Service worker weirdness in Chrome

I think I’ve found some more strange service worker behaviour in Chrome.

It all started when I was checking out the very nice new redesign of WebPageTest. I figured while I was there, I’d run some of my sites through it. I passed in a URL from The Session. When the test finished, I noticed that the “screenshot” tab said that something was being logged to the console. That’s odd! And the file doing the logging was the service worker script.

I fired up Chrome (which isn’t my usual browser), and started navigating around The Session with dev tools open to see what appeared in the console. Sure enough, there was a failed fetch attempt being logged. The only time my service worker script logs anything is in the catch clause of fetching pages from the network. So Chrome was trying to fetch a web page, failing, and logging this error:

The service worker navigation preload request failed with a network error.

But all my pages were loading just fine. So where was the error coming from?

After a lot of spelunking and debugging, I think I’ve figured out what’s happening…

First of all, I’m making use of navigation preloads in my service worker. That’s all fine.

Secondly, the website is a progressive web app. It has a manifest file that specifies some metadata, including start_url. If someone adds the site to their home screen, this is the URL that will open.

Thirdly, Google recently announced that they’re tightening up the criteria for displaying install prompts for progressive web apps. If there’s no network connection, the site still needs to return a 200 OK response: either a cached copy of the URL or a custom offline page.

So here’s what I think is happening. When I navigate to a page on the site in Chrome, the service worker handles the navigation just fine. It also parses the manifest file I’ve linked to and checks to see if that start URL would load if there were no network connection. And that’s when the error gets logged.

I only noticed this behaviour because I had specified a query string on my start URL in the manifest file. Instead of a start_url value of /, I’ve set a start_url value of /?homescreen. And when the error shows up in the console, the URL being fetched is /?homescreen.

Crucially, I’m not seeing a warning in the console saying “Site cannot be installed: Page does not work offline.” So I think this is all fine. If I were actually offline, there would indeed be an error logged to the console and that start_url request would respond with my custom offline page. It’s just a bit confusing that the error is being logged when I’m online.

I thought I’d share this just in case anyone else is logging errors to the console in the catch clause of fetches and is seeing an error even when everything appears to be working fine. I think there’s nothing to worry about.

Update: Jake confirmed my diagnosis and agreed that the error is a bit confusing. The good news is that it’s changing. In Chrome Canary the error message has already been updated to:

DOMException: The service worker navigation preload request failed due to a network error. This may have been an actual network error, or caused by the browser simulating offline to see if the page works offline: see https://w3c.github.io/manifest/#installability-signals

Much better!

minimum interesting service worker

An interesting idea from Tantek for an offline page that links off to an archived copy of the URL you’re trying to reach—useful for when you’re site goes down (though not for when the user’s internet connection is down).

as days pass by — Farmbound, or how I built an app in 2022

Stuart writes up the process up making a mobile game as a web app—not a native app. The Wordle effect reverberates.

It’s a web app. Works for everyone. And I thought it would be useful to explain why it is, why I think that’s the way to do things, and some of the interesting parts of building an app for everyone to play which is delivered over the web rather than via app stores and downloads.

Streets Series’ Articles - DEV Community 👩‍💻👨‍💻

This is a really excellent four-part series on web performance that really dives into the technical details and asks all the right questions:

1. Making the world’s fastest website, and other mistakes
2. The weirdly obscure art of Streamed HTML
3. Speed Needs Design, or: You can’t delight users you’ve annoyed
4. Routing: I’m not smart enough for a SPA

How to make MPAs that are as fast as SPAs | Go Make Things

The headline is a little misleading because if you follow this advice, your multi-page apps will be much much faster than single page apps, especially when you include that initial page load of a single page app.

Here’s a quick high-level summary of what I do…

1. Serve pre-rendered, mostly static HTML.
2. Inline everything, including CSS and JavaScript.
3. Use mostly platform-native JavaScript, and as little of it as possible.
4. Minify and gzip all the things.
5. Lean heavily on service workers.

That’s an excellent recipe for success right there!

Now THAT’S What I Call Service Worker! – A List Apart

This is terrific! Jeremy shows how you can implement a fairly straightforward service worker for performance gains, but then really kicks it up a notch with a recipe for turning a regular website into a speedy single page app without framework bloat.