Tags: service



Wednesday, July 11th, 2018

Tuesday, July 10th, 2018

Twitter and Instagram progressive web apps

Since support for service workers landed in Mobile Safari on iOS, I’ve been trying a little experiment. Can I replace some of the native apps I use with progressive web apps?

The two major candidates are Twitter and Instagram. I added them to my home screen, and banished the native apps off to a separate screen. I’ve been using both progressive web apps for a few months now, and I have to say, they’re pretty darn great.

There are a few limitations compared to the native apps. On Twitter, if you follow a link from a tweet, it pops open in Safari, which is fine, but when you return to Twitter, it loads anew. This isn’t any fault of Twitter—this is the way that web apps have worked on iOS ever since they introduced their weird web-app-capable meta element. I hope this behaviour will be fixed in a future update.

Also, until we get web notifications on iOS, I need to keep the Twitter native app around if I want to be notified of a direct message (the only notification I allow).

Apart from those two little issues though, Twitter Lite is on par with the native app.

Instagram is also pretty great. It too suffers from some navigation issues. If I click through to someone’s profile, and then return to the main feed, it also loads it anew, losing my place. It would be great if this could be fixed.

For some reason, the Instagram web app doesn’t allow uploading multiple photos …which is weird, because I can upload multiple photos on my own site by adding the multiple attribute to the input type="file" in my posting interface.

Apart from that, though, it works great. And as I never wanted notifications from Instagram anyway, the lack of web notifications doesn’t bother me at all. In fact, because the progressive web app doesn’t keep nagging me about enabling notifications, it’s a more pleasant experience overall.

Something else that was really annoying with the native app was the preponderance of advertisements. It was really getting out of hand.

Well …(looks around to make sure no one is listening)… don’t tell anyone, but the Instagram progressive web app—i.e. the website—doesn’t have any ads at all!

Here’s hoping it stays that way.

When 7 KB Equals 7 MB - Cloud Four

I remember Jason telling me about this weird service worker caching behaviour a little while back. This piece is a great bit of sleuthing in tracking down the root causes of this strange issue, followed up with a sensible solution.

Monday, July 9th, 2018

devMode.fm // Going Offline: Service Workers with Jeremy Keith

I talked for an hour about service workers ‘n’ stuff

(Also available on Huffduffer.)

Thursday, July 5th, 2018

The trimCache function in Going Offline

Paul Yabsley wrote to let me know about an error in Going Offline. It’s rather embarrassing because it’s code that I’m using in the service worker for adactio.com but for some reason I messed it up in the book.

It’s the trimCache function in Chapter 7: Tidying Up. That’s the reusable piece of code that recursively reduces the number of items in a specified cache (cacheName) to a specified amount (maxItems). On page 95 and 96 I describe the process of creating the function which, in the book, ends up like this:

 function trimCache(cacheName, maxItems) {
   cacheName.open( cache => {
     .then( items => {
       if (items.length > maxItems) {
           trimCache(cacheName, maxItems)
         ); // end delete then
       } // end if
     }); // end keys then
   }); // end open
 } // end function

See the problem? It’s right there at the start when I try to open the cache like this:

cacheName.open( cache => {

That won’t work. The open method only works on the caches object—I should be passing the name of the cache into the caches.open method. So the code should look like this:

caches.open( cacheName )
.then( cache => {

Everything else remains the same. The corrected trimCache function is here:

function trimCache(cacheName, maxItems) {
  .then( cache => {
    .then(keys => {
      if (keys.length > maxItems) {
          trimCache(cacheName, maxItems)
        ); // end delete then
      } // end if
    }); // end keys then
  }); // end open then
} // end function

Sorry about that! I must’ve had some kind of brainfart when I was writing (and describing) that one line of code.

You may want to deface your copy of Going Offline by taking a pen to that code example. Normally I consider the practice of writing in books to be barbarism, but in this case …go for it.

