Tags: serviceworkers



Tuesday, March 6th, 2018

Minimal viable service worker

I really, really like service workers. They’re one of those technologies that have such clear benefits to users that it seems like a no-brainer to add a service worker to just about any website.

The thing is, every website is different. So the service worker strategy for every website needs to be different too.

Still, I was wondering if it would be possible to create a service worker script that would work for most websites. Here’s the script I came up with.

The logic works like this:

  • If there’s a request for an HTML page, fetch it from the network and store a copy in a cache (but if the network request fails, try looking in the cache instead).
  • For any other files, look for a copy in the cache first but meanwhile fetch a fresh version from the network to update the cache (and if there’s no existing version in the cache, fetch the file from the network and store a copy of it in the cache).

So HTML files are served network-first, while all other files are served cache-first, but in both cases a fresh copy is always put in the cache. The idea is that HTML content will always be fresh (unless there’s a problem with the network), while all other content—images, style sheets, scripts—might be slightly stale, but get refreshed with every request.

My original attempt was riddled with errors. Jake came to my rescue and we revised the script into something that actually worked. In the process, my misunderstanding of how await works led Jake to write a great blog post on await vs return vs return await.

I got there in the end and the script seems solid enough. It’s a fairly simplistic strategy that could work for quite a few sites, but it has some issues…

Service workers don’t perform any automatic cleanup of caches—that’s up to you to do (usually during the activate event). This script doesn’t do any cleanup so the cache might grow and grow and grow. For that reason, I think the script is best suited for fairly small sites.

The strategy also assumes that a file will either be fetched from the network or the cache. There’s no contingency for when both attempts fail. So there’s no fallback offline page, for example.

I decided to test it in the wild, but I expanded it slightly to fix the fallback issue. The version on the Ampersand 2018 website includes a worst-case-scenario option to show a custom offline page that has been pre-cached. (By the way, if you haven’t got a ticket for Ampersand yet, get a ticket now—it’s going to be superb day of web typography nerdery.)

Anyway, this fairly basic script seems to be delivering some good performance improvements. If you’ve got a site that you think would benefit from this network/caching strategy, and it’s served over HTTPS, then:

  1. Feel free to download the script or copy and paste it into a file called serviceworker.js,
  2. Put that file in the root directory of your website,
  3. Add this in a script element at the bottom of your HTML pages:

if (navigator.serviceWorker && !navigator.serviceWorker.controller) { navigator.serviceWorker.register('/serviceworker.js'); }

You can also use the script as a starting point. You might find issues specific to your particular website. That’s okay—you can tweak and adjust the script to suit your needs.

If this minimal service worker script proves in any way useful to you, thank Jake.

Wednesday, February 28th, 2018

Offline itineraries with service workers

The Trivago website is a progressive web app. That means it

  1. is served over HTTPS,
  2. has a web app manifest JSON file, and it
  3. has a service worker script.

The service worker provides an opportunity for a nice bit of fun branding—if you lose your internet connection, the site provides a neat little maze game you can play. Cute!

That’s a fairly simple example of how service workers can enhance the user experience when the dreaded offline situation arises. But it strikes me that the travel industry is the perfect place to imagine other opportunities for offline enhancements.

Travel sites often provide itineraries—think airlines, trains, or hotels. The itineraries consist of places, times, and contact information. This is exactly the kind of information that you might find yourself trying to retrieve in an emergency situation, like maybe in a cab on the way to the airport or train station. Perhaps you’re stuck in traffic, in a tunnel. Or maybe you don’t have a data plan for the country you’re currently in. Either way, wouldn’t it be great if you could hit the website for your airline or hotel and get your itinerary, even if you’re offline.

Alright, let’s think this through…

Let’s assume that an individual itinerary has its own URL. That URL is a web page of information, mostly text, with perhaps an image or two (like a map). Now when you make your booking, let’s have the service worker cache that URL (and its assets) for offline access.

Hmm …but there’s a good chance that the device you make the booking on is not the same device that you’d have with you out and and about. Because caches are local to the browser, that’s a problem.

Okay, but of these kinds of sites have some kind of log-in mechanism. So we could update the log-in flow a bit: when a user logs in, check to see if they have any itineraries assigned to them, and if they do, fire off an event to the service worker (using postMessage) to cache the URLs of the itineraries.

