dotJS 2015

After dotCSS last Friday, I started my week with dotJS on Monday. Same organizers, but different Theatre. A bigger one was needed because dotJS is one full day of conferences for more than 1.000 attendees.

Overall, I liked dotCSS better (more compact, more human-sized, more great talks), but there have been some nice things said here as well, so I'll try to write all this here.


Christophe Porteneuve

To wake us all up, we started with Christophe Porteneuve, his non-French accent but very-English expressions, fast flow and way too bright slides.

At first I was a bit worried the talk was going to be something we had seen so many times. Speaking about callback hell and why it was bad. Then introducing promises and why it was good.

Then he moved further into unknown territory, using promises with generators. He started by introducing the yield concept, all the way to the await ES7 keyword. He said it himself that using promises with generators are a bit hackish and not the way they are meant to be used, but that was an interesting way to mix the concepts.

Slides are available here.

And Christophe does the same jokes in English than in French :)

By reading the docs, you're instantly part of the 1% best developers in the world.

Mathias Buus

Second talk was by Mathias Buus. There was a slide issue that forced him to redo the start of his talk, then there might have been a microphone issue because it was hard to hear what Mathias was saying. Or maybe he was just not speaking loudly enough.

It took me a while to understand what the topic of the talk was about. I managed to understand it was a project named hyperdrive, but it was only when he explained that it is a bittorent-like exchange format, done in JavaScript, that he got me hooked.

Hyperdrive uses something called smart diffing to split each file into chunks of data. It then does a checksum on each chunk, and store all the checksums into a table, a bit like a dictionary. In the end, to download a complete set of files, you only need to download each chunk listed in the dictionary and combine them to recreate the files. Doing so, you will never download twice the same content.

There is also another optimisation of creating a recursive hash on top of each set of hashes, in some kind of tree structure, but I will not be able to explain it clearly here.

He ended his talk with a demo of it in action, actually streaming a 200MB video file from one browser to another, just by sharing a hash representation of the tree of hashes.

The talk was a bit slow to start, but the end result was impressive, and I liked him taking the time to explain and demystify the bittorent logic (even if I did not remember everything).

Samuel Saccone

Third talk was by Samuel Saccone, that wasn't speaking really loudly either. Samuel told us about how you're supposed to debug a JavaScript application that becomes slow or unresponsive if used for a long period.


Such issues usually come from memory leaks. But as he so perfectly pointed out in his talk, it is not enough to know that. One actually have to be able to pinpoint the issue and fix it.

If you search for this issue on Google, you'll surely find some nice articles by Paul Irish or Addy Osmani and maybe even a Chrome dev tool video. Things looks so nice and easy when other are explaining it, but when it's your turn to actually understand the complex Chrome UI and untangle your messy code, this is a whole new story.

I was glad that I wasn't the only one struggling to understand how to fix that (and after discussing with other people, the feeling was shared). Fixing memory leaks is not something we want to do. This should be the browser or framework job. But as Samuel pointed, framework developers are humans as well, and they do not understand it either.

Usually, when you have a memory leak, it is because the garbage collector cannot get rid of some elements and keep them in memory. It cannot delete them because they still have a reference to an object that still exists. Most common examples are listeners to document or keeping a handy reference to _app in your views for examples (I plead guilty to this, back in my Backbone days).

He then walk us through the process of finding and fixing the bug, with a nice touch of humor. But the really nice information is that we can now write non-regression tests that checks for memory leaks.

By using drool, we have access to the current node count in the browser memory, so it's just a matter of running a set of actions repeatedly and see if that number grows or not.

I do not often have to debug memory leaks, but when I do, I always have to re-watch the same video tutorials and try to make it work with my current app. At least now I know that I'm not the only one finding this hard, and I'll even be able to write tests to prevent this from occurring again.

Rebecca Murphey

The last talk of the first session was by Rebecca Murphey, who had the difficult task of speaking between us and the lunch. Four sessions in the morning might be a bit too much, maybe 3 would have been better.


Anyway, she spoke a bit about HTTP/2, and what it will change. She, too, had some microphone issues and I had a hard time following what she was saying. I was a bit afraid she was going to do a list of what HTTP/2 was changing (something I had already seen several times recently -at ParisWeb and the perfUG for example-). But, no, instead she focussed on asking more down-to-earth and sensible questions.

First is: how will the server push data to the client? HTTP/2 lets the server pro-actively push data to the client. For example, if the client asks for an HTML page, the server can reply with a CSS, JavaScript and/or images along with the HTML, speculating that the user will ask for them later anyway.

