JavaScript Articles

Sort by:


  1. Creating a Multiplayer Game with TogetherJS and CreateJS

    Bubble Hell Duel is a multiplayer HTML5 dogfighting game. The object of the game is to dodge bubbles launched from your opponent while returning fire. This game was written mainly as a prototype for learning and the source code is available on GitHub. You can try the game out in single or multiplayer here. Currently the game does not contain any sound effects but uses CreateJS and TogetherJS.


    In this post I would like to share some of my experiences when developing the game. Please share your thoughts in the comments if you agree or have other suggestions.

    Game Engines

    When developing a 2d game you can write you own engine or make use of some fantastic libraries that are available. After spending a few days looking at the various options available I decided to use CreateJS. As I have some experience with Flash, CreateJS made sense for my needs as there was not much of a learning curve. I also wanted to make use of some Flash animations and CreateJS supported this feature. I will elaborate a bit more on animations later in the article.

    As I am a C++ developer I believe emscripten is also a good choice. It allows C/C++ code to be compiled to JavaScript, which can be executed in the browser. I am of the opinion that the static type checking and compile-time optimizations are great assets when developing large code bases. I have used emscripten before and it works very well, but for this project I wanted the fast and convenient prototyping capabilities of JavaScript. I also wanted to expand my JavaScript knowledge.

    I’d like to mention a few other libraries that seem very interesting: Cocos2d-x is making an emscripten port and they already support HTML5 binding. I also like pixi.js as it provides a webGL renderer but also supports Canvas fallback when the browser does not support webGL.

    C++ vs JavaScript

    At first I was a little bit worried about the performance of JavaScript, and that was the reason my decision between using CreateJS or emscripten was difficult. Fortunately a simple benchmark showed that a naive collision detection algorithm with about 400 balls on screen could still reach 40+ fps, which was enough for my simple experiment.

    As someone who has coded more in C++ than JavaScript I loved how quickly I could translate my thoughts into code and test them out on multiple browsers. On the other hand it was not very comfortable debugging my JavaScript. C++ compilers are quite good at pointing out misspellings and other mistakes that cause runtime issues. While the “use strict” directive and other mechanisms like closure compilers have their purpose they were not very helpful to me especially when variables became undefined. Rooting for the cause of errors can be somewhat difficult comparatively.

    As an example of difficult debugging, I encountered the following issue. I was using float numbers for coordinates and other geometric values like angles. These values were passed to the other player using the TogetherJS.send method for synchronization:

    var player = { x: 10.0, y: 10.0 };
    TogetherJS.hub.on('sync', function(msg){
        enemy.x = msg.x;
        enemy.y = msg.y;

    This worked, but lots of decimals were sent in this way, so I decided to relax the accuracy:

    TogetherJS.send({type:'sync', x:Math.round(player.x), y:Math.round(player.y) });

    Then I thought integers might not be accurate enough for collision detection, so I added more digits to the messages:

    TogetherJS.send({type:'sync', x:player.x.toFixed(2), y:player.y.toFixed(2) });

    While this seemed a reasonable solution, it actually induced a bug that was very hard to find and I did not notice it until I tested the game after implementing some more features. I noticed while playing the game the opponent would never move.

    It took me hours in debugging before I could locate the cause. I do not think I would have made this mistake using C++.

    If you would like to see this bug in action take a look at this jsFiddle project. Look at the three canvas tag outputs and you will notice the third canvas contains the bug. This issue occurs because toFixed returns a string representation.

    I am not sure using a closure compiler would have avoided this issue, but I did find in another project that it definitely helps with optimizations.

    Animation with Flash

    As with most games I wanted to use a good deal of animation. I was very familiar with creating animations in Flash and found that CreateJS supported several ways of consuming the Flash animations and presenting them in HTML5. CreateJS is a set of libraries and tools used to create interactive HTML5 content. So by using CreateJS I could consume my animations as well as use the other libraries available for loop handling, resource management and in the future, sound manipulation. For a quick introduction to CreateJS take a look at this video.

    CreateJS, which Mozilla now sponsors, offers great support for Flash animations.

    There are two ways of using Flash animations in HTML5 with CreateJS. The first option is to directly export the Flash animation in a way that you can access all the elements in their original form, including paths, transformations and tweens. The advantage to this approach is that it produces smaller files, and CreateJS allows you to transfer them into a sprite sheet on the client side, for faster rendering. Adobe Flash CS6 offers the CreateJS Toolkit plugin that allows the designer to export all the content of an animation to HTML5 files. This generally results in a JavaScript file with all the graphics and tweens, an HTML file, and a set of image files. You can open up the HTML document in your browser and see the animation.

    Another option is to export the animation into a sprite sheet, that is an image containing all the frames with a JavaScript file describing the position and size of each frame. These files can be easily integrated into HTML based games or applications via the SpriteSheet class in CreateJS. This is the approach I used for this game. To see the code where I use the SpriteSheet have a look at this link. If you want some more detail on this approach take a look at this video.

    I should also note that you can use a tool called Zoë to export directly to a sprite sheet or a JSON file from a Flash Animation as well.


    The above image is an example of a sprite sheet that I use in the game and was generated as described above. The original image came from the game Touhou Hisouten ~ Scarlet Weather Rhapsody, which is availabe at

    Multiplayer with TogetherJS

    On my first iteration of the code the game was not multiplayer. Originally it was a single-player bullet hell game, with a boss foe randomly moving across the screen. I could not last more than 30 seconds before succumbing to withering fire. It was interesting enough that I thought multiplayer would be exciting.

    I had heard of Together.js not long after it was released. The jsFiddle project is powered by Together.js and offers an impressive collaboration mode. This led me to using Together.js in my game. It is also very nice that Mozilla offers a default hub server simplifying the process of creating a multiplayer web based game. To learn more about Together.js be sure to check out this article.

    It was easy and comfortable integrating Together.js into my game, as it works like other event dispatcher/listeners frameworks.

    With Together.js, I was able to implement random match and invitation only multiplayer modes in the game. I did face a few design challenges that I had to overcome when designing the communication protocol.

    First off, I did not put code in to prevent cheating with two-party communications and assumed a certain level of trust between players. In the game design currently all collision detection of a player is done locally. Theoretically if you block corresponding messages you can mask that you have taken damage.

    Another area that I hacked a bit is that the bubbles of the enemy avatar are generated locally and randomly. This means that the bubbles seen from your character avatar are not necessarily the same as your opponent is seeing.

    In practice neither of these shortcuts should ruin the fun of the game.
    I did encounter a couple of issues or caveats with Together.JS.

    • I did not find a way to disable the cursor updating in Together.js. While this is useful in collaborative tools I did not need it in my game.
    • I am using Together.js in an asymmetric way, where both players see themselves as the red skirted Avatar (Reimu). This allows for easier placement of the player at the bottom of the screen and the opponent at the top. This also means that when you move the main player from an opponent’s view of the game your move is seen as the opponents move and vice versa.

    The Fun of Making Mistakes

    There are two visual effects in the game that came as unexpected surprises:

    • When a round finishes and the message ‘You Win’ or ‘You Lose’ appears, the time is frozen for a few seconds. This acts like a dramatic pause.
    • When a charge attack is released, the bullets are fixed and then gradually blown away toward the enemy.

    Neither of these effects was designed in this way. I didn’t want the pause and I wanted the bullets to continue rotating around the player upon releasing. However I made mistakes, and the result seemed to be much better than I had planned, so they made the final cut.

    Conclusion and Future Plans

    It is always fun learning new things. I like the fact that I could prototype and visualize pretty quickly. In the future I might add more patterns for the bullet curtains, and a few sound effects. In addition I will probably also draw more background images or possibly animate them.

    While developing the game I did realize in order to get a natural and intuitive feel required more effort than I expected. This is something I have always taken for granted while playing game.

    The code is open source, so feel free to fork and play. Be sure to comment if you have any suggestions for improving the game or the existing code.

  2. How fast is PDF.js?

    Hi, my name is Thorben and I work at Opera Software in Oslo, not at Mozilla. So, how did I end up writing for Mozilla Hacks? Maybe you know that there is no default PDF viewer in the Opera Browser, something we would like to change. But how to include one? Buy it from Adobe or Foxit? Start our own?