Wednesday, July 4th, 2018

Going Offline - Polytechnic

This is a lovely review of Going Offline from Garrett:

With his typical self-effacing humour (chapter titles include Making Fetch Happen and Cache Me If You Can), and easy manner, Jeremy explains how Service Workers, uh, work, the clever things you can do with them, and most importantly, how to build your own.

Best of all, he’s put it into action!

To that end, this site now has its own home-grown, organic, corn fed, Service Worker.

Tuesday, June 26th, 2018

10 Progressive Web App Examples that Brand Owners can Learn From - Iflexion

Adriana Blum lists progressive web apps that are doing very, very well from Twitter, Trivago, Starbucks, Forbes, Debebhams, West Elm, Washington Post, Pinterest, AliExpress, and Lancôme.

Instead of choosing between the immediacy of a mobile website and the rich experience offered by native apps, you can now offer your target audiences the best of both and improve the commercial performance of your business to boot.

Saturday, June 23rd, 2018

New tools for art direction on the web

I’m in Boston right now, getting ready to speak at An Event Apart. This will be my second (and last) Event Apart of the year—the other time was in Seattle back in April. After that event, I wrote about how inspired I was:

It was interesting to see repeating, overlapping themes. From a purely technical perspective, three technologies that were front and centre were:

  • CSS grid,
  • variable fonts, and
  • service workers.

From listening to other attendees, the overwhelming message received was “These technologies are here—they’ve arrived.”

I was itching to combine those technologies on a project. Coincidentally, it was around that time that I started planning to publish The Gęsiówka Story. I figured I could use that as an opportunity to tinker with those front-end technologies that I was so excited about.

But I was cautious. I didn’t want to use the latest exciting technology just for the sake of it. I was very aware of the gravity of the material I was dealing with. Documenting the story of Gęsiówka was what mattered. Any front-end technologies I used had to be in support of that.

First of all, there was the typesetting. I don’t know about you, but I find choosing the right typefaces to be overwhelming. Despite all the great tips and techniques out there for choosing and pairing typefaces, I still find myself agonising over the choice—what if there’s a better choice that I’m missing?

In this case, because I wanted to use a variable font, I had a constraint that helped reduce the possibility space. I started to comb through v-fonts.com to find a suitable typeface—I was fairly sure I wanted a serious serif.

I had one other constraint. The font file had to include English, Polish, and German glyphs. That pretty much sealed the deal for Source Serif. That only has one variable axis—weight—but I decided that this could also be an interesting constraint: how much could I wrangle out of a single typeface just using various weights?

I ended up using font weights of 75, 250, 315, 325, 340, 350, 400, and 525. Most of them were for headings or one-off uses, with a font-weight of 315 for the body copy.

(And can I just say once again how impressed I am that the founding fathers of CSS were far-sighted enough to keep those font weight ranges free for future use?)

Getting the typography right posed an interesting challenge. This was a fairly long piece of writing, so it really needed to be readable without getting tiring. But at the same time, I didn’t want it to be exactly pleasant to read—that wouldn’t do the subject matter justice. I wanted the reader to feel the seriousness of the story they were reading, without being fatigued by its weight.

Colour and type went a long way to communicating that feeling. The grid sealed the deal.

The Gęsiówka Story is mostly one single column of text, so on the face of it, there isn’t much opportunity to go crazy with CSS Grid. But I realised I could use a grid to create a winding effect for the text. I had to be careful though: I didn’t want it to become uncomfortable to read. I wanted to create a slightly unsettling effect.

Every section element is turned into a seven-column grid container:

section {
    display: grid;
    grid-column-gap: 2em;
    grid-template-columns: 2em repeat(5, 1fr) 2em;

The first and last columns are the same width as the gutters (2em), effectively creating “outer” gutters for the grid. Each paragraph within the section takes up six of the seven columns. I use nth-of-type to alternate which six columns are used (the first six or the last six). That creates the staggered indendation:

section > p {
    grid-column: 1/7;
section > p:nth-of-type(even) {
    grid-column: 2/8;

Staggered grid.