Now that the itineraries are cached, the final step is to create a custom offline page. As well as the usual “Sorry, the internet’s down” message, we can say “Sorry, the internet’s down …but here are your itineraries”. (This is kind of like the pattern you see on blogs like mine, Ethan’s, or Mike’s—a custom offline page that lists cached URLs of articles you’ve previously visited).

That’s just one pattern off the top of my head. It’s fun to imagine the different ways that service workers could be used to enhance the experience of just about any site, but they seem particularly relevant to travel sites—dodgy internet connections and travelling go hand-in-hand. At Clearleft, we’ve been working with quite a few travel-related clients lately so that’s why these scenarios are on my mind: booking holidays, flights, and so on. But, as I’ve said before and I’ll say again, every website can benefit from becoming a progressive web app.

Monday, February 26th, 2018

Reliable Experiences - PWA Roadshow - YouTube

A good hands-on introduction to service workers from Mariko.

Reliable Experiences - PWA Roadshow

Thursday, February 15th, 2018

Progressive Web App for FixMyStreet · Issue #1996 · mysociety/fixmystreet

Here’s a Github issue that turned into a good philosophical debate on how to build a progressive web app: should you enhance your existing site or creating a separate URL?

(For the record: I’m in favour of enhancing.)

Sunday, February 11th, 2018

Welcoming Progressive Web Apps to Microsoft Edge and Windows 10 - Microsoft Edge Dev BlogMicrosoft Edge Dev Blog

It’s really great to hear about how Microsoft will be promoting progressive web apps as first-class citizens …but it’s really unhelpful that they’re using this fudgy definition:

Progressive Web Apps are just great web sites that can behave like native apps—or, perhaps, Progressive Web Apps are just great apps, powered by Web technologies and delivered with Web infrastructure.

Although they also give a more technical definition:

Technologically speaking, PWAs are web apps, progressively enhanced with modern web technologies (Service Worker, Fetch networking, Cache API, Push notifications, Web App Manifest) to provide a more app-like experience.

Nice try, slipping notifications in there like that, but no. No, no, no. Let’s not fool ourselves into thinking that one of the most annoying “features” of native apps is even desirable on the web.

If you want to use notifications, fine. But they are absolutely not a requirement for a progressive web app.

(A responsive design, on the other hand, totally is.)

Friday, February 9th, 2018

Workers at Your Service | WebKit

Here’s an interesting insight on how WebKit is going to handle the cleanup of unused service workers and caches:

Service worker and Cache API stored information will grow as a user is browsing content. To keep only the stored information that is useful to the user, WebKit will remove unused service worker registrations after a period of a few weeks. Caches that do not get opened after a few weeks will also be removed.

Thursday, January 25th, 2018

Safari 11.1

Squee! The next time there’s an update for OS X and iOS, Safari will magically have service worker support! Not only that, but Safari on iOS will start using the information in web app manifests for adding to home screen.

That’s an impressive turnaround.

Thursday, December 21st, 2017

Release Notes for Safari Technology Preview 46 | WebKit

Well, that escalated quickly! Service workers are now available in Safari’s Technology Preview, which means it won’t be long before it lands in Safari proper.

Also: service workers also just landed in the insider release of Edge.

Everything offline’s coming up Milhouse!

Thursday, December 14th, 2017

Progressive Web Apps - My new book is available! | Dean Hume

This looks interesting—a new book by Dean Hume all about progressive web apps. A few chapters are available to download.

Monday, November 27th, 2017

Network based image loading using the Network Information API in Service Worker | justmarkup

This is clever—you can use the navigator.connection API from a service worker (because it’s asynchronous) which means you can have a service worker script that serves differently sized images based on bandwidth.

Thursday, November 23rd, 2017

Salva de la Puente - What is a PWA

Here’s a nice one-sentence definition for the marketing folk:

A Progressive Web App is a regular website following a progressive enhancement strategy to deliver native-like user experiences by using modern Web standards.

But if you’re talking to developers, I implore you to concretely define a Progressive Web App as the combination of HTTPS, a service worker, and a Web App Manifest.

Wednesday, November 15th, 2017

What is a Progressive Web App?

It seems like any new field goes through an inevitable growth spurt that involves “defining the damn thing.” For the first few years of the IA Summit, every second presentation seemed to be about defining what Information Architecture actually is. See also: UX. See also: Content Strategy.

Now it seems to be happening with Progressive Web Apps …which is odd, considering the damn thing is defined damn well.

