Sort by:


  1. Implementing Air Mozilla’s Related Events Feature

    Editor’s note: It’s not often that we hear from web developers and software engineers at the start their careers here on the Hacks blog. This post is a great reminder of what it’s like.

    Mozilla participates in Outreachy, and offers internship opportunities to bring women and other under-represented groups into the world of free and open source software. The application process for the upcoming round will open on September 22, 2015. The application deadline is October 26. Internship dates will be December 7, 2015 to March 7, 2016. If you’ve been thinking of applying, read this, be like Gloria – and go for it!

    How I became a Mozilla intern

    It was a spring night in Greece when I got the news I’d been selected by Mozilla to be an Outreachy intern. Outreachy is a project of the Software Freedom Conservancy that helps people from groups underrepresented in FOSS (free and open source software) to get involved by offering focused internship opportunities with a number of open source software organizations. That night, I was the happiest person in my town, I believe. I remember rushing to say a big thank you to my mentor Peter Bengtsson, before I got too overwhelmed with happiness to the point that I forgot to be grateful. It was a two-way street. I chose Mozilla and Mozilla chose me.

    I started contributing to Air Mozilla in March 2015, by solving beginner-friendly bugs. Initially I didn’t want to apply at all, because there were too many wonderful contributors, and in some way I felt overwhelmed by how rigorous the process was. I told several people I won’t apply because I am afraid I won’t be getting it, and sometimes it is okay to be afraid, but it is not okay to be afraid of something you are capable of achieving to the point of sabotaging yourself.

    After several pep talks, I went to view the list of the companies and decided to apply only for Mozilla. It will be a hit or miss, I told myself. I noticed it listed “Python” and I was aware that I didn’t know Python that well as it was something I self-studied briefly in the past, but I told myself to go ahead and try. Sometimes you need to do hard things to get the experience you want. Every expert was once a newbie. I went to check Bugzilla to see what I could fix. One bug looked very easy, so I thought I should work on that. That’s how it all started. Feedback after feedback, perseverance, pull, merge, merge conflicts, and getting a deeper understanding of how the code base functions made me able to contribute a lot more. One thing I loved about interning with open source is the fact that everything is open. I was asked to blog about what I worked on, and I didn’t need to hide anything that concerned it. Well… that doesn’t mean I shared my passwords too though! Passwords need to be protected and secret for a reason.

    Getting to work on Air Mozilla

    The internship day officially started on May 25th. I was told that after discussions I am being assigned to add a new feature. That new feature will act as a recommendation system suggesting similar events to Air Mozilla viewers while they view a specific event. Air Mozilla is a platform for Mozilla’s online multimedia presence that provides live and pre-recorded shows, interviews, news snippets, tutorial videos, and features about the Mozilla community.

    That sounded fascinating. I wasn’t nervous at all, it seemed pretty interesting and I always wondered how those things worked, that’s until I started getting really stuck down the road. Then I realized the complexity of it. I had to either build my own algorithms and computations to do the job, or find an open source tool we could use instead of reinventing the wheel. After several days of research we decided Elastic Search seemed like a good fit. Elastic Search, is a search server. It has a function called “More Like This” and that helps you search for similar events based on your set parameters. I went and read the documentation of Elastic Search but to be honest, I had a very hard time navigating it. Mostly because I couldn’t find enough examples on how to use it with Python. At some point we decided pyelasticsearch might make things easier; it’s a Python library version of Elastic Search that has the functionalities we were looking for.

    For a short recap, Elastic Search creates a mapping and indexes all the events on Air Mozilla. When you visit an event page, with the “More Like This” feature, we ask it to search the indexed events that are similar to the event you are currently viewing and return back the title, id, tags and its similarity scores on the back-end. For events to be considered related, their tags, channel and titles must be very much alike, but that’s not where we stop.

    We also need to be able to set someone’s level of access: a person who is logged in as a Mozilla employee will have more access than an anonymous guest would. A logged in volunteer contributor will have more access than an anonymous guest, but less than a Mozilla employee. You get the idea. Each event has its own access level setting. Some events are private and limited to employees, while the majority of Air Mozilla events are accessible to the public or to members of the entire Mozilla community. Regardless, the Related Events feature lets people see a list of similar events, filtered for their level of access, with the most similar “related events” appearing on top.