That might seem like overkill just to indent every second paragraph by 4em, but I then used the same grid dimensions to layout figure elements with images and captions.

section > figure {
    display: grid;
    grid-column-gap: 2em;
    grid-template-columns: 2em repeat(5, 1fr) 2em;

Then I can lay out differently proportioned images across different ranges of the grid:

section > figure.landscape > img {
    grid-column: 1/5;
section > figure.landscape > figcaption {
    grid-column: 5/8;
section > figure.portrait > img {
    grid-column: 1/4;
section > figure.portrait > figcaption {
    grid-column: 4/8;

Because they’re positioned on the same grid as the paragraphs, everything lines up nicely (and yes, if subgrid existed, I wouldn’t have to redeclare the grid dimensions for the figures).

Finally, I wanted to make sure that the whole thing could be read offline. After all, once you’ve visited the URL once, there’s really no reason to make any more requests to the server. Static documents—and books—are the perfect candidates for an “offline first” approach: always look in the cache, and only go to the network as a last resort.

In this case I used a variation of my minimal viable service worker script, and the result is a very short set of instructions. There’s a little bit of pre-caching going on: I grab the variable font and the HTML page itself (which includes the CSS inlined).

So there you have it: variable fonts, CSS grid, and service workers: three exciting front-end technologies, all of which can be applied as progressive enhancements on top of the core content.

Once again, I find that it’s personal projects that offer the most opportunities to try out new or interesting techniques. And The Gęsiówka Story is a very personal project indeed.

I discovered a browser bug - JakeArchibald.com

Jake’s blow-by-blow account of uncovering a serious browser vulnerability is fascinating. But if you don’t care for the technical details, skip ahead to to how different browser makers handled the issue—it’s very enlightening. (And if you do care for the technical details, make sure you click on the link to the PDF version of this post.)

Monday, June 18th, 2018

Praise for Going Offline

I’m very, very happy to see that my new book Going Offline is proving to be accessible and unintimidating to a wide audience—that was very much my goal when writing it.

People have been saying nice things on their blogs, which is very gratifying. It’s even more gratifying to see people use the knowledge gained from reading the book to turn those blogs into progressive web apps!

Sara Soueidan:

It doesn’t matter if you’re a designer, a junior developer or an experienced engineer — this book is perfect for anyone who wants to learn about Service Workers and take their Web application to a whole new level.

I highly recommend it. I read the book over the course of two days, but it can easily be read in half a day. And as someone who rarely ever reads a book cover to cover (I tend to quit halfway through most books), this says a lot about how good it is.

Eric Lawrence:

I was delighted to discover a straightforward, very approachable reference on designing a ServiceWorker-backed application: Going Offline by Jeremy Keith. The book is short (I’m busy), direct (“Here’s a problem, here’s how to solve it“), opinionated in the best way (landmine-avoiding “Do this“), and humorous without being confusing. As anyone who has received unsolicited (or solicited) feedback from me about their book knows, I’m an extremely picky reader, and I have no significant complaints on this one. Highly recommended.

Ben Nadel:

If you’re interested in the “offline first” movement or want to learn more about Service Workers, Going Offline by Jeremy Keith is a really gentle and highly accessible introduction to the topic.

Daniel Koskine:

Jeremy nails it again with this beginner-friendly introduction to Service Workers and Progressive Web Apps.

Donny Truong

Jeremy’s technical writing is as superb as always. Similar to his first book for A Book Apart, which cleared up all my confusions about HTML5, Going Offline helps me put the pieces of the service workers’ puzzle together.

People have been saying nice things on Twitter too…

Aaron Gustafson:

It’s a fantastic read and a simple primer for getting Service Workers up and running on your site.

Ethan Marcotte:

Of course, if you’re looking to take your website offline, you should read @adactio’s wonderful book

Lívia De Paula Labate:

Ok, I’m done reading @adactio’s Going Offline book and as my wife would say, it’s the bomb dot com.

If that all sounds good to you, get yourself a copy of Going Offline in paperbook, or ebook (or both).