I’ve written before about the naming of Progressive Web Apps. On the whole, I think it’s a pretty good term, especially if you’re trying to convince the marketing team.

Regardless of the specifics of the name, what I like about Progressive Web Apps is that they have a clear definition. It reminds me of Responsive Web Design. Whatever you think of that name, it comes with a clear list of requirements:

  1. A fluid layout,
  2. Fluid images, and
  3. Media queries.

Likewise, Progressive Web Apps consist of:

  1. HTTPS,
  2. A service worker, and
  3. A Web App Manifest.

There’s more you can do in addition to that (just as there’s plenty more you can do on a responsive site), but the core definition is nice and clear.

Except, for some reason, that clarity is being lost.

Here’s a post by Ben Halpern called What the heck is a “Progressive Web App”? Seriously.

I have a really hard time describing what a progressive web app actually is.

He points to Google’s intro to Progressive Web Apps:

Progressive Web Apps are user experiences that have the reach of the web, and are:

  • Reliable - Load instantly and never show the downasaur, even in uncertain network conditions.
  • Fast - Respond quickly to user interactions with silky smooth animations and no janky scrolling.
  • Engaging - Feel like a natural app on the device, with an immersive user experience.

Those are great descriptions of the benefits of Progressive Web Apps. Perfect material for convincing your clients or your boss. But that appears on developers.google.com …surely it would be more beneficial for that audience to know the technologies that comprise Progressive Web Apps?

Ben Halpern again:

Google’s continued use of the term “quality” in describing things leaves me with a ton of confusion. It really seems like they want PWA to be a general term that doesn’t imply any particular implementation, and have it be focused around the user experience, but all I see over the web is confusion as to what they mean by these things. My website is already “engaging” and “immersive”, does that mean it’s a PWA?

I think it’s important to use the right language for the right audience.

If you’re talking to the business people, tell them about the return on investment you get from Progressive Web Apps.

If you’re talking to the marketing people, tell them about the experiential benefits of Progressive Web Apps.

But if you’re talking to developers, tell them that a Progressive Web App is a website served over HTTPS with a service worker and manifest file.

Thursday, November 9th, 2017

Jeremy Keith on Evaluating Technology at SmashingConf Barcelona 2017 on Vimeo

I think this is the best delivery of this talk I’ve ever given. It was something about being in that wonderful venue.

I got quite worked up around the the 32 minute mark.

Jeremy Keith on Evaluating Technology at SmashingConf Barcelona 2017

Monday, November 6th, 2017

Installing Progressive Web Apps

When I was testing the dConstruct Audio Archive—which is now a Progressive Web App—I noticed some interesting changes in how Chrome on Android offers the “add to home screen” prompt.

It used to literally say “add to home screen.”

Getting the “add to home screen” prompt for https://huffduffer.com/ on Android Chrome. And there’s the “add to home screen” prompt for https://html5forwebdesigners.com/ HTTPS + manifest.json + Service Worker = “Add to Home Screen” prompt. Add to home screen.

Now it simply says “add.”

The dConstruct Audio Archive is now a Progressive Web App

I vaguely remember there being some talk of changing the labelling, but I could’ve sworn it was going to change to “install”. I’ve got to be honest, just having the word “add” doesn’t seem to provide much context. Based on the quick’n’dirty usability testing I did with some co-workers, it just made things confusing. “Add what?” “What am I adding?”

Additionally, the prompt appeared immediately on the first visit to the site. I thought there was supposed to be an added “engagement” metric in order for the prompt to appear; that the user needs to visit the site more than once.

You’d think I’d be happy that users will be presented with the home-screen prompt immediately, but based on the behaviour I saw, I’m not sure it’s a good thing. Here’s what I observed:

  1. The user types the URL archive.dconstruct.org into the address bar.
  2. The site loads.
  3. The home-screen prompt slides up from the bottom of the screen.
  4. The user immediately moves to dismiss the prompt (cue me interjecting “Don’t close that!”).

This behaviour is entirely unsurprising for three reasons:

  1. We web designers and web developers have trained users to dismiss overlays and pop-ups if they actually want to get to the content. Nobody’s going to bother to actually read the prompt if there’s a 99% chance it’s going to say “Sign up to our newsletter!” or “Take our survey!”.
  2. The prompt appears below the “line of death” so there’s no way to tell it’s a browser or OS-level dialogue rather than a JavaScript-driven pop-up from the site.
  3. Because the prompt now appears on the first visit, no trust has been established between the user and the site. If the prompt only appeared on later visits (or later navigations during the first visit) perhaps it would stand a greater chance of survival.