    Implementing Elastic Search

    In the code below we are using the more_like_this (mlt) query. We break the tags and the titles into different queries because we want to boost the relevance score of each one to a different degree. In this case we want a similar title to have a higher relevance score than the similar tags. The field parameter is used to determine which field we are trying to run mlt against, the docs parameter is the documents we are trying to find similar events for. You can read more about this at the More Like This Query page.

    mlt_query1 = { 
        'more_like_this': { 
            'fields': ['title'], 
            'docs': [ 
                    '_index': index, 
                    '_type': doc_type, 
            'min_term_freq': 1, 
            'max_query_terms': 20, 
            'min_doc_freq': 1, 
            'boost': 1.0, 
    mlt_query2 = { 
        'more_like_this': { 
            'fields': ['tags'], 
            'docs': [ 
                    '_index': index, 
                    '_type': doc_type, 
            'min_term_freq': 1, 
            'max_query_terms': 20, 
            'min_doc_freq': 1, 
            'boost': -0.5, 


    Below we are asking that one or more of the given mlt queries should match the documents, i.e., list of events:

    query_ = { 
        'bool': { 
            'should': [mlt_query1, mlt_query2], 


    Below, with request.user.is_active we are checking if the user is logged in and what their access level is. If the user is logged in as a volunteer the events that appear must not contain events that are restricted to Mozilla employees only.

    If the user is logged in and is not a volunteer, that means they are Mozilla paid staff members. Their “related events” results should contain all events that are similar to the event we are currently viewing.

    In the case of an anonymous user, the “related events” results must contain only events that are accessible to everyone.

    if request.user.is_active: 
      query = { 
        'fields': fields, 
        'query': query_ 
      query = { 
        'fields': fields, 
        'query': query_, 
        "filter": { 
          "bool": { 
            "must": { 
              "term": {"privacy": Event.PRIVACY_PUBLIC} 

    In the code above you see we used the more_like_this query along with a filter. More_like_this does return similar events based on title, tags and channels but it doesn’t ensure that the events returned are filtered by access level. So this is where “filter” comes in. It makes sure to filter the relevant events and allow only the ones the user can view based on their level of access.

    With the use of cron jobs I completely delete and re-index the index for the week, and every 10 minutes we re-index (without first deleting) all events that have changed in the last 10 minutes.

    AJAX queries were used to get the thumbnails and links of the related events. AJAX loads asynchronously, so running those queries doesn’t slow the pageload. When I view an event, I want it to load the page fully without waiting for Elastic Search to finish running the queries. Elastic Search queries run pretty fast but it still takes some time. Sometimes, it can be slow. The relevant events section appears under the events’ details. Ideally, someone watching an event won’t mind if there is a slight delay in the loading of the related events while they view the event, however a page that loads slowly as a whole can be frustrating. Through AJAX our Elastic Search queries can run in the background after the page has loaded.

    In the screenshots below, you can see how it works. I have added red arrows to the title of event we are on and the title of the first recommended event. If you compare both pictures you can see how the events we are viewing, and/or recommending, relate to each other.

    Example of Related Events for volunteers and staff

    A second example of Related Events

    Examples of related events for signed-in users


    4 key things that make you realize how much you enjoyed your internship:

    • Your mentor made sure you took the right steps to learn a lot.
    • You feel that every portion of it, even the frustrating moments, made you grow not only in skills but as a person.
    • You were extremely happy you got it.
    • You are extremely sad it is ending.

    That’s exactly how it’s been for me. I want to take a moment to thank Outreachy, my mentor Peter, and the other Mozillians I met on IRC and in email during my internship. Thank you for being helpful, welcoming, and for contributing to my learning experience.

    What made the pensive ending of my internship bright and merry is a motto Mozilla takes pride in: Once a Mozillian, Always a Mozillian.

  2. Scroll snapping explained

    Have you ever tried to snap your page’s contents after scrolling? There are many JavaScript libraries out there providing this functionality. Here are a few examples:

    As this is a common use case related to page layout and behavior, the W3C has published a pure CSS approach to scroll snapping.

    CSS scroll snapping, (available since July’s Firefox 39 release), allows you to control where to stop on an overflowing element when it’s scrolled. This lets you section your page into logical divisions and thus create smoother, easier-to-interact-with user interfaces. Touch devices in particular benefit from this feature, where it is easier for people to pan through pages instead of tapping through hierarchical structures.

    Image gallery

    Image galleries are surely the most common use case for scroll snapping: Users can flip through the images, viewing one image at a time by swiping or scrolling the page. So let’s see how this can be achieved with the new properties:

    img {
      width: 200px;
    .photoGallery {
      width: 200px;
      overflow: auto;
      white-space: nowrap;
      scroll-snap-points-x: repeat(100%);
      scroll-snap-type: mandatory;

    The related HTML code looks like this:

    <div class="photoGallery">
      <img src="img1.png"><img src="img2.png"><img src="img3.png">

    Here’s a live demo:

    See the Pen wKvYdK by Potch (@potch) on CodePen.

    The code above creates a simple image gallery with three images, which can be scrolled through horizontally.

    In this case the size of the images and their containing <div> are set to 200 pixels. The overflow: auto; displays a scrollbar on clients that support it. white-space: nowrap; serves to keep all images horizontally aligned. The definition “scroll-snap-points-x: repeat(100%);” sets a repeated horizontal snap point at 100% of the viewport width of the container <div>, in this case at 200 pixel intervals. Setting scroll-snap-type: mandatory; has the effect that snapping is forced. The viewport will always snap at a snap point, so the display will never stay in between two images.