    Introducing PDF.js

    While investigating our options we quickly stumbled upon PDF.js. The project aims to create a full-featured PDF viewer in the browser using JavaScript and Canvas. Yeah, it sounds a bit crazy, but it makes sense: browsers need to be good at processing text, images, fonts, and vector graphics — exactly the things a PDF viewer has to be good at. The draw commands in PDFs are a subset of Postscript, and they are not so different from what Canvas offers. Also security is virtually no issue: using PDF.js is as secure as opening any other website.

    Working on PDF.js

    So Christian Krebs, Mathieu Henri and myself began looking at PDF.js in more detail and were impressed: it’s well designed, seems fast and big parts of the code are just wow!

    But we also discovered some problems, mainly with performance on very large or graphics-heavy PDFs. We decided that the best way to get to know PDF.js better and to push the project further, was to help the project and address the major issues we found. This gave us a pretty good understanding of the project and its high potential. We were also very impressed by how much the performance of PDF.js improved while we worked on it. This is an active and well managed project.

    Benchmarking PDF.js

    Of course, our tests gave us the wrong impression about performance. We tried to find super large, awkward and hard-to-render PDFs, but that is not what most people want to view. Most PDFs you actually want to view in PDF.js are fine. But how to test that?

    Well, you could check the most popular PDFs on the Internet – as these are the ones you probably want to view – and benchmark them. A snapshot of 5 to 10k PDFs should be enough … but how do you get them?

    I figured that search engines would be my friend. If you tell them to search for PDFs only, they give you the most relevant PDFs for that keyword, which in turn are probably the most popular ones. And if you use the most searched keywords you end up with a good approximation.

    Benchmarking that many PDFs is a big task. So I got myself a small cluster of old computers and built a nice server application that supplied them with tasks. The current repository has almost 7000 PDFs and benchmarking one version of PDF.js takes around eight hours.

    The results

    Let’s skip to the interesting part with the pretty pictures. This graph


    gives us almost all the interesting results at one look. You see a histogram of the time it took to process all the pages in the PDFs in relation to the average time it takes to process the average page of the Tracemonkey Paper (the default PDF you see when opening PDF.js). The User Experience when viewing the Tracemonkey Paper is good and from my tests even 3 to 4 times slower is still okay. That means from all benchmarked pages over 96% (exclude pdfs that crashed) will translate to a good user experience. That is really good news! Or to use a very simple pie chart (in % of pages):


    You probably already noticed the small catch: around 0.8% of the PDFs crashed PDF.js when we tested them. We had a closer look at most of them and at least a third are actually so heavily damaged that probably no PDF viewer could ever display them.

    And this leads us to another good point: we have to keep in mind that these results just stand here without comparison. There are some PDFs on the Internet that are so complex that there is no hope that even native PDF viewers could display them nice and fast. The slowest tested PDF is an incredibly detailed vector map of the public transport system of Lisbon. Try to open it in Adobe Reader, it’s not fun!


    From these results we concluded that PDF.js is a very valid candidate to be used as the default PDF viewer in the Opera Browser. There is still a lot of work to do to integrate PDF.js nicely into it, but we are working right now on integrating it behind an experimental flag (BTW: There is an extension that adds PDF.js with the default Mozilla viewer. The “nice” integration I am talking about would be deeper and include a brand new viewer). Thanks Mozilla! We are looking forward to working on PDF.js together with you guys!

    PS: Both the code of the computational system and the results are publicly available. Have a look and tell us if you find them useful!

    PPS: If anybody works at a big search engine company and could give me a list with the actual 10k most used PDFs, that would be awesome :)

    Appendix: What’s next?

    The corpus and the computational framework I described, could be used to do all kinds of interesting things. In the next step, we hope to classify PDFs by used fonts formats, image formats and the like. So you can quickly get PDFs to test a new feature with. We also want to look at which drawing instructions are used with which frequency in the Postscript so we can better optimise for the very common ones, like we did with HTML in browsers. Let’s see what we can actually do ;)

  3. asm.js performance improvements in the latest version of Firefox make games fly!

    The latest version of Firefox which launched last week includes a major update to the user interface as well as to features like Sync. Another area in which this release brings significant improvements is in asm.js performance, which as we will see below is very important for things like games. To put that aspect of Firefox’s performance in context, we’ll take a look at benchmark results comparing Firefox to other browsers, which show that Firefox is faster at executing asm.js code.

    asm.js speedups

    asm.js is a subset of JavaScript that is very easy to optimize and is particularly useful for porting code in C or C++ to the Web. We’ve blogged about how Firefox can optimize asm.js code using 32-bit floating point operations, which, together with all the other work on optimizing asm.js, allows it to run at around 1.5x slower than the speed of the same C/C++ when compiled natively. So, while not quite native speed yet, things are getting very close. At the time of that blog post those optimizations were only on nightly builds, but they are now reaching hundreds of millions of Firefox users in Firefox 29, which is now the release version of Firefox.

    Another important set of asm.js optimizations concern startup speed. As blogged about by Luke a few months ago, Firefox performs ahead of time (AOT) compilation and can cache the results, for significant speedups in startup times. Those optimizations also shipped to users in Firefox 29.

    Web browser comparisons

    Now that all those optimizations have shipped, it’s interesting to look at up-to-date browser comparisons on asm.js code. The above graph shows the Emscripten benchmark suite running the latest stable versions of Google Chrome, Internet Explorer and Firefox on Windows 8.1. Lower numbers are better in all the results here, which are real-world codebases compiled to asm.js (see notes in the graph).

    Unity, Emscripten and asm.js

    asm.js is a subset of JavaScript, so it is just one of many styles of JavaScript out there. But it represents an important use case. As we announced at GDC, Unity, one of the most popular game creation tools on the market, will support the Web by using Emscripten to compile their engine to asm.js.

    But videos are no substitute for the real thing! You can try the games shown there in your browser right now, with Unity’s recently released Dead Trigger 2 and Angry Bots demos. If you run those in the latest version of Firefox, you’ll see many of the asm.js optimizations mentioned earlier in action. For example, if you visit one of those links more than once then asm.js caching will allow it to avoid recompiling the game (so it starts up faster), and also gameplay will be smoother due to faster asm.js execution.

    Being able to execute asm.js-style code efficiently makes it possible for games like this to run well on the Web, without proprietary, nonstandard plugins. That’s why it’s exciting to see more asm.js optimizations reach Firefox users in Firefox 29. And while benchmark results can sometimes seem like nothing more than abstract numbers, speedups on asm.js benchmarks directly improve things like games, where performance is extremely important and very noticeable.

    (Thanks to Marc Schifer for helping with the benchmark measurements.)

  4. Coordinate Conversion Made Easy – the power of GeometryUtils

    In a previous post we introduced the GeometryUtils interface and the getBoxQuads() API for retrieving the CSS box geometry of a DOM node. GeometryUtils also takes care of another important problem: converting coordinates reliably from one DOM node to another. For example, you might want to find the bounding-box of one element relative to another element, or you might want to convert event coordinates from the viewport to some arbitrary element.

    Existing APIs

    Until now, simple cases could be handled using getBoundingClientRect() and some math, but complex cases (e.g. involving CSS transforms) were almost impossible to handle using standard APIs. The nonstandard APIs webkitConvertPointToPage and webkitConvertPageToPoint are a big improvement, but apart from not being standardized, they’re not as powerful as they need to be. In particular it’s more convenient and more robust to provide an API for directly converting coordinates from one element to another.[1]

    New APIs

    GeometryUtils introduces three new methods for coordinate conversion:

    • to.convertPointFromNode(point, from) converts a a point relative to the top-left of the first border-box of “from” to a point relative to the top-left of the first border-box of “to”. The point is a DOMPointInit, which means you can pass a DOMPoint or a JS object such as {x:0, y:0}.
    • to.convertRectFromNode(rect, from) converts a a DOMRect relative to the top-left of the first border-box of “from” to a DOMQuad relative to the top-left of the first border-box of “to” by converting the vertices of the DOMRect. It converts to a DOMQuad to ensure that the result is accurate even if it needs to be rotated or skewed by CSS transforms.
    • to.convertQuadFromNode(quad, from) converts a DOMQuad from “from” to “to”. It’s just like convertRectFromNode except for taking a DOMQuad.

    As with getBoxQuads, a node can be an Element, TextNode or Document; when a Document is used, the coordinates are relative to the document’s viewport.