All the needed syntax is available in the protocol to allow that, but how will the server know what to send? Will it be some kind of heuristic guessing based on previous requests? Will it be some configuration in our nginx files, or a LUA module that can process a bit of logic? Nothing is yet implemented to do it in any webserver.

There are a few POC webservers that lets you do it, but they exist only so we can test the various syntaxes and see which one is best. Nothing is ready yet, this is a new story we'll have to write.

Second question was: how are we going to debug HTTP/2 requests? The current Chrome and Firefox dev tools do not display the server push in the waterfall. Also, HTTP/2 being wrapped a binary protocol, all previous HTTP tool will need an upgrade to work with HTTP/2.

Next is, how are we going to transition from HTTP to HTTP/2? Most of the webperf best practices in HTTP are either useless or bad practice in HTTP/2. Are we going to have two builds, and redirect to one or the other based on the user support of HTTP?

If we look at how CDNs are currently handling HTTP/2, we see that they are not ready either. At the moment, only Cloudflare implements it, but it does not (yet) provides a way to do server push.

At first, the low voice volume, hungry belly and generic explanation of HTTP/2 make me fear a boring talk. In the end, the questions asked were clever and made me think. We wanted HTTP/2. Now we have it. But we still have to build our tools to correctly use it and the next few years will be spend toying with it, discovering usages, building stuff and emerging best practices. Can't wait to get started.

You can find the slides here

Lunch break and lightning talks

As usual, the food was excellent, and the mountain of cheese is always a nice touch. As usual, the main hall is also too crowded. Everybody is crammed between the food tables and the sponsors booths.


After eating, but before starting with the real speakers, we had the lightning talks session. There was much more of them than in dotCSS, which is nice.

Johannes Fiala showed us swagger-codegen which is a tool to generate a SDK for any API that is exposed through Swagger.

Vincent Voyer, my colleague, shared with us the best way to publish ES6 modules today. Any module should be loadable either from a require or a script, and be easily pushed to npm. The browser support is too low to directly push stuff in ES6, and the js dev environment is too fragmented to safely push a browserify or webpack module.

The trick is to push to npm and the CDNs an ES5-compatible version, the one that is built by Babel for example, while still maintaining an ES6 code base for developers.

After than, Etienne Margraff, did a demo of Vorlon.js, a local webserver to debug any application, on any device by redirecting all debug messages to the webserver UI.

Then Maxime Salnikov tried to explain how Angular 2 was working. I say try because I did not get it. Maybe it comes from my aversion to Angular and the strong Russian accent.

Finally, Nicolas GreniƩ did a commercial presentation of Amazon Lambda. This was not supposed to be commercial I guess, just a way to explain how serverless microservices are working and why it's a good thing, but in reality as it was only talking about Amazon lambda this felt weird. Oh and the serverless part only means that the server is on somebody else infrastructure. Nevertheless, I was convinced by the power of the approach and would like to tinker with it.

Nicolas Bevaqua

After that, we got back to the main speakers. And this is also when it started to get really hot in the theater. This only got worse the more we advanced into the evening, but man it was hot. And as I'm complaining I might add that the available space to put my leg was too small, even on the first floor and I didn't have much space to put my elbows either which made it quite hard (and sometimes painful) to take notes.


Anyway, back to real business. Nicolas might better be known under the ponyfoo alias. You might know him for his extensive and very useful serie of blog posts about ES6.

Actually if that's how you know him, you pretty much already know everything he was saying in his talk. Basically he went over all the goodness that makes writing ES6 code so much enjoyable than ES5: arrow functions, spread/destructuring, default function values, rest parameters, string interpolation, let, const and other shiny new stuff.

I won't detail it here, but I strongly invite you to read on the subject, npm install --save-dev babel and start using it right away.

Andre Medeiros

The next one totally deserves the award of best talk of the day. Andre had some really simple, clear and beautiful slides, he told us a nice programming story in small increments and managed to convey complex concepts to the audience.

I was hooked really quickly and it's only at the end of the talk that he told us that he just taught us what reactive programming is, how it works, what problem it solves. Thanks a lot for that, this is the talk I enjoyed the most, and one I will remember for a long time. I even stopped taking notes at the end to keep my focus on the talk.

He walked us through the story of two linked variables, and how changing one would affect the other, and the window of inconsistency this could create. He then changed our point of view and represented those changes on a timeline where we do not need to know the value of each var at any given time, but only the event that lead to some changes in the value. He compared it to the difference between our age and our birthday. We do not need to know how old we are for any second of our live. We just need to know our birthday.