    Item lists

    You’ve probably seen plenty of online product pages listing different features with an image of each feature and a description next to it, or an interface displaying a series of user testimonials.

    In these cases, scroll snapping lets you align the sections so that maximal display space is used.

    .features {
      width: 400px;
      height: 250px;
      padding: 0;
      overflow: auto;
      scroll-snap-type: proximity;
      scroll-snap-destination: 0 16px;
    .features > section {
      clear: both;
      margin: 20px 0;
      scroll-snap-coordinate: 0 0;
    img {
      width: 50px;
      height: 50px;
      margin: 5px 10px;
      float: left;
    section:last-child {
      margin-bottom: 60px;

    And here’s the related HTML code:

    <div class="features">
      <section id="feature1">
        <img src="feature1.png"/>
        <p>Lorem ipsum...</p>
      <section id="feature2">
        <img src="feature2.png"/>
        <p>Lorem ipsum...</p>

    Here’s a live demo:

    See the Pen NGWOjN by Potch (@potch) on CodePen.

    When you scroll within the example above, each feature section is positioned so that the top is aligned with the top of the viewport to display as much text as possible. This is achieved by applying scroll-snap-coordinate: 0 0; to the sections. The two zeros refer to the x and y coordinates of the element where it will snap to the container element. scroll-snap-destination: 0 16px; defines the offset position within the container element to which the inner elements should snap. In this case, that’s 16 pixels below the top of the container so that the text at top of the section has some margin at the top.

    In addition to the properties currently defined within the CSS Scroll Snap Points specification, Gecko implements the additional properties scroll-snap-type-x and scroll-snap-type-y, for setting the snap type individually per axis. These long-hand properties may be added to the specification in the future.

    Currently snap points can only be set through coordinates, either referring to the start edge of the container element or the ones within it. Sometimes this requires some calculation to set them at the right position. Future extensions to this feature may extend the functionality to be able to set snap points on the box model instead, which would make it easier to place them. There’s already a discussion about this within the www-style mailing list.

    Has this new functionality caught your interest? Then it’s time to give it a try! And if you don’t remember how to use the different properties, you can always refer to the documentation on MDN.

  3. Flash-Free Clipboard for the Web

    As part of our effort to grow the Web platform and make it accessible to new devices, we are trying to reduce the Web’s dependence on Flash. As part of that effort, we are standardizing and exposing useful features which are currently only available to Flash to the entirety of the Web platform.

    One of the reasons why many sites still use Flash is because of its copy and cut clipboard APIs. Flash exposes an API for programmatically copying text to the user’s clipboard on a button press. This has been used to implement handy features, such as GitHub’s “clone URL” button. It’s also useful for things such as editor UIs, which want to expose a button for copying to the clipboard, rather than requiring users to use keyboard shortcuts or the context menu.

    Unfortunately, Web APIs haven’t provided the functionality to copy text to the clipboard through JavaScript, which is why visiting GitHub with Flash disabled shows an ugly grey box where the button is supposed to be. Fortunately, we have a solution. The editor APIs provide document.execCommand as an entry point for executing editor commands. The "copy" and cut" commands have previously been disabled for web pages, but with Firefox 41, which is currently in Beta, and slated to move to release in mid-September, it is becoming available to JavaScript within user-action initiated callbacks.

    Using execCommand("cut"/"copy")

    The execCommand("cut"/"copy") API is only available during a user-triggered callback, such as a click. If you try to call it at a different time, execCommand will return false, meaning that the command failed to execute. Running execCommand("cut") will copy the current selection to the clipboard, so let’s go about implementing a basic copy-to-clipboard button.

    // button which we are attaching the event to
    var button = ...;
    // input containing the text we want to copy 
    var input = ...;
    button.addEventListener("click", function(event) {
      // Select the input node's contents;
      // Copy it to the clipboard

    That code will trigger a copy of the text in the input to the clipboard upon the click of the button in Firefox 41 and above. However, you probably want to also handle failure situations, potentially to fallback to another Flash-based approach such as ZeroClipboard, or even just to tell the user that their browser doesn’t support the functionality.

    The execCommand method will return false if the action failed, for example, due to being called outside of a user-initiated callback, but on older versions of Firefox, we would also throw a security exception if you attempted to use the "cut" or "copy" APIs. Thus, if you want to be sure that you capture all failures, make sure to surround the call in a try-catch block, and also interpret an exception as a failure.