    <div id="d" style="position:absolute; transform:rotate(45deg); left:100px; top:100px; width:100px; height:100px;"></div>
    <div id="e" style="position:absolute; left:100px; top:100px; width:100px; height:100px;"></div>
    var p1 = document.convertPointFromNode({
        x:0, y:0
      }, document.getElementById("e")
    // p1.x == 100, p1.y == 100
    var p2 = document.convertPointFromNode({
        x:0, y:0
      }, document.getElementById("d")
    // p2.x == 150, p2.y == 150 - 50*sqrt(2) (approx)
    p2 = document.getElementById("e").convertPointFromNode({
        x:0, y:0
      }, document.getElementById("d")
    // p2.x == 50, p2.y == 50 - 50*sqrt(2) (approx)
    var q1 = document.convertRectFromNode(
      new DOMRect(0, 0, 50, 50),
    // q1.p1.x == 100, q1.p1.y == 100
    // q1.p2.x == 150, q1.p2.y == 100
    // q1.p3.x == 150, q1.p3.y == 150
    // q1.p4.x == 100, q1.p4.y == 150
    var q2 = document.convertQuadFromNode(
      new DOMQuad({
        x:60, y:50
      }, {
        x:90, y:50
      }, {
        x:100, y:100
      }, {
        x:50, y:100
    // q2.p1.x == 100, q2.p1.y == 100
    // q2.p2.x == 150, q2.p2.y == 100
    // q2.p3.x == 140, q2.p3.y == 150
    // q2.p4.x == 110, q2.p4.y == 150

    Sometimes it’s useful to convert to or from an element’s CSS content-box, padding-box or margin-box. This is supported via an optional ConvertCoordinateOptions dictionary with the following options:

    • fromBox: one of "content", "padding", "border" or "margin", selecting which CSS box of the first fragment of the from node the input point(s) are relative to.
    • toBox: selects which CSS box of the first fragment of the to node the returned point(s) are relative to.

    As a special case, this makes it easy to convert points between different
    CSS box types of the same element. For example, to convert a point from an
    element’s border-box to be relative to its content-box, use
    element.convertPointFromNode(point, element, {toBox:"content"}).


    <div id="e" style="position:absolute; padding:20px; left:100px; top:100px; width:60px; height:60px;"></div>
    var p1 = document.convertPointFromNode({
        x:0, y:0
      }, document.getElementById("e"),
    // p1.x == 120, p1.y == 120
    p1 = document.getElementById("e").convertPointFromNode({
        x:120, y:120
      }, document,
    // p1.x == 0, p1.y == 0
    p1 = document.getElementById("e").convertPointFromNode({
        x:0, y:0
      }, document.getElementById("e"),
    // p1.x == 20, p1.y == 20
    p1 = document.getElementById("e").convertPointFromNode({
        x:20, y:20
      }, document.getElementById("e"),
    // p1.x == 0, p1.y == 0
    e content-box
    e border-box

    These APIs are available in Firefox nightly builds and should be released in Firefox 31. Firefox is the first browser to implement these APIs.


    [1] Consider the following example:

    <div style="transform:scale(0)">
      <div id="a">...<>
      <div id="b">...<>

    In this case, converting a point relative to a to be relative to b by converting first to page coordinates and then back to b doesn’t work, because the scale(0) maps every point in a to a single point in the page.

  5. Rormix – Discover Emerging Music Videos with Firefox OS

    Rormix is a platform for discovering emerging music videos. Music videos are tagged by genre and similar commercial artists, making it easy to discover new music videos.

    The Rormix app was made using PhoneGap and released on iOS and Android. Development took just over a month from the first line of code, to the app submissions in the app stores. The Firefox OS port took one developer just one day!

    Listed below are a few things we learnt along the way:

    What screen sizes am I developing for?

    When you develop an open web app you can install it in the actual desktop browser, the Android Firefox browser or Firefox OS devices.

    If you want to support all of them in one app, responsive designs are a must (you can also select just the platform you want to support). The current crop of Firefox OS phones have a resolution of 320×480. They have a pixel density of 1 so no special graphics need to be produced.

    Back Button?

    iOS devices don’t have a back button, Android devices have a hardware back button, so where does Firefox OS stand? It has a software back button that you can optionally hide or show when building the manifest for the app. The back button can be hidden at the bottom of the screen however it can be hard to press.

    I recommend that you build a back button into your app and hide the default one to make the app easier to navigate.

    //jQuery example

    Stateful design

    As a back button has a presence in Firefox OS you need to build a stateful application in order to go back in state when the user presses the back button.
    A simple way to implement this is using one of the various JS frameworks that use fragment identifiers to load different states (e.g. Sammy JS).

    //jQuery example
    //Sammy app
    var app;
        app = Sammy(function() {
            this.get('#/', function() {
                //Load default content
            this.get('#/trending', function() {
                //Get trending content
            this.get('#/fresh', function() {
                //Get fresh content
    //Load the default content on app load'#/');
    //Go to fresh content

    Creating a menu

    The trick with making menus for Firefox OS is to use CSS3 transforms for speed, but also making them simple enough to limit the redraw cycle when the menu comes into play. Firefox OS phones have the same width in reference pixels as all iPhones (at the time of writing), and the same pixel height as iPhones previous to the iPhone 5, so if you have a design that works for iOS then you’re all set.

    Adding some Firefox OS flavour

    There are a set of design guidelines that give you an idea of the colour scheme etc of the Firefox OS platform. They also detail how to make the icon for your app, the fonts used etc.

    Submitting your app

    When you have finished building your app you have a choice of how to submit it. You can package it up in a zip file:

    zip -r *

    You can send this zip to the Marketplace or you can host it yourself.

    The other option is to simply host the code as a web page (rather than zip it), and with a little extra JS prompt the user to download the app to their phone.

    Aside: Using PhoneGap / Cordova and HTML5

    Building web apps allows you to quickly and easily build cross platform apps. Even better, with responsive designs it can all be in one project. Advancing tools and workflows (Sass and Yeoman for example) makes developing apps even easier.

    PhoneGap / Cordova supports Firefox OS from version 3.4 (more information in Building Cordova apps for Firefox OS). The biggest advantage of using PhoneGap is that you only need to support a single codebase for all your apps. We all know some browsers have niggles, and PhoneGap has a built in merge mechanism that allows you to put platform specific code aside from the main code and it will merge them when building the app.

    PhoneGap also has a bunch of libraries for accessing native properties of the phone (native dialogue boxes for example) and this code is the same across all platforms, minimising duplicate code.

    The best thing about PhoneGap is the ability for you to create your own plugins, harnessing the power of mobile devices in a really easy way, effortlessly switching between JS and native mobile code.


  6. Introducing the getBoxQuads API

    Web developers often need to determine where an element has been placed in the page, or more generally, where it is relative to another element. Existing APIs for doing this have significant limitations. The new GeometryUtils interface and its supporting interfaces DOMPoint, DOMRect and DOMQuad provide Web-standard APIs to address these problems. Firefox is the first browser to implement these APIs; they are available in Firefox 31 Nightly builds.

    Current best standardized APIs for retrieving element geometry

    Currently the best standardized DOM APIs for retrieving element geometry are element.getBoundingClientRect() and element.getClientRects(). These return the border-box rectangle(s) for an element relative to the viewport of the containing document. These APIs are supported cross-browser but have several limitations:

    • When complex CSS transforms are present, they return the smallest axis-aligned rectangle enclosing the transformed border-box. This loses information.
    • There is no way to obtain the coordinates of the content-box, padding-box or border-box. In simple cases you can add or subtract computed style values from the results of getBoundingClientRect()/getClientRects() but this is clumsy and difficult to get right. For example, when a <span> breaks into several fragments, its left border is only added to one of the fragments — either the first or the last, depending on the directionality of the text.
    • There is no way to obtain box geometry relative to another element.

    Introducing getBoxQuads()

    The GeometryUtils.getBoxQuads() method, implemented on Document, Element and TextNode, solves these problems. It returns a list of DOMQuads, one for each CSS fragment of the object (normally this list would just have a single


    <div id="d"
      style="position:absolute; left:100px; top:100px; width:100px; height:100px;">
    var quads = document.getElementById("d").getBoxQuads();
    // quads.length == 1
    // quads[0].p1.x == 100
    // quads[0].p1.y == 100
    // quads[0].p3.x == 200
    // quads[0].p3.y == 200

    Using bounds

    A DOMQuad is a collection of four DOMPoints defining the corners of an arbitrary quadrilateral. Returning DOMQuads lets getBoxQuads() return accurate information even when arbitrary 2D or 3D transforms are present. It has a handy bounds attribute returning a DOMRectReadOnly for those cases where you just want an axis-aligned bounding rectangle.