It’s still possible to add a Progressive Web App to the home screen, but the option to do that is hidden behind the mysterious three-dots-vertically-stacked icon (I propose we call this the shish kebab icon to distinguish it from the equally impenetrable hamburger icon).

I was chatting with Andreas from Mozilla at the View Source conference last week, and he was filling me in on how Firefox on Android does the add-to-homescreen flow. Instead of a one-time prompt, they’ve added a persistent icon above the “line of death” (the icon is a combination of a house and a plus symbol).

When a Firefox 58 user arrives on a website that is served over HTTPS and has a valid manifest, a subtle badge will appear in the address bar: when tapped, an “Add to Home screen” confirmation dialog will slide in, through which the web app can be added to the Android home screen.

This kind of badging also has issues (without the explicit text “add to home screen”, the user doesn’t know what the icon does), but I think a more persistently visible option like this works better than the a one-time prompt.

Firefox is following the lead of the badging approach pioneered by the Samsung Internet browser. It provides a plus symbol that, when pressed, reveals the options to add to home screen or simply bookmark.

What does it mean to be an App?

I don’t think Chrome for Android has any plans for this kind of badging, but they are working on letting the site authors provide their own prompts. I’m not sure this is such a good idea, given our history of abusing pop-ups and overlays.

Sadly, I feel that any solution that relies on an unrequested overlay is doomed. That’s on us. The way we’ve turned browsing the web—especially on mobile—into a frustrating chore of dismissing unwanted overlays is a classic tragedy of the commons. We blew it. Users don’t trust unrequested overlays, and I can’t blame them.

For what it’s worth, my opinion is that ambient badging is a better user experience than one-time prompts. That opinion is informed by a meagre amount of testing though. I’d love to hear from anyone who’s been doing more detailed usability testing of both approaches. I assume that Google, Mozilla, and Samsung are doing this kind of testing, and it would be really great to see the data from that (hint, hint).

But it might well be that ambient badging is just too subtle to even be noticed by the user.

On one end of the scale you’ve got the intrusiveness of an add-to-home-screen prompt, but on the other end of the scale you’ve got the discoverability problem of a subtle badge icon. I wonder if there might be a compromise solution—maybe a badge icon that pulses or glows on the first or second visit?

Of course that would also need to be thoroughly tested.

Thursday, November 2nd, 2017

The dConstruct Audio Archive works offline

The dConstruct conference is as old as Clearleft itself. We put on the first event back in 2005, the year of our founding. The last dConstruct was in 2015. It had a good run.

I’m really proud of the three years I ran the show—2012, 2013, and 2014—and I have great memories from each event. I’m inordinately pleased that the individual websites are still online after all these years. I’m equally pleased with the dConstruct audio archive that we put online in 2012. Now that the event itself is no longer running, it truly is an archive—a treasury of voices from the past.

I think that these kinds of online archives are eminently suitable for some offline design. So I’ve added a service worker script to the dConstruct archive.


To start with, there’s the no-brainer: as soon as someone hits the website, pre-cache static assets like CSS, JavaScript, the logo, and icon images. Now subsequent page loads will be quicker—those assets are taken straight from the cache.

But what about the individual pages? For something like Resilient Web Design—another site that won’t be updated—I pre-cache everything. I could do that with the dConstruct archive. All of the pages with all of the images add up to less than two megabytes; the entire site weighs less than a single page on Wired.com or The Verge.

In the end, I decided to go with a cache-as-you-go strategy. Every time a page or an image is fetched from the network, it is immediately put in a cache. The next time that page or image is requested, the file is served from that cache instead of the network.

Here’s the logic for fetch requests:

  1. First, look to see if the file is in a cache. If it is, great! Serve that.
  2. If the file isn’t in a cache, make a network request and serve the response …but put a copy of a file in the cache.
  3. The next time that file is requested, go to step one.

Save for offline

That caching strategy works great for pages, images, and other assets. But there’s one kind of file on the dConstruct archive that’s a bit different: the audio files. They can be fairly big, so I don’t want to cache those unless the user specifically requests it.

If you end up on the page for a particular talk, and your browser supports service workers, you’ll get an additional UI element in the list of options: a toggle to “save offline” (under the hood, it’s a checkbox). If you activate that option, then the audio file gets put into a cache.