    // button which we are attaching the event to
    var button = ...;
    // input containing the text we want to copy
    var input = ...;
    button.addEventListener("click", function(event) {
      event.preventDefault();; // Select the input node's contents
      var succeeded;
      try {
        // Copy it to the clipboard
        succeeded = document.execCommand("copy");
      } catch (e) {
        succeeded = false;
      if (succeeded) {
        // The copy was successful!
      } else {
        // The copy failed :(

    The "cut" API is also exposed to web pages through the same mechanism, so just s/copy/cut/, and you’re all set to go!

    Feature testing

    The editor APIs provide a method document.queryCommandSupported("copy") intended to allow API consumers to determine whether a command is supported by the browser. Unfortunately, in versions of Firefox prior to 41, we returned true from document.queryCommandSupported("copy") even though the web page was unable to actually perform the copy operation. However, attempting to execute document.execCommand("copy") would throw a SecurityException. So, attempting to copy on load, and checking for this exception is probably the easiest way to feature-detect support for document.execCommand("copy") in Firefox.

    var supported = document.queryCommandSupported("copy");
    if (supported) {
      // Check that the browser isn't Firefox pre-41
      try {
      } catch (e) {
        supported = false;
    if (!supported) {
      // Fall back to an alternate approach like ZeroClipboard

    Support in other browsers

    Google Chrome and Internet Explorer both also support this API. Chrome uses the same restriction as Firefox (that it must be run in a user-initiated callback). Internet Explorer allows it to be called at any time, except it first prompts the user with a dialog, asking for permission to access the clipboard.

    For more information about the API and browser support, see MDN documentation for document.execCommand().

  4. Developer Edition 42: Wifi Debugging, Win10, Multiprocess Firefox, ReactJS tools, and more

    Firefox 42 has arrived! In this release, we put a lot of effort into the quality and polish of the Developer Edition browser. Although many of the bugs resolved this release don’t feature in the Release Notes, these small fixes make the tools faster and more stable. But there’s still a lot to report, including a major change to how Firefox works.

    Debugging over wifi

    Now, with remote website debugging, you can debug Firefox for Android devices over wifi – no USB cable or ADB needed.

    Multiprocess is enabled by default

    Multiprocess Firefox (aka E10s) has been enabled by default in Developer Edition. When it’s enabled, Firefox renders and executes web-related content in a single background content process. If you experience any issues with addons after updating to Developer Edition 42, try disabling incompatible addons or reverting to a single process mode using about:preferences.

    Windows 10 theme support

    The Developer Edition theme has a new look in Windows 10 to match the OS styling. Take a look:

    Screenshot of the dark Developer Edition theme in Windows 10

    Dark Developer Edition theme – Windows 10

    Screenshot of the light Developer Edition theme in Windows 10

    Light Developer Edition theme – Windows 10

    React Developer Tools support for Firefox

    If you’re developing with ReactJS, you may have noticed that the React project recently released a beta for their developer tools extension, including initial support for Firefox. While there are no official builds yet of the Firefox version, the source is available on github.

    Other notable changes

    • Asynchronous call stacks now allow you to follow the code flow through setTimeout, DOM event handlers, and Promise handlers. (Bug 981514)
    • There is a new configurable Firefox OS simulator page in WebIDE. From here, you can change a simulator to run with a custom profile and screen size, using a list of presets from reference devices. (Bug 1156834)
    • CSS filter presets are now available in the inspector. (Bug 1153184)
    • The MDN tooltip now uses syntax highlighting for code samples. (Bug 1154469)
    • When using the “copy” keyboard shortcut in the inspector, the outerHTML of the selected node is now copied onto the clipboard. (Bug 968241)
    • New UX improvements have landed in the style editor’s search feature. (Bug 1159001, Bug 1153474)
    • CSS variables are now treated as normal declarations in the inspector. (Bug 1142206)
    • CSS autocomplete popup now supports pressing ‘down’ to list all results in an empty value field (Bug 1142206)

    Thanks to everyone who contributed time and energy to help the DevTools team in this release of Firefox Developer Edition 42! Each release takes a lot of effort from people writing patches, testing, documenting, reporting bugs, sending feedback, discussing features, etc. You can help set our priorities by sharing constructive feedback and letting us know what you’d like from Firefox Developer Tools.

    You can download Firefox Developer Edition now, for free.

  5. ES6 In Depth: The Future

    ES6 In Depth is a series on new features being added to the JavaScript programming language in the 6th Edition of the ECMAScript standard, ES6 for short.

    Last week’s article on ES6 modules wrapped up a 4-month survey of the major new features in ES6.

    This post covers over a dozen more new features that we never got around to talking about at length. Consider it a fun tour of all the closets and oddly-shaped upstairs rooms in this mansion of a language. Maybe a vast underground cavern or two. If you haven’t read the other parts of the series, take a look; this installment may not be the best place to start!

    (a picture of the Batcave, inexplicably)

    “On your left, you can see typed arrays…”

    One more quick warning: Many of the features below are not widely implemented yet.

    OK. Let’s get started.