Sunday, June 17th, 2018

Detecting image requests in service workers

In Going Offline, I dive into the many different ways you can use a service worker to handle requests. You can filter by the URL, for example; treating requests for pages under /blog or /articles differently from other requests. Or you can filter by file type. That way, you can treat requests for, say, images very differently to requests for HTML pages.

One of the ways to check what kind of request you’re dealing with is to see what’s in the accept header. Here’s how I show the test for HTML pages:

if (request.headers.get('Accept').includes('text/html')) {
    // Handle your page requests here.

So, logically enough, I show the same technique for detecting image requests:

if (request.headers.get('Accept').includes('image')) {
    // Handle your image requests here.

That should catch any files that have image in the request’s accept header, like image/png or image/jpeg or image/svg+xml and so on.

But there’s a problem. Both Safari and Firefox now use a much broader accept header: */*

My if statement evaluates to false in those browsers. Sebastian Eberlein wrote about his workaround for this issue, which involves looking at file extensions instead:

if (request.url.match(/\.(jpe?g|png|gif|svg)$/)) {
    // Handle your image requests here.

So consider this post a patch for chapter five of Going Offline (page 68 specifically). Wherever you see:

if (request.headers.get('Accept').includes('image'))

Swap it out for:

if (request.url.match(/\.(jpe?g|png|gif|svg)$/))

And feel to add any other image file extensions (like webp) in there too.

Tuesday, June 12th, 2018

Any Site can be a Progressive Web App - Jeremy Keith | DeltaV 2018 - YouTube

Here’s a really quick (ten minute) talk about the offline user experience that I gave at the Delta V conference recently. I’m quite happy with how it turned out—there’s something to be said for having a short and snappy time slot.

There’s a common misconception that making a Progressive Web App means creating a Single Page App with an app-shell architecture. But the truth is that literally any website can benefit from the performance boost that results from the combination of HTTPS + Service Worker + Web App Manifest.

Any Site can be a Progressive Web App - Jeremy Keith | DeltaV 2018

Friday, June 8th, 2018

Registering service workers

In chapter two of Going Offline, I talk about registering your service worker wrapped up in some feature detection:

if (navigator.serviceworker) {

But I also make reference to a declarative way of doing this that isn’t very widely supported:

<link rel="serviceworker" href="/serviceworker.js">

No need for feature detection there. Thanks to the liberal error-handling model of HTML (and CSS), browsers will just ignore what they don’t understand, which isn’t the case with JavaScript.

Alas, it looks like that nice declarative alternative isn’t going to be making its way into browsers anytime soon. It has been removed from the HTML spec. That’s a shame. I have a preference for declarative solutions where possible—they’re certainly easier to teach. But in this case, the JavaScript alternative isn’t too onerous.

So if you’re reading Going Offline, when you get to the bit about someday using the rel value, you can cast a wistful gaze into the distance, or shed a tiny tear for what might have been …and then put it out of your mind and carry on reading.

Tuesday, June 5th, 2018

Clearleft.com is a progressive web app

What’s that old saying? The cobbler’s children have no shoes that work offline. Or something.

It’s been over a year since the Clearleft site relaunched and I listed some of the next steps I had planned:

Service worker. It’s a no-brainer. Now that the Clearleft site is (finally!) running on HTTPS, having a simple service worker to cache static assets like CSS, JavaScript and some images seems like the obvious next step.

You know how it is. Those no-brainer tasks are exactly the kind of thing that end up on a to-do list without ever quite getting to-done. Meanwhile I’ve been writing and speaking about how any website can be a progressive web app. I think Alanis Morissette used to sing about this sort of situation.

Enough is enough! Clearleft.com is now a progressive web app. It has a manifest file and a service worker script.

The service worker logic is fairly straightforward, and taken almost verbatim from Going Offline. As you navigate around the site, the service worker applies different logic depending on the kind of file you’re requesting:

  • Pages are served fresh from the network, falling back to the cache when there’s a problem.
  • Everything else is served from the cache where possible, resorting to the network only if there’s no match in the cache—quite the performance boost!