I could try to put into words what I learned from his talk, but this wouldn't do it justice. Instead, I encourage you to wait for the video of his talk, book 18 minutes in your busy schedule and watch it. It's worth it.

All of the concepts he talked about, and much much more, are implemented in RxJS.

Did I told you how great that talk was?

Eric Shoftall

After another break, we continued with Eric Shoftall. The theater was getting hotter and hotter and it was getting really uncomfortable and harder to concentrate.

Still, Eric talk was interesting so it made it easier. Eric is a fun guy (who ran for mayor of SF and did lobbying using mechanical turks), created gulp, and now tries to make WebRTC work everywhere.

WebRTC is still a cutting-edge technology. It is only really well supported by Chrome, and is hard to implement. There are a large number of quirks to know. The basic dance to get two nodes to connect is a bit long (but can be abstract in a few lines through the Peer module).

But where things are really getting complicated, it's when you need to make it work in Android, iOS and IE. But you need to, because as Eric said:

If you build an app that works only in Chrome, it's not an app, it's a demo.

Recent Android phone ships with Chrome, so it works well. But old version have a crappy browser installed by default. Fortunately, if you package your app in a Cordova bundle, using Crosswalk, you can force the webview to use the latest Chrome.

For iOS, we enter the hackish zone. There is something called iosrtc, which is a WebRTC polyfill written in Swift. It re-implement the methods and protocol in another thread, which makes it integrate with the webview quite challenging. For example, to play a video using it, you have to manually display the video from the other thread with an absolute positioning on top of the webview.

This reminds me of the old hacks to do file uploads in HTML by overlaying a transparent SWF file above a file upload to allow uploading several files at once. This was so hacky, on so many levels...

For IE, there is a proprietary IE plugin called Temasys that users need to manually install before being able to use WebRTC. And even once installed, you have to deal with the same positioning hacks than for iOS.

In the end, Eric created WebRTC everywhere, that packs all the solutions to the common issues he found into one neat bundle. I liked his talk because it is always interesting to see the creative ways people find to fix complex issues.

Forbes Lindesay

On the next talk, Forbes, creator of the Jade HTML preprocessor walked through the various parts that compose a transpiler (compiler? post-processor? I don't know).


We're used to using this kind of tools today. CoffeeScript, Babel, SCSS, postCSS and Jade (that actually had to be renamed to Pug because of legal issues...).

All of this tools (as we've already seen in the postCSS talk at dotCSS) are made up of three parts. First a Lexer to parse the string code into a list of tokens. Then a Parser that convert those tokens into a structured tree. And finally a code generator that will output it back as a string.

I already had a general idea of how lexers and parsers were working, and I guess people that did a computer school had to build one at some point. But it was nice to not assume that everybody knows everything, and re-explain this kind of stuff.

It might have been a bit too long sometimes, and could have been made shorter because some parts really seemed obvious. Anyway, as he said at the end of the talk, now the audience knows how this works and can contribute to Pug :)

Actually, Pug seems to be to HTML what postCSS is to CSS. You can build your own plugin to add to the Pug pipeline and maybe define your own HTML custom tags or attributes to transpile (compile? post-process? damn.) them into something else.

I still do not know how to build my own lexer/parser after this talk, but I liked the idea of making Pug modular and more high level. This also does a great echo to all the good things that have been said on postCSS.

Tim Caswell

Tim (aka @creationix and his son Jack then did a live-coding session involving colored LEDs, arduino and, maybe, JavaScript, but I'm not sure. By that time, I had moved to the ground level where the air was fresher.


I must say that I did not get what the message of this talk was. Tim explained that there are no programmers in his town, and he wanted to make coding easy and teach it around him, especially to kids. This is all well and a very nice idea... But then I failed to really understand the rest of the talk :/

Tim's son, Jack, managed to live-code something in a weird programming language to make some leds blink and a robot move. The language itself looked so low level to me that the performance seemed more to be how the kid managed to remember all the magic numbers to use. Really, having to input stuff like istart 0x70 or write 6 is absolutely not the idea of programming I would like to show to people that don't know anything about it.

Henrik Joreteg

So, after this small WTF talk, we're back to one of the best talks of the day, done by Henrik Joreteg.

He started his talk with something that makes a lot of sense. When you use a mobile phone, you're actually using a physical object, and when you use its touch screen you want it to react immediately. That's how we're used to have physical objects to react; immediately.