    Features you may already be using

    ES6 standardizes some features that were previously in other standards, or widely implemented but nonstandard.

    • Typed arrays, ArrayBuffer, and DataView. These were all standardized as part of WebGL, but they’ve been used in many other APIs since then, including Canvas, the Web Audio API, and WebRTC. They’re handy whenever you need to process large volumes of raw binary or numeric data.

      For example, if the Canvas rendering context is missing a feature you want, and if you’re feeling sufficiently hardcore about it, you can just implement it yourself:

      var context = canvas.getContext("2d");
      var image = context.getImageData(0, 0, canvas.width, canvas.height);
      var pixels =;  // a Uint8ClampedArray object
      // ... Your code here!
      // ... Hack on the raw bits in `pixels`
      // ... and then write them back to the canvas:
      context.putImageData(image, 0, 0);

      During standardization, typed arrays picked up methods like .slice(), .map(), and .filter().

    • Promises. Writing just one paragraph about promises is like eating just one potato chip. Never mind how hard it is; it barely even makes sense as a thing to do. What to say? Promises are the building blocks of asynchronous JS programming. They represent values that will become available later. So for example, when you call fetch(), instead of blocking, it returns a Promise object immediately. The fetch goes on in the background, and it’ll call you back when the response arrives. Promises are better than callbacks alone, because they chain really nicely, they’re first-class values with interesting operations on them, and you can get error handling right with a lot less boilerplate. They’re polyfillable in the browser. If you don’t already know all about promises, check out Jake Archibald’s very in-depth article.

    • Functions in block scope. You shouldn’t be using this one, but it’s possible you have been. Maybe unintentionally.

      In ES1-5, this code was technically illegal:

      if (temperature > 100) {
        function chill() {
          return fan.switchOn().then(obtainLemonade);

      That function declaration inside an if block was supposedly forbidden. They were only legal at toplevel, or inside the outermost block of a function.

      But it worked in all major browsers anyway. Sort of.

      Not compatibly. The details were a little different in each browser. But it sort of worked, and many web pages still use it.

      ES6 standardizes this, thank goodness. The function is hoisted to the top of the enclosing block.

      Unfortunately, Firefox and Safari don’t implement the new standard yet. So for now, use a function expression instead:

      if (temperature > 100) {
        var chill = function () {    
          return fan.switchOn().then(obtainLemonade);

      The only reason block-scoped functions weren’t standardized years ago is that the backward-compatibility constraints were incredibly complicated. Nobody thought they could be solved. ES6 threads the needle by adding a very strange rule that only applies in non-strict code. I can’t explain it here. Trust me, use strict mode.

    • Function names. All the major JS engines have also long supported a nonstandard .name property on functions that have names. ES6 standardizes this, and makes it better by inferring a sensible .name for some functions that were heretofore considered nameless:

      > var lessThan = function (a, b) { return a < b; };

      For other functions, such as callbacks that appear as arguments to .then methods, the spec still can’t figure out a name. is then the empty string.

    Nice things

    • Object.assign(target, ...sources). A new standard library function, similar to Underscore’s _.extend().

    • The spread operator for function calls. This is nothing to do with Nutella, even though Nutella is a tasty spread. But it is a delicious feature, and I think you'll like it.

      Back in May, we introduced rest parameters. They’re a way for functions to receive any number of arguments, a more civilized alternative to the random, clumsy arguments object.

      function log(...stuff) {  // stuff is the rest parameter.
        var rendered =; // It's a real array.

      What we didn’t say is that there’s matching syntax for passing any number of arguments to a function, a more civilized alternative to fn.apply():

      // log all the values from an array

      Of course it works with any iterable object, so you can log all the stuff in a Set by writing log(...mySet).

      Unlike rest parameters, it makes sense to use the spread operator multiple times in a single argument list:

      // kicks are before trids
      log("Kicks:", ...kicks, "Trids:", ...trids);

      The spread operator is handy for flattening an array of arrays:

      > var smallArrays = [[], ["one"], ["two", "twos"]];
      > var oneBigArray = [].concat(...smallArrays);
      > oneBigArray
          ["one", "two", "twos"]

      ...but maybe this one of those pressing needs that only I have. If so, I blame Haskell.

    • The spread operator for building arrays. Also back in May, we talked about “rest” patterns in destructuring. They’re a way to get any number of elements out of an array:

      > var [head, ...tail] = [1, 2, 3, 4];
      > head
      > tail
          [2, 3, 4]

      Guess what! There’s matching syntax for getting any number of elements into an array:

      > var reunited = [head, ...tail];
      > reunited
          [1, 2, 3, 4]

      This follows all the same rules as the spread operator for function calls: you can use the spread operator many times in the same array, and so on.

    • Proper tail calls. This one is too amazing for me to try to explain here.