    For example:

    <div id="d"
      style="transform:rotate(45deg); position:absolute; left:100px; top:100px; width:100px; height:100px;">
    var quads = document.getElementById("d").getBoxQuads();
    // quads[0].p1.x == 150
    // quads[0].p1.y == 150 - 50*sqrt(2) (approx)
    // quads[0].p3.x == 150
    // quads[0].p3.y == 150 + 50*sqrt(2) (approx)
    // quads[0].bounds.width == 100*sqrt(2) (approx)

    Passing in options

    By default getBoxQuads() returns border-boxes relative to the node’s document viewport, but this can be customized by passing in an optional
    options dictionary with the following (optional) members:

    • box: one of "content", "padding", "border" or "margin", selecting which CSS box type to return.
    • relativeTo: a Document, Element or TextNode; getBoxQuads() returns coordinates relative to the top-left of the border-box of that node (the border-box of the first fragment, if there’s more than one fragment). For documents, the origin of the document’s viewport is used.


    <div id="d"
      style="position:absolute; left:100px; top:100px; width:150px; height:150px;">
        <div id="e"
          style="position:absolute; padding:20px; left:0; top:0; width:100px; height:100px;">
    var quads = document.getElementById("e").getBoxQuads({
    // quads[0].p1.x == 0
    // quads[0].p1.y == 0
    quads = document.getElementById("e").getBoxQuads({
    // quads[0].p1.x == 20
    // quads[0].p1.y == 20
    e content-box
    e border-box

    The relativeTo node need not be an ancestor of the node receiving getBoxQuads(). The nodes can even be in different documents, although they must be in the same toplevel browsing context (i.e. browser tab).

    Scratching the surface

    If you’ve read this far, you’re probably observant enough to have noticed additional methods in GeometryUtils — methods for coordinate conversion. These will be covered in a future blog post.

  7. Introducing the Canvas Debugger in Firefox Developer Tools

    The Canvas Debugger is a new tool we’ll be demoing at the Game Developers Conference in San Francisco. It’s a tool for debugging animation frames rendered on a Canvas element. Whether you’re creating a visualization, animation or debugging a game, this tool will help you understand and optimize your animation loop. It will let you debug either a WebGL or 2D Canvas context.

    Canvas Debugger Screenshot

    You can debug an animation using a traditional debugger, like our own JavaScript Debugger in Firefox’ Developer Tools. However, this can be difficult as it becomes a manual search for all of the various canvas methods you may wish to step through. The Canvas Debugger is designed to let you view the rendering calls from the perspective of the animation loop itself, giving you a much better overview of what’s happening.

    How it works

    The Canvas Debugger works by creating a snapshot of everything that happens while rendering a frame. It records all canvas context method calls. Each frame snapshot contains a list of context method calls and the associated JavaScript stack. By inspecting this stack, a developer can trace the call back to the higher level function invoked by the app or engine that caused something to be drawn.

    Certain types of Canvas context functions are highlighted to make them easier to spot in the snapshot. Quickly scrolling through the list, a developer can easily spot draw calls or redundant operations.

    Canvas Debugger Call Highlighting Detail

    Each draw call has an associated screenshot arranged in a timeline at the bottom of the screen as a “film-strip” view. You can “scrub” through this film-strip using a slider to quickly locate a draw call associated with a particular bit of rendering. You can also click a thumbnail to be taken directly to the associated draw call in the animation frame snapshot.

    Canvas Debugger Timeline Picture

    The thumbnail film-strip gives you get a quick overview of the drawing process. You can easily see how the scene is composed to get the final rendering.

    Stepping Around

    You might notice a familiar row of buttons in the attached screenshot. They’ve been borrowed from the JavaScript Debugger and provide the developer a means to navigate through the animation snapshot. These buttons may change their icons at final release, but for now, we’ll describe them as they currently look.

    Canvas Debugger Buttons image

    • “Resume” – Jump to the next draw call.
    • “Step Over” – Goes over the current context call.
    • “Step Out” – Jumps out of the animation frame (typically to the next requestAnimationFrame call).
    • “Step In” – Goes to the next non-context call in the JavaScript debugger

    Jumping to the JavaScript debugger by “stepping in” on a snapshot function call, or via a function’s stack, allows you to add a breakpoint and instantly pause if the animation is still running. Much convenience!

    Future Work

    We’re not done. We have some enhancements to make this tool even better.

    • Add the ability to inspect the context’s state at each method call. Highlight the differences in state between calls.
    • Measure Time spent in each draw call. This will readily show expensive canvas operations.
    • Make it easier to know which programs and shaders are currently in use at each draw call, allowing you to jump to the Shader Editor and tinkering with shaders in real time. Better linkage to the Shader Editor in general.
    • Inspecting Hit Regions by either drawing individual regions separately, colored differently by id, or showing the hit region id of a pixel when hovering over the preview panel using the mouse.

    And we’re just getting started. The Canvas Debugger should be landing in Firefox Nightly any day now. Watch this space for news of its landing and more updates.

  8. Flambe Provides Support For Firefox OS

    Flambe is a performant cross-platform open source game engine based on the Haxe programming language. Games are compiled to HTML5 or Flash and can be optimized for desktop or mobile browsers. The HTML5 Renderer uses WebGL, but provides fallback to the Canvas tag and functions nicely even on low-end phones. Flash Rendering uses Stage 3D and native Android and iOS apps are packaged using Adobe AIR.

    Flambe provides many other features, including:

    • simple asset loading
    • scene management
    • touch support
    • complete physics library
    • accelerometer access

    It has been used to create many of the Nickelodeon games available at and To see other game examples, and some of the other well-known brands making use of the engine, have a look at the Flambe Showcase.

    In the last few weeks, the developers of the Flambe engine have been working to add support for Firefox OS. With the 4.0.0 release of Flambe, it is now possible to take Flambe games and package them into publication-ready Firefox OS applications, complete with manifest.

    Firefox Marketplace Games

    To get an idea of what is possible with the Flambe engine on the Firefox OS platform, take a look at two games that were submitted recently to the Firefox Marketplace. The first — The Firefly Game written by Mark Knol — features a firefly that must navigate through a flock of hungry birds. The game’s use of physics, sound and touch are very effective.

    The second game, entitled Shoot’em Down, tests the player’s ability to dodge fire while shooting down as many enemy aircraft as possible. The game was written by Bruno Garcia, who is the main developer of the Flambe engine. The source for this game is available as one of the engine’s demo apps.

    Building a Firefox OS App using Flambe

    Before you can begin writing games using the Flambe engine, you will need to install and setup a few pieces of software:

    1. Haxe. Auto installers are available for OSX, Windows and Linux on the download page.
    2. Node.js for building projects. Version 0.8 or greater is required
    3. A Java runtime.

    Once those prerequisites are met, you can run the following command to install Flambe:

    # Linux and Mac may require sudo
    npm install -g flambe
    flambe update

    This will install Flambe and you can begin writing apps with the engine.

    Create a Project

    To create a new project, run the following command.

    flambe new

    This will create a directory named whatever you supplied for ProjectName. In this directory you will have several files and other directories for configuring and coding your project. By default the new command creates a very simple project that illustrates loading and animating an image.

    A YAML (flambe.yaml) file within the project directory defines several characteristics of the project for build purposes. This file contains tags for developer, name and version of the app, and other project meta-data, such as description. In addition it contains the main class name that will be fired as the entry point to your application. This tag needs to be set to a fully qualified Haxe Class name. I.e., if you use a package name in your Haxe source file, you need to prepend the package name in this tag like this: packagename.Classname. (The default example uses urgame.Main.) You can also set the orientation for your app within the YAML file.

    Of specific note for Firefox OS developers, a section of the YAML file contains a partial manifest.webapp that can be altered. This data is merged into a complete manifest.webapp when the project is built.

    The main project folder also contains a directory for assets (images, sounds, animations, and particle effects files). The icons folder contains the icons that will be used with your app. The src folder contains the Haxe source code for your application.

    Build the Project

    Flambe provides a build method to compile your code to the appropriate output. To build the app run:

    flambe build <output>

    Where output is html, flash, android, ios, or firefox. Optionally you can add the –debug option to the build command, producing output more suitable for debugging. For Firefox OS this will produce non-minified JavaScript files. The build process will add a build directory to your application. Inside of the build directory a firefox directory will be created containing your Firefox OS app.