In both cases, if a page or a file is retrieved from the network, it’s gets put into a cache. I’ve got one cache for pages, and another for everything else. And even if a file is retrieved from that cache, I still fire off a fetch request to grab a fresh copy for the cache. So while there’s a chance that a stale file might be served up, it will only ever be slightly stale, and the next time it’s requested, it’ll be fresh.

In the worst-case scenario, when a page can’t be retrieved from the network or the cache, you end up seeing a custom offline page. There you can see a list of any pages that are cached (meaning you can revisit them even without an internet connection).

A custom offline page showing a list of URLs.

It’s not ideal—page titles would be friendlier than URLs—but it’s a start. I’m sure I’ll revisit it soon. Honest.

Oh, and after a year of procrastinating about doing this, guess how long it took? About half a day. Admittedly, this isn’t my first progressive web app, and the more you build ‘em, the easier it gets. Still, it’s a classic example of a small investment of time leading to a big improvement in performance and user experience.

If you think your company’s website could benefit from being a progressive web app (and believe me, it definitely could), you have a couple of options:

  1. Arm yourself with a copy of Going Offline and give it a go yourself. Or
  2. Get in touch with Clearleft. We can help you. (See, I can say that with a straight face now that we’re practicing what we preach.)

Either way, don’t dilly dally …like I did.

Fresher service workers, by default

“Ah, this is good news!”, I thought, reading this update about how service worker scripts won’t be cached.

And that was the moment when I realised what an utter nerd I had become.

Monday, June 4th, 2018

Progressive Web Games – Mozilla Hacks – the Web developer blog

I was just talking about how browser-based games are the perfect use-case for service workers. Andrzej Mazur breaks down how that would work:

  • Add to Home screen
  • Offline capabilities
  • Progressive loading


I noticed something interesting recently about how I browse the web.

It used to be that I would notice if a site were responsive. Or, before responsive web design was a thing, I would notice if a site was built with a fluid layout. It was worthy of remark, because it was exceptional—the default was fixed-width layouts.

But now, that has flipped completely around. Now I notice if a site isn’t responsive. It feels …broken. It’s like coming across an embedded map that isn’t a slippy map. My expectations have reversed.

That’s kind of amazing. If you had told me ten years ago that liquid layouts and media queries would become standard practice on the web, I would’ve found it very hard to believe. I spent the first decade of this century ranting in the wilderness about how the web was a flexible medium, but I felt like the laughable guy on the street corner with an apocalyptic sandwich board. Well, who’s laughing now

Anyway, I think it’s worth stepping back every now and then and taking stock of how far we’ve come. Mind you, in terms of web performance, the trend has unfortunately been in the wrong direction—big, bloated websites have become the norm. We need to change that.

Now, maybe it’s because I’ve been somewhat obsessed with service workers lately, but I’ve started to notice my expectations around offline behaviour changing recently too. It’s not that I’m surprised when I can’t revisit an article without an internet connection, but I do feel disappointed—like an opportunity has been missed.

I really notice it when I come across little self-contained browser-based games like

Those games are great! I particularly love Battleship Solitaire—it has a zen-like addictive quality to it. If I load it up in a browser tab, I can then safely go offline because the whole game is delivered in the initial download. But if I try to navigate to the game while I’m offline, I’m out of luck. That’s a shame. This snack-sized casual games feel like the perfect use-case for working offline (or, even if there is an internet connection, they could still be speedily served up from a cache).

I know that my expectations about offline behaviour aren’t shared by most people. The idea of visiting a site even when there’s no internet connection doesn’t feel normal …yet.

But perhaps that expectation will change. It’s happened before.

(And if you want to be ready when those expectations change, I’ve written a Going Offline for you.)

Friday, May 18th, 2018

How to display a “new version available” of your Progressive Web App

This is a good walkthrough of the flow you’d need to implement if you want to notify users of an updated service worker.