      To understand this feature, there’s no better place to start than page 1 of Structure and Interpretation of Computer Programs. If you enjoy it, just keep reading. Tail calls are explained in section 1.2.1, “Linear Recursion and Iteration”. The ES6 standard requires that implementations be “tail-recursive”, as the term is defined there.

      None of the major JS engines have implemented this yet. It’s hard to implement. But all in good time.


    • Unicode version upgrade. ES5 required implementations to support at least all the characters in Unicode version 3.0. ES6 implementations must support at least Unicode 5.1.0. You can now use characters from Linear B in your function names!

      Linear A is still a bit risky, both because it was not added to Unicode until version 7.0 and because it might be hard to maintain code written in a language that has never been deciphered.

      (Even in JavaScript engines that support the emoji added in Unicode 6.1, you can’t use 😺 as a variable name. For some reason, the Unicode Consortium decided not to classify it as an identifier character. 😾)

    • Long Unicode escape sequences. ES6, like earlier versions, supports four-digit Unicode escape sequences. They look like this: \u212A. These are great. You can use them in strings. Or if you’re feeling playful and your project has no code review policy whatsoever, you can use them in variable names. But then, for a character like U+13021 (𓀡), the Egyptian hieroglyph of a guy standing on his head, there's a slight problem. The number 13021 has five digits. Five is more than four.

      In ES5, you had to write two escapes, a UTF-16 surrogate pair. This felt exactly like living in the Dark Ages: cold, miserable, barbaric. ES6, like the dawn of the Italian Renaissance, brings tremendous change: you can now write \u{13021}.

    • Better support for characters outside the BMP. The .toUpperCase() and .toLowerCase() methods now work on strings written in the Deseret alphabet!

      In the same vein, String.fromCodePoint(...codePoints) is a function very similar to the older String.fromCharCode(...codeUnits), but with support for code points beyond the BMP.

    • Unicode RegExps. ES6 regular expressions support a new flag, the u flag, which causes the regular expression to treat characters outside the BMP as single characters, not as two separate code units. For example, without the u, /./ only matches half of the character "😭". But /./u matches the whole thing.

      Putting the u flag on a RegExp also enables more Unicode-aware case-insensitive matching and long Unicode escape sequences. For the whole story, see Mathias Bynens’s very detailed post.

    • Sticky RegExps. A non-Unicode-related feature is the y flag, also known as the sticky flag. A sticky regular expression only looks for matches starting at the exact offset given by its .lastIndex property. If there isn’t a match there, rather than scanning forward in the string to find a match somewhere else, a sticky regexp immediately returns null.

    • An official internationalization spec. ES6 implementations that provide any internationalization features must support ECMA-402, the ECMAScript 2015 Internationalization API Specification. This separate standard specifies the Intl object. Firefox, Chrome, and IE11+ already fully support it. So does Node 0.12.


    • Binary and octal number literals. If you need a fancy way to write the number 8,675,309, and 0x845fed isn’t doing it for you, you can now write 0o41057755 (octal) or 0b100001000101111111101101 (binary).

      Number(str) also now recognizes strings in this format: Number("0b101010") returns 42.

      (Quick reminder: number.toString(base) and parseInt(string, base) are the original ways to convert numbers to and from arbitrary bases.)

    • New Number functions and constants. These are pretty niche. If you’re interested, you can browse the standard yourself, starting at Number.EPSILON.

      Maybe the most interesting new idea here is the “safe integer” range, from −(253 - 1) to +(253 - 1) inclusive. This special range of numbers has existed as long as JS. Every integer in this range can be represented exactly as a JS number, as can its nearest neighbors. In short, it’s the range where ++ and -- work as expected. Outside this range, odd integers aren’t representable as 64-bit floating-point numbers, so incrementing and decrementing the numbers that are representable (all of which are even) can’t give a correct result. In case this matters to your code, the standard now offers constants Number.MIN_SAFE_INTEGER and Number.MAX_SAFE_INTEGER, and a predicate Number.isSafeInteger(n).

    • New Math functions. ES6 adds hyperbolic trig functions and their inverses, Math.cbrt(x) for computing cube roots, Math.hypot(x, y) for computing the hypotenuse of a right triangle, Math.log2(x) and Math.log10(x) for computing logarithms in common bases, Math.clz32(x) to help compute integer logarithms, and a few others.

      Math.sign(x) gets the sign of a number.

      ES6 also adds Math.imul(x, y), which does signed multiplication modulo 232. This is a very strange thing to want... unless you are working around the fact that JS does not have 64-bit integers or big integers. In that case it’s very handy. This helps compilers. Emscripten uses this function to implement 64-bit integer multiplication in JS.

      Similarly Math.fround(x) is handy for compilers that need to support 32-bit floating-point numbers.

    The end

    Is this everything?