    Debug the Project

    You can debug your application in the Firefox App Manager. See Using the App Manager for details on installing and debugging using the App Manager. Within the App Manager you can add the built app using the Add Packaged App button and selecting the ProjectName/build/firefox directory. Debugging for other platforms is described in the Flambe documentation.
    The -debug option can provide additional insight for debugging and performance tuning. In addition to being able to step through the generated JavaScript, Flambe creates a source map that allows you to look look through the original Haxe files while debugging.
    To see the original Haxe files in the debugger, select the Debugger options icon in the far right corner of the debugger and choose Show Original Sources.
    Also, when using the -debug option you can use a shortcut key (Ctrl + O) to initiate a view of your app that illustrates overdraw — this measures the number of times a pixel is being drawn in a frame. The brighter the pixel the more times it is being drawn. By reducing the amount of overdraw, you should be able to improve the performance of your game.

    A Bit about Haxe and Flambe

    Haxe is an object-oriented, class-based programing language that can be compiled to many other languages. In Flambe, your source code needs to be written using Haxe-specific syntax. Developers familiar with Java, C++ or JavaScript will find learning the language relatively straightforward. The Haxe website contains a reference guide that nicely documents the language. For editing, there are many options available for working with Haxe. I am using Sublime with the Haxe plugin.

    Flambe offers some additional classes that need to be used when building your app. To get a better understanding of these classes, let’s walk through the simple app that is created when you run the flambe new command. The Main.hx file created in the source directory contains the Haxe source code for the Main Class. It looks like this:

    package urgame;
    import flambe.Entity;
    import flambe.System;
    import flambe.asset.AssetPack;
    import flambe.asset.Manifest;
    import flambe.display.FillSprite;
    import flambe.display.ImageSprite;
    class Main
      private static function main ()
        // Wind up all platform-specific stuff
        // Load up the compiled pack in the assets directory named "bootstrap"
        var manifest = Manifest.fromAssets("bootstrap");
        var loader = System.loadAssetPack(manifest);
      private static function onSuccess (pack :AssetPack)
        // Add a solid color background
        var background = new FillSprite(0x202020, System.stage.width, System.stage.height);
        System.root.addChild(new Entity().add(background));
        // Add a plane that moves along the screen
        var plane = new ImageSprite(pack.getTexture("plane"));
        plane.x._ = 30;
        plane.y.animateTo(200, 6);
        System.root.addChild(new Entity().add(plane));

    Haxe Packages and Classes

    The package keyword provides a way for classes and other Haxe data types to be grouped and addressed by other pieces of code, organized by directory. The import keyword is used to include classes and other Haxe types within the file you are working with. For example, import flambe.asset.Manifest will import the Manifest class, while import flambe.asset.* will import all types defined in the asset package. If you try to use a class that you have not imported into your code and run the build command, you will receive an error message stating that the particular class could not be found. All of the Flambe packages are documented on the Flambe website.

    Flambe Subsystem Setup and Entry point

    The main function is similar to other languages and acts as the entry point into your app. Flambe applications must have one main function and only one per application. In the main function the System.init() function is called to setup all the subsystems that will be needed by your code and the Flambe engine.

    Flambe Asset Management

    Flambe uses a dynamic asset management system that allows images, sound files, etc. to be loaded very simply. In this particular instance the fromAssets function defined in the Manifest class examines the bootstrap folder located in the assets directory to create a manifest of all the available files. The loadAssetPack System function creates an instance of the AssetPack based on this manifest. One of the functions of AssetPack is get, which takes a function parameter to call when the asset pack is loaded into memory. In the default example, the only asset is an image named plane.png.

    Flambe Entities and Components

    Flambe uses an abstract concept of Entities and Components to describe and manipulate game objects. An Entity is essentially just a game object with no defining characteristics. Components are characteristics that are attached to entities. For example an image component may be attached to an entity. Entities are also hierarchal and can be nested. For example, entity A can be created and an image could be attached to it. Entity B could then be created with a different image. Entity A could then be attached to the System root (top level Entity) and Entity B could then be attached to Entity A or the System root. The entity nest order is used for rendering order, which can be used to make sure smaller visible objects are not obscured by other game objects.

    Creating Entities and Components in the Sample App

    The onSuccess function in the default sample is called by the loader instance after the AssetPack is loaded. The function first creates an instance of a FillSprite Component, which is a rectangle defined by the size of the display viewport width and height. This rectangle is colored using the hex value defined in the first parameter. To actually have the FillSprite show up on the screen you first have to create an Entity and add the Component to it. The new Entity().add(background) method first creates the Entity and then adds the FillSprite Component. The entire viewport hierarchy starts at the System.root, so the addChild command adds this new Entity to the root. Note this is the first Entity added and it will be the first rendered. In this example this entity represents a dark background.

    Next the plane image is created. This is done by passing the loaded plane image to the ImageSprite Component constructor. Note that the AssetPack class’s getTexture method is being used to retrieve the loaded plane image. The AssetPack class contains methods for retrieving other types of Assets as well. For example, to retrieve and play a sound you would use pack.getSound("bounce").play();.

    Flambe Animated Data Types

    Flambe wraps many of the default Haxe data types in classes and introduces a few more. One of these is the AnimatedFloat class. This class essentially wraps a float and provides some utility functions that allow the float to be altered in a specific way. For example, one of the functions of the AnimatedFloat class is named animateTo, which takes parameters to specify the final float value and the time in which the animation will occur. Many components within the Flambe system use AnimatedFloats for property values. The plane that is loaded in the default application is an instance of the ImageSprite Component. Its x and y placement values are actually AnimatedFloats. AnimatedFloat values can be set directly but special syntax has to be used (value._).

    In the example, the x value for the ImageSprite is set to 30 using this syntax: plane.x._ = 30;. The y value for the ImageSprite is then animated to 200 over a 6 second period. The x and y values for an ImageSprite represent the upper left corner of the image when placed into the viewport. You can alter this using the centerAnchor function of the ImageSprite class. After this call, the x and y values will be in reference to the center of the image. While the default example does not do this, it could be done by calling plane.centerAnchor();. The final line of code just creates a new Entity, adds the plane Component to the Entity and then adds the new Entity to the root. Note that this is the second Entity added to the root and it will render after the background is rendered.

    Flambe Event Model

    Another area of Flambe that is important to understand is its event model. Flambe uses a signal system where the subsystems, Components and Entities have available signal properties that can be connected to in order to listen for a specific signal event. For example, resizing the screen fires a signal. This event can be hooked up using the following code.

    System.stage.resize.connect(function onResize() {
      //do something

    This is a very nice feature when dealing with other components within apps. For example, to do something when a user either clicks on or touches an ImageSprite within your app you would use the following code:

    //ImageSprite Component has pointerDown signal property
    myBasketBallEntity.get(ImageSprite).pointerDown.connect(function (event) {

    In this case the pointerDown signal is fired when a user either uses a mouse down or touch gesture.

    Demo Apps

    The Flambe repository also contains many demo apps that can be used to further learn the mechanics and APIs for the engine. These demos have been tested on Firefox OS and perform very well. Pictured below are several screenshots taken on a Geeksphone Keon running Firefox OS.

    Of particular note in the demos are the physics and particles demos. The physics demo uses the Nape Haxe library and allows for some very cool environments. The Nape website contains documentation for all the packages available. To use this library you need to run the following command:

    haxelib install nape

    The particle demo illustrates using particle descriptions defined in a PEX file within a Flambe-based game. PEX files can be defined using a particle editor, like Particle Designer.

    Wrapping Up

    If you are a current Flambe game developer with one or more existing games, why not use the new version of the engine to compile and package them for Firefox OS? If you are a Firefox OS developer and are looking for a great way to develop new games for the platform, Flambe offers an excellent means for developing engaging, performant games for Firefox OS–and many other platforms besides!

    And, if you are interested in contributing to Flambe, we’d love to hear from you as well.

  9. The Making of the Time Out Firefox OS app

    A rash start into adventure

    So we told our client that yes, of course, we would do their Firefox OS app. We didn’t know much about FFOS at the time. But, hey, we had just completed refactoring their native iOS and Android apps. Web applications were our core business all along. So what was to be feared?

