Tags: es6



Going offline at Indie Web Camp Düsseldorf

I’ve just come back from a ten-day trip to Germany. The trip kicked off with Indie Web Camp Düsseldorf over the course of a weekend.

IndieWebCamp Düsseldorf 2017

Once again the wonderful people at Sipgate hosted us in their beautiful building, and once again myself and Aaron helped facilitate the two days.

IndieWebCamp Düsseldorf 2017

Saturday was the BarCamp-like discussion day. Plenty of interesting topics were covered. I led a session on service workers, and that’s also what I decided to work on for the second day—that’s when the talking is done and we get down to making.

IndieWebCamp Düsseldorf 2017 IndieWebCamp Düsseldorf 2017 IndieWebCamp Düsseldorf 2017 IndieWebCamp Düsseldorf 2017

I like what Ethan is doing on his offline page. He shows a list of pages that have been cached, but instead of just listing URLs, he shows a title and description for each page.

I’ve already got a separate cache for pages that gets added to as the user browses around my site. I needed to figure out a way to store the metadata for those pages so that I could then display it on the offline page. I came up with a workable solution, and interestingly, it involved no changes to the service worker script at all.

When you visit any blog post, I put metadata about the page into localStorage (after first checking that there’s an active service worker):

if (navigator.serviceWorker && navigator.serviceWorker.controller) {
  window.addEventListener('load', function() {
    var data = {
      "title": "A minority report on artificial intelligence",
      "description": "Revisiting Spielberg’s films after a decade and a half.",
      "published": "May 7th, 2017",
      "timestamp": "1494171049"

In my case, I’m outputting the metadata from the server, but you could just as easily grab some from the DOM like this:

var data = {
  "title": document.querySelector("title").innerText,
  "description": document.querySelector("meta[name='description']").getAttribute("contents")

Meanwhile in my service worker, when you visit that same page, it gets added to a cache called “pages”. Both localStorage and the cache API are using URLs as keys. I take advantage of that on my offline page.

The nice thing about writing JavaScript on my offline page is that I know the page will only be seen by modern browsers that support service workers, so I can use all sorts of fancy from ES6, or whatever we’re calling it now.

I start by looping through the keys of the “pages” cache (that’s right—the cache API isn’t just for service workers; you can access it from any script). Then I check to see if there is a corresponding localStorage key with the same string (a URL). If there is, I pull the metadata out of local storage and add it to an array called browsingHistory:

const browsingHistory = [];
.then( cache => {
  .then(keys => {
    keys.forEach( request => {
      let data = JSON.parse(localStorage.getItem(request.url));
        if (data) {
          data['url'] = request.url;

Then I sort the list of pages in reverse chronological order:

browsingHistory.sort( (a,b) => {
  return b.timestamp - a.timestamp;

Now I loop through each page in the browsing history list and construct a link to each URL, complete with title and description:

let markup = '';
browsingHistory.forEach( data => {
  markup += `
<h2><a href="${ data.url }">${ data.title }</a></h2>
<p>${ data.description }</p>
<p class="meta">${ data.published }</p>

Finally I dump the constructed markup into a waiting div in the page with an ID of “history”:

let container = document.getElementById('history');
container.insertAdjacentHTML('beforeend', markup);

All those steps need to be wrapped inside the then clause attached to caches.open("pages") because the cache API is asynchronous.

There you have it. Now if you’re browsing adactio.com and your network connection drops (or my server goes offline), you can choose from a list of pages you’ve previously visited.

The current situation isn’t ideal though. I’ve got a clean-up operation in my service worker to limit the number of items stored in my “pages” cache. The cache never gets bigger than 35 items. But there’s no corresponding clean-up of metadata stored in localStorage. So there could be a lot more bits of metadata in local storage than there are pages in the cache. It’s not harmful, but it’s a bit wasteful.

I can’t do a clean-up of localStorage from my service worker because service workers can’t access localStorage. There’s a very good reason for that: the localStorage API is synchronous, and everything that happens in a service worker needs to be asynchronous.

Service workers can access indexedDB: it’s asynchronous. I could use indexedDB instead of localStorage, but I’m not a masochist. My best bet would be to use the localForage library, which wraps indexedDB in the simple syntax of localStorage.

Maybe I’ll do that at the next Homebrew Website Club here in Brighton.

Class teacher

ES6 introduced a whole bunch of new features to JavaScript. One of those features is the class keyword. This introduction has been accompanied by a fair amount of concern and criticism.

Here’s the issue: classes in JavaScript aren’t quite the same as classes in other programming languages. In fact, technically, JavaScript doesn’t really have classes at all. But some say that technically isn’t important. If it looks like a duck, and quacks like a duck, shouldn’t we call it a duck even if technically it’s somewhat similar—but not quite the same—species of waterfowl?

The argument for doing this is that classes are so familiar from other programming languages, that having some way of using classes in JavaScript—even if it isn’t technically the same as in other languages—brings a lot of benefit for people moving over to JavaScript from other programming languages.

But that comes with a side-effect. Anyone learning about classes in JavaScript will basically be told “here’s how classes work …but don’t look too closely.”

Now if you believe that outcomes matter more than understanding, then this is a perfectly acceptable trade-off. After all, we use computers every day without needing to understand the inner workings of every single piece of code under the hood.

It doesn’t sit well with me, though. I think that understanding how something works is important (in most cases). That’s why I favour learning underlying technologies first—HTML, CSS, JavaScript—before reaching for abstractions like frameworks and libraries. If you understand the way things work first, then your choice of framework, library, or any other abstraction is an informed choice.

The most common way that people refer to the new class syntax in JavaScript is to describe it as syntactical sugar. In other words, it doesn’t fundamentally introduce anything new under the hood, but it gives you a shorter, cleaner, nicer way of dealing with objects. It’s an abstraction. But because it’s an abstraction taken from other programming languages that work differently to JavaScript, it’s a bit of fudge. It’s a little white lie. The class keyword in JavaScript will work just fine as long as you don’t try to understand it.

My personal opinion is that this isn’t healthy.

I’ve come across two fantastic orators who cemented this view in my mind. At Render Conf in Oxford earlier this year, I had the great pleasure of hearing Ashley Williams talk about the challenges of teaching JavaScript. Skip to the 15 minute mark to hear her introduce the issues thrown up classes in JavaScript.

More recently, the mighty Kyle Simpson was on an episode of the JavaScript Jabber podcast. Skip to the 17 minute mark to hear him talk about classes in JavaScript.

(Full disclosure: Kyle also some very kind things about some of my blog posts at the end of that episode, but you can switch it off before it gets to that bit.)

Both Ashley and Kyle bring a much-needed perspective to the discussion of language design. That perspective is the perspective of a teacher.

In his essay on W3C’s design principles, Bert Bos lists learnability among the fundamental driving forces (closely tied to readability). Learnability and teachability are two sides of the same coin, and I find it valuable to examine any language decisions through that lens. With that mind, introducing a new feature into a language that comes with such low teachability value as to warrant a teacher actively telling a student not to learn how things really work …well, that just doesn’t seem right.