    Well, no. I didn’t even mention the object that’s the common prototype of all built-in iterators, the top-secret GeneratorFunction constructor,, v2), how Symbol.species helps support subclassing builtins like Array and Promise, or how ES6 specifies details of how multiple globals work that have never been standardized before.

    I’m sure I missed a few things, too.

    But if you’ve been following along, you have a pretty good picture of where we’re going. You know you can use ES6 features today, and if you do, you’ll be opting in to a better language.

    A few days ago, Josh Mock remarked to me that he had just used eight different ES6 features in about 50 lines of code, without even really thinking about it. Modules, classes, argument defaults, Set, Map, template strings, arrow functions, and let. (He missed the for-of loop.)

    This has been my experience, too. The new features hang together very well. They end up affecting almost every line of JS code you write.

    Meanwhile, every JS engine is hurrying to implement and optimize the features we’ve been discussing for the past few months.

    Once we’re done, the language will be complete. We’ll never have to change anything again. I’ll have to find something else to work on.

    Just kidding. Proposals for ES7 are already picking up steam. Just to pick a few:

    • Exponentation operator. 2 ** 8 will return 256. Implemented in Firefox Nightly.

    • Array.prototype.includes(value). Returns true if this array contains the given value. Implemented in Firefox Nightly; polyfillable.

    • SIMD. Exposes 128-bit SIMD instructions provided by modern CPUs. These instructions do an arithmetic operation on 2, or 4, or 8 adjacent array elements at a time. They can dramatically speed up a wide variety of algorithms for streaming audio and video, cryptography, games, image processing, and more. Very low-level, very powerful. Implemented in Firefox Nightly; polyfillable.

    • Async functions. We hinted at this feature in the post on generators. Async functions are like generators, but specialized for asynchronous programming. When you call a generator, it returns an iterator. When you call an async function, it returns a promise. Generators use the yield keyword to pause and produce a value; async functions instead use the await keyword to pause and wait for a promise.

      It’s hard to describe them in a few sentences, but async functions will be the landmark feature in ES7.

    • Typed Objects. This is a follow-up to typed arrays. Typed arrays have elements that are typed. A typed object is simply an object whose properties are typed.

      // Create a new struct type. Every Point has two fields
      // named x and y.
      var Point = new TypedObject.StructType({
        x: TypedObject.int32,
        y: TypedObject.int32
      // Now create an instance of that type.
      var p = new Point({x: 800, y: 600});
      console.log(p.x); // 800

      You would only do this for performance reasons. Like typed arrays, typed objects offer a few of the benefits of typing (compact memory usage and speed), but on a per-object, opt-in basis, in contrast to languages where everything is statically typed.

      They’re are also interesting for JS as a compilation target.

      Implemented in Firefox Nightly.

    • Class and property decorators. Decorators are tags you add to a property, class, or method. An example shows what this is about:

      import debug from "jsdebug";
      class Person {
        hasRoundHead(assert) {
          return this.head instanceof Spheroid;

      @debug.logWhenCalled is the decorator here. You can imagine what it does to the method.

      The proposal explains how this would work in detail, with many examples.

    There’s one more exciting development I have to mention. This one is not a language feature.

    TC39, the ECMAScript standard committee, is moving toward more frequent releases and a more public process. Six years passed between ES5 and ES6. The committee aims to ship ES7 just 12 months after ES6. Subsequent editions of the standard will be released on a 12-month cadence. Some of the features listed above will be ready in time. They will “catch the train” and become part of ES7. Those that aren’t finished in that timeframe can catch the next train.

    It’s been great fun sharing the staggering amount of good stuff in ES6. It’s also a pleasure to be able to say that a feature dump of this size will probably never happen again.

    Thanks for joining us for ES6 In Depth! I hope you enjoyed it. Keep in touch.

  6. Flying a drone in your browser with WebBluetooth

    There are tons of devices around us, and the number is only growing. And more and more of these devices come with connectivity. From suitcases to plants to eggs. This brings new challenges: how can we discover devices around us, and how can we interact with them?

    Currently device interactions are handled by separate apps running on mobile phones. But this does not solve the discoverability issue. I need to know which devices are around me before I know which app to install. When I’m standing in front of a meeting room I don’t care about which app to install, or even what the name or ID of the meeting room is. I just want to make a booking or see availability, and as fast as possible.


    Scott Jenson from Google has been thinking about discoverability for a while, and came up with the Physical Web project, whose premise is:

    Walk up and use anything

    The idea is that you use Bluetooth Smart, the low energy variant of bluetooth, to broadcast URLs to the world. Your phone picks up the advertisment package, decodes it, and shows some information to the user. One click and the user is redirected to a web page with relevant content. This can be used for a variety of things:

    • A meeting room can broadcast a URL to its calendar for scheduling.
    • A movie poster can broadcast a URL to show viewing times and trailers.
    • A prescription medicine can broadcast a URL with information about the medication or how to refill it.
    • Look around you. Examples of other use cases are everywhere, waiting to be implemented.