Now if you lose your network connection while browsing the site, you’ll get a custom offline page with the option to listen to any audio files you saved for offline listening. You’ll also see this collection of talks on the homepage, regardless of whether you’ve got an internet connection or not.

So if you’ve got a long plane journey ahead of you, have a browse around the dConstruct archive and select some talks for your offline listening pleasure.

Or just enjoy the speediness of browsing the site.

Turning another website into a Progressive Web App.

Sunday, October 29th, 2017

Can You Afford It?: Real-world Web Performance Budgets – Infrequently Noted

Alex looks at the mindset and approaches you need to adopt to make a performant site. There’s some great advice in here for setting performance budgets for JavaScript.

JavaScript is the single most expensive part of any page in ways that are a function of both network capacity and device speed. For developers and decision makers with fast phones on fast networks this is a double-whammy of hidden costs.

Sunday, October 22nd, 2017

Pattern Libraries, Performance, and Progressive Web Apps

Ever since its founding in 2005, Clearleft has been laser-focused on user experience design.

But we’ve always maintained a strong front-end development arm. The front-end development work at Clearleft is always in service of design. Over the years we’ve built up a wealth of expertise on using HTML, CSS, and JavaScript to make better user experiences.

Recently we’ve been doing a lot of strategic design work—the really in-depth long-term engagements that begin with research and continue through to design consultancy and collaboration. That means we’ve got availability for front-end development work. Whether it’s consultancy or production work you’re looking for, this could be a good opportunity for us to work together.

There are three particular areas of front-end expertise we’re obsessed with…

Pattern Libraries

We caught the design systems bug years ago, way back when Natalie started pioneering pattern libraries as our primary deliverable (or pattern portfolios, as we called them then). This approach has proven effective time and time again. We’ve spent years now refining our workflow and thinking around modular design. Fractal is the natural expression of this obsession. Danielle and Mark have been working flat-out on version 2. They’re very eager to share everything they’ve learned along the way …and help others put together solid pattern libraries.

Danielle Huntrods Mark Perkins


Thinking about it, it’s no surprise that we’re crazy about performance at Clearleft. Like I said, our focus on user experience, and when it comes to user experience on the web, nothing but nothing is more important than performance. The good news is that the majority of performance fixes can be done on the front end—images, scripts, fonts …it’s remarkable how much a good front-end overhaul can make to the bottom line. That’s what Graham has been obsessing over.

Graham Smith

Progressive Web Apps

Over the years I’ve found myself getting swept up in exciting new technologies on the web. When Clearleft first formed, my head was deep into DOM Scripting and Ajax. Half a decade later it was HTML5. Now it’s service workers. I honestly think it’s a technology that could be as revolutionary as Ajax or HTML5 (maybe I should write a book to that effect).

I’ve been talking about service workers at conferences this year, and I can’t hide my excitement:

There’s endless possibilities of what you can do with this technology. It’s very powerful.

Combine a service worker with a web app manifest and you’ve got yourself a Progressive Web App. It’s not just a great marketing term—it’s an opportunity for the web to truly excel at delivering the kind of user experiences previously only associated with native apps.

Jeremy Keith

I’m very very keen to work with companies and organisations that want to harness the power of service workers and Progressive Web Apps. If that’s you, get in touch.

Whether it’s pattern libraries, performance, or Progressive Web Apps, we’ve got the skills and expertise to share with you.

Thursday, October 19th, 2017

Mozilla Developer Roadshow Asia Jeremy Keith - YouTube

At the 14 minute mark I had to deal with an obstreperous member of the audience. He wasn’t heckling exactly …he just had a very bad experience with web components, and I think my talk was triggering for him.

Mozilla Developer Roadshow Asia: Jeremy Keith

Monday, October 9th, 2017

Service Worker Registration  |  Web Fundamentals  |  Google Developers

Hmm …seems like I should probably wait for the load event before triggering navigator.serviceworker.register().

Monday, September 25th, 2017

Network Information API

It looks like this is landing in Chrome. The navigator.connection.type property will allow us to progressively enhance based on connection type:

A web application that makes use of a service worker to cache resources during installation might have different bundles of assets that it might cache: a list of crucial assets that are cached unconditionally, and a bundle of larger, optional assets that are only cached ahead of time when navigator.connection.type is 'ethernet' or 'wifi'.

There are potential security issues around fingerprinting that are addressed in this document.