    More than we thought, it turned out. Some of the dragons along the way we fought and defeated ourselves. At times we feared that we wouldn’t be able to rescue the princess in time (i.e. before MWC 2013). But whenever we got really lost in detail forest, the brave knights from Mozilla came to our rescue. In the end, it all turned out well and the team lived happily ever after.

    But here’s the full story:

    Mission & challenge

    Just like their iOS and Android apps, Time Out‘s new Firefox OS app was supposed to allow browsing their rich content on bars, restaurants, things to do and more by category, area, proximity or keyword search, patient zero being Barcelona. We would need to show results as illustrated lists as well as visually on a map and have a decent detail view, complete with ratings, access details, phone button and social tools.

    But most importantly, and in addition to what the native apps did, this app was supposed to do all of that even when offline.

    Oh, and there needed to be a presentable, working prototype in four weeks time.

    Cross-platform reusability of the code as a mobile website or as the base of HTML5 apps on other mobile platforms was clearly prio 2 but still to be kept in mind.

    The princess was clearly in danger. So we arrested everyone on the floor that could possibly be of help and locked them into a room to get the basics sorted out. It quickly emerged that the main architectural challenges were that

    • we had a lot of things to store on the phone, including the app itself, a full street-level map of Barcelona, and Time Out’s information on every venue in town (text, images, position & meta info),
    • at least some of this would need to be loaded from within the app; once initially and synchronizable later,
    • the app would need to remain interactively usable during these potentially lengthy downloads, so they’d need to be asynchronous,
    • whenever the browser location changed, this would be interrupted

    In effect, all the different functionalities would have to live within one single HTML document.

    One document plus hash tags

    For dynamically rendering, changing and moving content around as required in a one-page-does-all scenario, JavaScript alone didn’t seem like a wise choice. We’d been warned that Firefox OS was going to roll out on a mix of devices including the very low cost class, so it was clear that fancy transitions of entire full-screen contents couldn’t be orchestrated through JS loops if they were to happen smoothly.

    On the plus side, there was no need for JS-based presentation mechanics. With Firefox OS not bringing any graveyard of half-dead legacy versions to cater to, we could (finally!) rely on HTML5 and CSS3 alone and without fallbacks. Even beyond FFOS, the quick update cycles in the mobile environment didn’t seem to block the path for taking a pure CSS3 approach further to more platforms later.

    That much being clear, which better place to look for best practice examples than Mozilla Hacks? After some digging, Thomas found Hacking Firefox OS in which Luca Greco describes the use of fragment identifiers (aka hashtags) appended to the URL to switch and transition content via CSS alone, which we happily adopted.

    Another valuable source of ideas was a list of GAIA building blocks on Mozilla’s website, which has since been replaced by the even more useful Building Firefox OS site.

    In effect, we ended up thinking in terms of screens. Each physically a <div>, whose visibility and transitions are governed by :target CSS selectors that draw on the browser location’s hashtag. Luckily, there’s also the onHashChange event that we could additionally listen to in order to handle the app-level aspects of such screen changes in JavaScript.

    Our main HTML and CSS structure hence looked like this:

    And a menu

    We modeled the drawer menu very similarily, just that it sits in a <nav> element on the same level as the <section> container holding all the screens. Its activation and deactivation works by catching the menu icon clicks, then actively changing the screen container’s data-state attribute from JS, which triggers the corresponding CSS3 slide-in / slide-out transition (of the screen container, revealing the menu beneath).

    This served as our “Hello, World!” test for CSS3-based UI performance on low-end devices, plus as a test case for combining presentation-level CSS3 automation with app-level explicit status handling. We took down a “yes” for both.


    By the time we had put together a dummy around these concepts, the first design mockups from Time Out came in so that we could start to implement the front end and think about connecting it to the data sources.

    For presentation, we tried hard to keep the HTML and CSS to the absolute minimum. Mozilla’s GAIA examples being a very valuable source of ideas once more.

    Again, targeting Firefox OS alone allowed us to break free of the backwards compatibility hell that we were still living in, desktop-wise. No one would ask us Will it display well in IE8? or worse things. We could finally use real <section>, <nav>, <header>, and <menu> tags instead of an army of different classes of <div>. What a relief!

    The clear, rectangular, flat and minimalistic design we got from Time Out also did its part to keep the UI HTML simple and clean. After we were done with creating and styling the UI for 15 screens, our HTML had only ~250 lines. We later improved that to 150 while extending the functionality, but that’s a different story.

    Speaking of styling, not everything that had looked good on desktop Firefox even in its responsive design view displayed equally well on actual mobile devices. Some things that we fought with and won:

    Scale: The app looked quite different when viewed on the reference device (a TurkCell branded ZTE device that Mozilla had sent us for testing) and on our brand new Nexus 4s:

    After a lot of experimenting, tearing some hair and looking around how others had addressed graceful, proportional scaling for a consistent look & feel across resolutions, we stumbled upon this magic incantation:

    <meta name="viewport" content="user-scalable=no, initial-scale=1,
    maximum-scale=1, width=device-width" />

    What it does, to quote an article at Opera, is to tell the browser that there is “No scaling needed, thank you very much. Just make the viewport as many pixels wide as the device screen width”. It also prevents accidental scaling while the map is zoomed. There is more information on the topic at MDN.

    Then there are things that necessarily get pixelated when scaled up to high resolutions, such as the API based venue images. Not a lot we could do about that. But we could at least make the icons and logo in the app’s chrome look nice in any resolution by transforming them to SVG.

    Another issue on mobile devices was that users have to touch the content in order to scroll it, so we wanted to prevent the automatic highlighting that comes with that:

    li, a, span, button, div
        -moz-tap-highlight-color: transparent;
        -moz-user-select: none;

    We’ve since been warned that suppressing the default highlighting can be an issue in terms of accessibility, so you might wanted to consider this carefully.

    Connecting to the live data sources

    So now we had the app’s presentational base structure and the UI HTML / CSS in place. It all looked nice with dummy data, but it was still dead.

    Trouble with bringing it to life was that Time Out was in the middle of a big project to replace its legacy API with a modern Graffiti based service and thus had little bandwidth for catering to our project’s specific needs. The new scheme was still prototypical and quickly evolving, so we couldn’t build against it.

    The legacy construct already comprised a proxy that wrapped the raw API into something more suitable for consumption by their iOS and Android apps, but after close examination we found that we better re-re-wrap that on the fly in PHP for a couple of purposes:

    • Adding CORS support to avoid XSS issues, with the API and the app living in different subdomains of,
    • stripping API output down to what the FFOS app really needed, which we could see would reduce bandwidth and increase speed by magnitude,
    • laying the foundation for harvesting of API based data for offline use, which we already knew we’d need to do later

    As an alternative to server-side CORS support, one could also think of using the SystemXHR API. It is a mighty and potentially dangerous tool however. We also wanted to avoid any needless dependency on FFOS-only APIs.

    So while the approach wasn’t exactly future proof, it helped us a lot to get to results quickly, because the endpoints that the app was calling were entirely of our own choice and making, so that we could adapt them as needed without time loss in communication.

    Populating content elements

    For all things dynamic and API-driven, we used the same approach at making it visible in the app:

    • Have a simple, minimalistic, empty, hidden, singleton HTML template,
    • clone that template (N-fold for repeated elements),
    • ID and fill the clone(s) with API based content.
    • For super simple elements, such as <li>s, save the cloning and whip up the HTML on the fly while filling.

    As an example, let’s consider the filters for finding venues. Cuisine is a suitable filter for restaurants, but certainly not for museums. Same is true for filter values. There are vegetarian restaurants in Barcelona, but certainly no vegetarian bars. So the filter names and lists of possible values need to be asked of the API after the venue type is selected.

    In the UI, the collapsible category filter for bars & pubs looks like this:

    The template for one filter is a direct child of the one and only

    <div id="templateContainer">

    which serves as our central template repository for everything cloned and filled at runtime and whose only interesting property is being invisible. Inside it, the template for search filters is:

    <div id="filterBoxTemplate">

    So for each filter that we get for any given category, all we had to do was to clone, label, and then fill this template:

    '#categoryResultScreen .filter-container');
    $("#" +'.filter-button').html(;

    As you certainly guessed, we then had to to call the API once again for each filter in order to learn about its possible values, which were then rendered into <li> elements within the filter‘s <ul> on the fly:

    $("#" + filterId).children('.filter_options').html(
    '<li><span>Loading ...</span></li>');, function (filterOptions)
      $.each(filterOptions, function(key, option)
        var entry = $('<li filterId="' + + '"><span>'
          + + '</span></li>');
        if (selectedOptionId && selectedOptionId == filterOptionId)
        $("#" + filterId).children('.filter_options').append(entry);

    DOM based caching

    To save bandwidth and increase responsiveness in on-line use, we took this simple approach a little further and consciously stored more application level information in the DOM than needed for the current display if that information was likely needed in the next step. This way, we’d have easy and quick local access to it without calling – and waiting for – the API again.