Wednesday, May 16th, 2018

HTTPS + service worker + web app manifest = progressive web app

I gave a quick talk at the Delta V conference in London last week called Any Site can be a Progressive Web App. I had ten minutes, but frankly I only needed enough time to say the title of the talk because, well, that was also the message.

There’s a common misconception that making a Progressive Web App means creating a Single Page App with an app-shell architecture. But the truth is that literally any website can benefit from the performance boost that results from the combination of HTTPS + Service Worker + Web App Manifest.

See how I define a progressive web app as being HTTPS + service worker + web app manifest? I’ve been doing that for a while. Here’s a post from last year called Progressing the web:

Literally any website can be a progressive web app:

That last step can be tricky if you’re new to service workers, but it’s not unsurmountable. It’s certainly a lot easier than completely rearchitecting your existing website to be a JavaScript-driven single page app.

Later I wrote a post called What is a Progressive Web App? where I compared the definition to responsive web design.

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.

But here’s the thing. Outside of the confines of my own website, it’s hard to find that definition anywhere.

On Google’s developer site, their definition uses adjectives like “reliable”, “fast”, and “engaging”. Those are all great adjectives, but more useful to a salesperson than a developer.

Over on the Mozilla Developer Network, their section on progressive web apps states:

Progressive web apps use modern web APIs along with traditional progressive enhancement strategy to create cross-platform web applications. These apps work everywhere and provide several features that give them the same user experience advantages as native apps. 

Hmm …I’m not so sure about that comparison to native apps (and I’m a little disturbed that the URL structure is /Apps/Progressive). So let’s click through to the introduction:

PWAs are web apps developed using a number of specific technologies and standard patterns to allow them to take advantage of both web and native app features.

Okay. Specific technologies. That’s good to hear. But instead of then listing those specific technologies, we’re given another list of adjectives (discoverable, installable, linkable, etc.). Again, like Google’s chosen adjectives, they’re very nice and desirable, but not exactly useful to someone who wants to get started making a progressive web app. That’s why I like to cut to the chase and say:

  • You need to be running on HTTPS,
  • Then you can add a service worker,
  • And don’t forget to add a web app manifest file.

If you do that, you’ve got a progressive web app. Now, to be fair, there’a lot that I’m leaving out. Your site should be fast. Your site should be responsive (it is, after all, on the web). There’s not much point mucking about with service workers if you haven’t sorted out the basics first. But those three things—HTTPS + service worker + web app manifest—are specifically what distinguishes a progressive web app. You can—and should—have a reliable, fast, engaging website before turning it into a progressive web app.

Jason has been thinking about progressive web apps a lot lately (he should write a book or something), and he said to me:

I agree with you on the three things that comprise a PWA, but as far as I can tell, you’re the first to declare it as such.

I was quite surprised by that. I always assumed that I was repeating the three ingredients of a progressive web app, not defining them. But looking through all the docs out there, Jason might be right. It’s surprising because I assumed it was obvious why those three things comprise a progressive web app—it’s because they’re testable.

Lighthouse, PWA Builder, Sonarwhal and other tools that evaluate your site will measure its progressive web app score based on the three defining factors (HTTPS, service worker, web app manifest). Then there’s Android’s Add to Home Screen prompt. Here finally we get a concrete description of what your site needs to do to pass muster:

  • Includes a web app manifest…
  • Served over HTTPS (required for service workers)
  • Has registered a service worker with a fetch event handler

(Although, as of this month, Chrome will no longer show the prompt automatically—you also have to write some JavaScript to handle the beforeinstallprompt  event).

Anyway, if you’re looking to turn your website into a progressive web app, here’s what you need to do (assuming it’s already performant and responsive):

  1. Switch over to HTTPS. Certbot can help you here.
  2. Add a web app manifest.
  3. Add a service worker to your site so that it responds even when there’s no network connection.

That last step might sound like an intimidating prospect, but help is at hand: I wrote Going Offline for exactly this situation.