But the web of yesterday has been thought for the desktop, with desktop hardware and speed. The web of today is, in the best of cases, mobile-first. Still, this is not enough because the web of tomorrow will be mobile-only. There are more and more users everyday that uses smartphones only and have ditched their desktop browser.

Still, we, the web developer community, build our websites on a desktop machine. And we test our code on the desktop as well, only testing on mobile emulators later in the cycle and on real devices really at the end of the chain while we should actually do the opposite. We should all have physical phones near to our work station and take them in our hand when we code for it. This will make us really feel what we are coding for.

He also quoted a tweet saying:

If you want to write fast code, use a slow computer

Which is absolutely true. I think we're guilty of thinking along the lines of "oh yes it is a bit slow on mobile, but mobiles are getting more powerful every 6 months so this should be ok soon". But that's not true. Not everybody has the latest iPhone, nor a fast bandwidth, but they still want a nice experience.

Google set some rules to their products, based on the number 6. I did not manage to write them all down, but it was something like:

  • max 60KB of HTML
  • max 60KB of CSS
  • max 60KB of JavaScript
  • 60fps
  • max .6s to load the page

They managed to achieve it, and I think they are sensible values that we could all try to reach. Forcing us to work under a budget will help us make things better. It might be hard, but not impossible.

He then continued by giving some advices on what we should do, and most of all what we should stop doing.

First of all, we should get back to the server-side rendering that we should never have stopped doing. Browser are really fast at parsing HTML, much more than they are at parsing and executing js then building the DOM. There is no need to go all isomorphic/universal/whatever. Just push the HTML you know is going to be needed to the page. There's even a webpack config that can do that for you.

Second point is to really think if you need the whole framework you're using or even if you need a framework at all. Do we need a templating system when we have JSX? No need to parse and modify DOM element when we have HTML-to-JavaScript transforms at build time.

Also, React proved that re-rendering the whole UI whenever the app state changed could actually be really lightweight as long as you use a virtual DOM. If you strip everything down, in the end, your whole app can be simplified as newVDom = app(state). You have only one state for your whole app, you process it, and it returns a virtual DOM. If you really need a little more structure on how the data flows, you can add Redux which is only 2KB.

React is nice, but the real gold nugget in it is the virtual DOM. You can extract only this part from the React core for only 10% of the total size.

The last trick is to use the main browser thread for the UI rendering (vdom to DOM) and make all the heavier computation asynchronously in WebWorkers on the side. The UI only pass actions to the WebWorkers that yield the new vdom back to the UI thread when they are done.

You can see this in action on a website named, which apparently works also really well on old mobile devices.

I liked that talk as well, because he did not hesitate to challenge what we take as granted. It's healthy once in a while to cleanup what we think we now about our tools, remove the dust and change the broken pieces. React introduced some really clever ideas, and even if the framework as a whole works nice, you can still cherry-pick parts of it to go with the bare minimum.

Brendan Eich

And finally the last talk of the day was done by Brendan Eich, and was really really weird. I had to ask people afterwards to explain me what it was about.

What was missing for this talk was a previously, on Brendan Eich. I felt like I had to catch up with a story without knowing the context. He talked about TC39 and asm.js, without explaining what it is. In no specific order he also talked about how FirefoxOS and Tizen, that were huge hopes of game changers failed in the last years. He explained that it was due to the fact that those platform did not have any apps, and that people today wants app. But app developers doesn't want to code apps for platforms that have very few users, which creates a vicious circle.

He then continue saying that if you build something for the web, you cannot do it in isolation, it has to fit in the existing landscape. He went on joking about Douglas Crockford and its minimalist views.

Then, he started killing zombies with chicken equipped with machine guns. And that crashed. So he started killing pigs with an axe.

To be honest, this talk made absolutely no sense to me, I am completely unable to synthesize the point of the talk.


I think I'm gonna say the same thing as last year. dotJS is for me less interesting than dotCSS. Proportionally, there was way much less inspirational talks, the sound wasn't great, the place is less comfortable and was getting hotter and hotter along the day. On a very personal note I also realized that my eyes are getting worse, and even with my glasses on, the slides were a bit blurry for me when I sat in the back.


Last year I said "maybe I won't come next year", but I still came. This time I'm saying it again, removing the maybe. This is just no longer adapted to what I'm looking for in a conference. I'll still come to dotCSS and dotScale, though.

Tags : #dotJS

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