    The technical way we did so was a funny hack. Let’s look at the transition from the search result list to the venue detail view to illustrate:

    As for the filters above, the screen class for the detailView has an init() method that populates the DOM structure based on API input as encapsulated on the application level. The trick now is, while rendering the search result list, to register anonymous click handlers for each of its rows, which – JavaScript passing magic – contain a copy of, rather than a reference to, the venue objects used to render the rows themselves:

    renderItems: function (itemArray)
      $.each(itemArray, function(key, itemData)
        var item = screen.dom.resultRowTemplate.clone().attr('id',
        $('.result-name', item).text(;
        $('.result-type-label', item).text(itemData.section);
        $('.result-type', item).text(itemData.subSection);
    showDetails: function (venue)
      require(['screen/detailView'], function (detailView)

    In effect, there’s a copy of the data for rendering each venue’s detail view stored in the DOM. But neither in hidden elements nor in custom attributes of the node object, but rather conveniently in each of the anonymous pass-by-value-based click event handlers for the result list rows, with the added benefit that they don’t need to be explicitly read again but actively feed themselves into the venue details screen as soon a row receives a touch event.

    And dummy feeds

    Finishing the app before MWC 2013 was pretty much a race against time, both for us and for Time Out’s API folks, who had an entirely different and equally – if not more so – sportive thing to do. Therefore they had very limited time for adding to the (legacy) API that we were building against. For one data feed, this meant that we had to resort to including static JSON files into the app’s manifest and distribution; then use relative, self-referencing URLs as fake API endpoints. The illustrated list of top venues on the app’s main screen was driven this way.

    Not exactly nice, but much better than throwing static content into the HTML! Also, it kept the display code already fit for switching to the dynamic data source that eventually materialized later, and compatible with our offline data caching strategy.

    As the lack of live data on top venues then extended right to their teaser images, we made the latter physically part of the JSON dummy feed. In Base64 :) But even the low-end reference device did a graceful job of handling this huge load of ASCII garbage.

    State preservation

    We had a whopping 5M of local storage to spam, and different plans already (as well as much higher needs) for storing the map and application data for offline use. So what to do with this liberal and easily accessed storage location? We thought we could at least preserve the current application state here, so you’d find the app exactly as you left it when you returned to it.


    A city guide is the very showcase of an app that’s not only geo aware but geo centric. Maps fit for quick rendering and interaction in both online and offline use were naturally a paramount requirement.

    After looking around what was available, we decided to go with Leaflet, a free, easy to integrate, mobile friendly JavaScript library. It proved to be really flexible with respect to both behaviour and map sources.

    With its support for pinching, panning and graceful touch handling plus a clean and easy API, Leaflet made us arrive at a well-usable, decent-looking map with moderate effort and little pain:

    For a different project, we later rendered the OSM vector data for most of Europe into terabytes of PNG tiles in cloud storage using on-demand cloud power. Which we’d recommend as an approach if there’s a good reason not to rely on 3rd party hosted apps, as long as you don’t try this at home; Moving the tiles may well be slower and more costly than their generation.

    But as time was tight before the initial release of this app, we just – legally and cautiously(!) – scraped ready-to use OSM tiles off

    The packaging of the tiles for offline use was rather easy for Barcelona because about 1000 map tiles are sufficient to cover the whole city area up to street level (zoom level 16). So we could add each tile as a single line into the manifest.appache file. The resulting, fully automatic, browser-based download on first use was only 10M.

    This left us with a lot of lines like


    in the manifest and wishing for a $GENERATE clause as for DNS zone files.

    As convenient as it may seem to throw all your offline dependencies’ locations into a single file and just expect them to be available as a consequence, there are significant drawbacks to this approach. The article Application Cache is a Douchebag by Jake Archibald summarizes them and some help is given at Html5Rocks by Eric Bidleman.

    We found at the time that the degree of control over the current download state, and the process of resuming the app cache load in case that the initial time users spent in our app didn’t suffice for that to complete was rather tiresome.

    For Barcelona, we resorted to marking the cache state as dirty in Local Storage and clearing that flag only after we received the updateready event of the window.applicationCache object but in the later generalization to more cities, we moved the map away from the app cache altogether.

    Offline storage

    The first step towards offline-readiness was obviously to know if the device was online or offline, so we’d be able to switch the data source between live and local.

    This sounds easier than it was. Even with cross-platform considerations aside, neither the online state property (window.navigator.onLine), the events fired on the <body> element for state changes (“online” and “offline”, again on the <body>), nor the navigator.connection object that was supposed to have the on/offline state plus bandwidth and more, really turned out reliable enough.

    Standardization is still ongoing around all of the above, and some implementations are labeled as experimental for a good reason :)

    We ultimately ended up writing a NetworkStateService class that uses all of the above as hints, but ultimately and very pragmatically convinces itself with regular HEAD requests to a known live URL that no event went missing and the state is correct.

    That settled, we still needed to make the app work in offline mode. In terms of storage opportunities, we were looking at:

    Storage Capacity Updates Access Typical use
    App / app cache, i.e. everything listed in the file that the value of appcache_path in the app‘s webapp.manifest points to, and which is and therefore downloaded onto the device when the app is installed. <= 50M. On other platforms (e.g. iOS/Safari), user interaction required from 10M+. Recommendation from Moziila was to stay <2M. Hard. Requires user interaction / consent, and only wholesale update of entire app possible. By (relative) path HTML, JS, CSS, static assets such as UI icons
    LocalStorage 5M on UTF8-platforms such as FFOS, 2.5M in UTF16, e.g. on Chrome. Details here Anytime from app By name Key-value storage of app status, user input, or entire data of modest apps
    Device Storage (often SD card) Limited only by hardware Anytime from app (unless mounted as UDB drive when cionnected to desktop computer) By path, through Device Storage API Big things
    FileSystem API Bad idea
    Database Unlimited on FFOS. Mileage on other platforms varies Anytime from app Quick and by arbitrary properties Databases :)

    Some aspects of where to store the data for offline operation were decided upon easily, others not so much:

    • the app, i.e. the HTML, JS, CSS, and UI images would go into the app cache
    • state would be maintained in Local Storage
    • map tiles again in the app cache. Which was a rather dumb decision, as we learned later. Barcelona up to zoom level 16 was 10M, but later cities were different. London was >200M and even reduced to max. zoom 15 still worth 61M. So we moved that to Device Storage and added an actively managed download process for later releases.
    • The venue information, i.e. all the names, locations, images, reviews, details, showtimes etc. of the places that Time Out shows in Barcelona. Seeing that we needed lots of space, efficient and arbitrary access plus dynamic updates, this had to to go into the Database. But how?

    The state of affairs across the different mobile HTML5 platforms was confusing at best, with Firefox OS already supporting IndexedDB, but Safari and Chrome (considering earlier versions up to Android 2.x) still relying on a swamp of similar but different sqlite / WebSQL variations.

    So we cried for help and received it, as always when we had reached out to the Mozilla team. This time in the form of a pointer to pouchDB, a JS-based DB layer that at the same time wraps away the different native DB storage engines behind a CouchDB-like interface and adds super easy on-demand synchronization to a remote CouchDB-hosted master DB out there.

    Back last year it still was in pre-alpha state but very usable already. There were some drawbacks, such as the need for adding a shim for WebSql based platforms. Which in turn meant we couldn’t rely on storage being 8 bit clean, so that we had to base64 our binaries, most of all the venue images. Not exactly pouchDB’s fault, but still blowing up the size.


    The DB platform being chosen, we next had to think how we’d harvest all the venue data from Time Out’s API into the DB. There were a couple of endpoints at our disposal. The most promising for this task was proximity search with no category or other restrictions applied, as we thought it would let us harvest a given city square by square.

    Trouble with distance metrics however being that they produce circles rather than squares. So step 1 of our thinking would miss venues in the corners of our theoretical grid

    while extending the radius to (half the) the grid’s diagonal, would produce redundant hits and necessitate deduplication.

