instead of asserting
-          // @ts-ignore
-          this.promisedDevPagesManifest = fetch(
-            `${this.assetPrefix}/_next/static/development/_devPagesManifest.json`
-          )
-            .then((res) => res.json())
-            .then((manifest: { pages: string[] }) => {
-              window.__DEV_PAGES_MANIFEST = manifest
-              return manifest.pages
-            })
-            .catch((err) => {
-              console.log(`Failed to fetch devPagesManifest`, err)
-            })
-        }
-        // TODO Remove this assertion as this could be undefined
-        return this.promisedDevPagesManifest!
+        this.promisedDevPagesManifest ||= fetch(
+          `${this.assetPrefix}/_next/static/development/_devPagesManifest.json`
+        )
+          .then((res) => res.json())
+          .then((manifest: { pages: string[] }) => {
+            window.__DEV_PAGES_MANIFEST = manifest
+            return manifest.pages
+          })
+          .catch((err) => {
+            console.log(`Failed to fetch devPagesManifest:`, err)
+            throw new Error(
+              `Failed to fetch _devPagesManifest.json. Is something blocking that network request?\n` +
+                'Read more: https://nextjs.org/docs/messages/failed-to-fetch-devpagesmanifest'
+            )
+          })
+        return this.promisedDevPagesManifest
       }
     }
   }