    In the end, we simply searched by proximity to a city center location, paginating through the result indefinitely, so that we could be sure to to encounter every venue, and only once:

    Technically, we built the harvester in PHP as an extension to the CORS-enabled, result-reducing API proxy for live operation that was already in place. It fed the venue information in to the master CouchDB co-hosted there.

    Time left before MWC 2013 getting tight, we didn’t spend much time on a sophisticated data organization and just pushed the venue information into the DB as one table per category, one row per venue, indexed by location.

    This allowed us to support category based and area / proximity based (map and list) browsing. We developed an idea how offline keyword search might be made possible, but it never came to that. So the app simply removes the search icon when it goes offline, and puts it back when it has live connectivity again.

    Overall, the app now

    • supported live operation out of box,
    • checked its synchronization state to the remote master DB on startup,
    • asked, if needed, permission to make the big (initial or update) download,
    • supported all use cases but keyword search when offline.

    The involved components and their interactions are summarized in this diagram:

    Organizing vs. Optimizing the code

    For the development of the app, we maintained the code in a well-structured and extensive source tree, with e.g. each JavaScript class residing in a file of its own. Part of the source tree is shown below:

    This was, however, not ideal for deployment of the app, especially as a hosted Firefox OS app or mobile web site, where download would be the faster, the fewer and smaller files we had.

    Here, Require.js came to our rescue.

    It provides a very elegant way of smart and asynchronous requirement handling (AMD), but more importantly for our purpose, comes with an optimizer that minifies and combines the JS and CSS source into one file each:

    To enable asynchronous dependency management, modules and their requirements must be made known to the AMD API through declarations, essentially of a function that returns the constructor for the class you’re defining.

    Applied to the search result screen of our application, this looks like this:

      // new class being definied
      // its dependencies
      ['screens/abstractResultScreen', 'app/applicationController'],
      // its anonymous constructor
      function (AbstractResultScreen, ApplicationController)
        var SearchResultScreen = $.extend(true, {}, AbstractResultScreen,
          // properties and methods
            resultRowTemplate: $('#searchResultRowTemplate'),
            list: $('#search-result-screen-inner-list'),
        return SearchResultScreen;

    For executing the optimization step in the build & deployment process, we used Rhino, Mozilla’s Java-based JavaScript engine:

    java -classpath ./lib/js.jar:./lib/compiler.jar ./lib/r.js -o /tmp/timeout-webapp/

    CSS bundling and minification is supported, too, and requires just another call with a different config.


    Four weeks had been a very tight timeline to start with, and we had completely underestimated the intricacies of taking HTML5 to a mobile and offline-enabled context, and wrapping up the result as a Marketplace-ready Firefox OS app.

    Debugging capabilities in Firefox OS, especially on the devices themselves, were still at an early stage (compared to clicking about:app-manager today). So the lights in our Cologne office remained lit until pretty late then.

    Having built the app with a clear separation between functionality and presentation also turned out a wise choice when a week before T0 new mock-ups for most of the front end came in :)

    But it was great and exciting fun, we learned a lot in the process, and ended up with some very useful shiny new tools in our box. Often based on pointers from the super helpful team at Mozilla.

    Truth be told, we had started into the project with mixed expectations as to how close to the native app experience we could get. We came back fully convinced and eager for more.

    In the end, we made the deadline and as a fellow hacker you can probably imagine our relief. The app finally even received its 70 seconds of fame, when Jay Sullivan shortly demoed it at Mozilla’s MWC 2013 press conference as a showcase for HTML5′s and Firefox OS’s offline readiness (Time Out piece at 7:50). We were so proud!

    If you want to play with it, you can find the app in the marketplace or go ahead try it online (no offline mode then).

    Since then, the Time Out Firefox OS app has continued to evolve, and we as a team have used the chance to continue to play with and build apps for FFOS. To some degree, the reusable part of this has become a framework in the meantime, but that’s a story for another day..

    We’d like to thank everyone who helped us along the way, especially Taylor Wescoatt, Sophie Lewis and Dave Cook from Time Out, Desigan Chinniah and Harald Kirschner from Mozilla, who were always there when we needed help, and of course Robert Nyman, who patiently coached us through writing this up.

  10. Ember.JS – What it is and why we need to care about it

    This is a guest post by Sourav Lahoti and his thoughts about Ember.js

    Developers increasingly turn to client-side frameworks to simplify development, and there’s a big need for good ones in this area. We see a lot of players in this field, but for lots of functionality and moving parts, very few stand out in particular — Ember.js is one of them.

    So what is Ember.js? Ember.js is a MVC (Model–View–Controller) JavaScript framework which is maintained by the Ember Core Team (including Tom Dale, Yehuda Katz, and others). It helps developers create ambitious single-page web applications that don’t sacrifice what makes the web great: URI semantics, RESTful architecture, and the write-once, run-anywhere trio of HTML, CSS, and JavaScript.

    Why do we need to care

    Ember.js is tightly coupled with the technologies that make up the web today. It doesn’t attempt to abstract that away. Ember.js brings a clean and consistent application development model. If one needs to migrate from HTML to any other technology, Ember.js framework will evolve along with the current trends in web front end technology.

    It makes it very easy to create your own “component” and “template views” that are easy to understand, create and update. Coupled with its consistent way of managing bindings and computed properties, Ember.js does indeed offer much of the boilerplate code that a web framework needs.

    The core concept

    There are some nominal terms that you will find very common when you use ember.js and they form the basics of Ember.js:

    A Route object basically represents the state of the application and corresponds to a url.
    Every route has an associated Model object, containing the data associated with the current state of the application.
    Controllers are used to decorate models with display logic.

    A controller typically inherits from ObjectController if the template is associated with a single model record, or an ArrayController if the template is associated with a list of records.

    Views are used to add sophisticated handling of user events to templates or to add reusable behavior to a template.
    Components are a specialized view for creating custom elements that can be easily reused in templates.

    Hands-on with Ember.js

    Data Binding:

    <script type=”text/x-handlebars”>
        <label>Insert your name:</label>
        {{input type=”text” value=name}}
      <p><strong>Echo: {{name}}</strong></p>
    App = Ember.Application.create();

    Final result when the user interacts with the web app

    Ember.js does support data binding as we can see in the above example. What we type into the input is bound to name, as is the text after Echo: . When you change the text in one place, it automatically updates everywhere.

    But how does this happen? Ember.js uses Handlebars for two-way data binding. Templates written in handlebars get and set data from their controller. Every time we type something in our input, the name property of our controller is updated. Then, automatically, the template is updated because the bound data changed.

    A simple Visiting card demo using Handlebars

    We can create our own elements by using Handlebars.


    <script type="text/x-handlebars">
      {{v-card myname=name street-address=address locality=city zip=zipCode email=email}}
      <h2 class="subheader">Enter Your information:</h2>
      <label>Enter Your Name:</label>
      {{input type="text" value=name}}
      <label>Enter Your Address:</label>
      {{input type="text" value=address}}
      <label>Enter Your City:</label>
      {{input type="text" value=city}}
      <label>Enter Your Zip Code:</label>
      {{input type="text" value=zipCode}}
      <label>Enter Your Email address:</label>
      {{input type="text" value=email}}
    <script type="text/x-handlebars" data-template-name="components/v-card">
      <ul class="vcard">
        <li class="myname">{{myname}}</li>
        <li class="street-address">{{street-address}}</li>
        <li class="locality">{{locality}}</li>
        <li><span class="state">{{usState}}</span>, <span class="zip">{{zip}}</span></li>
        <li class="email">{{email}}</li>


    .vcard {
      border: 1px solid #dcdcdc;
      max-width: 12em;
      padding: 0.5em;
    .vcard li {
      list-style: none;
    .vcard .name {
      font-weight: bold;
    .vcard .email {
      font-family: monospace;
    label {
      display: block;
      margin-top: 0.5em;


    App = Ember.Application.create();
    App.ApplicationController = Ember.Controller.extend({
        name: 'Sourav',
        address: '123 M.G Road.',
        city: 'Kolkata',
        zipCode: '712248',
        email: ''

    The component is defined by opening a new <script type="text/x-handlebars">, and setting its template name using the data-template-name attribute to be components/[NAME].

    We should note that the web components specification requires the name to have a dash in it in order to separate it from existing HTML tags.

    There is much more to it, I have just touched the surface. For more information, feel free to check out the Ember.js Guides.