    However, the material world is not a one-way street, and this presents a problem. Broadcasting a URL is great for informing me about things like movie times, but it does not allow me to interact more deeply with the device. If I want to fly a drone I don’t just want to discover that there’s a drone near me, I also want to interact with the drone straight away. We need to have a way for web pages to communicate back to devices.

    Enter the work of the Web Bluetooth W3C group, that includes representatives of Mozilla’s Bluetooth team, who are working on bringing bluetooth APIs to the browser. If the Physical Web allows us to walk up to any device and get the URL of a web app, then WebBluetooth allows the web app to connect to the device and talk back to it.

    At this point, there’s still a lot of work to be done. The bluetooth API is only exposed to certified content on Firefox OS, and thus is not currently accessible to ordinary web content. Until security issues have been cleared this will continue to be the case. A second issue is that Physical Web beacons broadcast a URL. How can a specific web resource know which specific device has broadcast the URL?

    As you can see, lots of work remains to be done, but this blog is called Mozilla Hacks for a reason. Let’s start hacking!

    Adding Physical Web support to Firefox OS

    Since most of the work around WebBluetooth has been done for Firefox OS, I’ve made it my weapon of choice. I want the process of discovering devices to be as painless and obvious as possible. I figured the lockscreen would be the best possible place. Whenever you have bluetooth enabled on your Firefox OS phone, a new notification would then pop up asking you to search for devices (tracking bug).

    Tap, tap, tap

    navigator.mozBluetooth.defaultAdapter.startLeScan([]).then(handle => {
      handle.ondevicefound = e => {
        console.log('Found', e.device, e.scanRecord);
      setTimeout(() => {
      }, 5000);
    }, err => console.error(err));

    As you can see on the third line, we have a scanRecord. This is the advertisement package that the device broadcasts. It’s nothing more than a set of bytes, and you are free to declare your own protocol. For our purpose—broadcasting URLs over bluetooth—Google has already developed two ways of encoding: UriBeacon and EddyStone, both of which can be found in the wild today.

    Parsing the advertisement package is pretty straightforward. Here’s some code I wrote to parse UriBeacons. Parsing the UriBeacon will give you a URL, which is often shortened, because of limited bytes in the advertisement package —this makes for an uninformative UI:

    So what the hack (pun intended) is this device?

    To get some information about the web page behind the beacon we can do an AJAX request and parse the content of the page to enhance the information displayed on the lockscreen:

    function resolveURI(uri, ele) {
    var x = new XMLHttpRequest({ mozSystem: true });
    x.onload = e => {
      var h = document.createElement('html');
      h.innerHTML = x.responseText;
      // After following 301/302s, this contains the last resolved URL
      console.log('url is', x.responseURL);
      var titleEl = h.querySelector('title');
      var metaEl = h.querySelector('meta[name="description"]');
      var bodyEl = h.querySelector('body');
      if (titleEl && titleEl.textContent) {
        console.log('title is', titleEl.textContent);
      if (metaEl && metaEl.content) {
        console.log('description is', metaEl.content);
      else if (bodyEl && bodyEl.textContent) {
        console.log('description is', bodyEl.textContent);
    x.onerror = err => console.error('Loading', uri, 'failed', err);'GET', uri);

    This yields a nicer notification that actually describes the beacon.

    Much nicer

    A drone that doesn’t broadcast a URL

    Unfortunately not all BLE devices broadcast URLs at this point. All of this new technology is experimental and very cool, but not yet fully implemented. We’ve got high hopes that this will change in the near future. Because I still want to be able to fly my drone now, I added some code that transforms the data a drone broadcasts into a URL.

    The web application

    Now that we’ve solved the issue of discoverability, we need a way to control the drone from the browser. Since bluetooth access is not available for web content, we need to make some changes to Gecko, where the Firefox OS security model is implemented. If you are interested in the changes, here’s the commit. We also needed a sneaky hack to make sure the tab’s process is run with the right Linux permissions.

    With these changes in place, we open up navigator.mozBluetooth to all content, and run every tab in Firefox in a process that is part of the ‘bluetooth’ Linux group, ensuring access to the hardware. If you’re playing around with this build later, please note that with my “sneaky” hack implemented, you are now running a build where no security is guaranteed. Using a build hack like this, with security disabled, is fine for IoT experimentation, but is definitely not recommended as a production solution. When the Web Bluetooth spec is finalized, and official support lands in Gecko, proper security will be implemented.

    With the API in place, we can start writing the application. When you tap on the Physical Web notification on the lockscreen, we pass the device address in as a parameter. This is subject to change. For the ongoing discussion take a look at the Eddystone -> Web Bluetooth handoff.

    var address = 'aa:bb:cc:dd:ee'; // parsed from URL
    var counter = 0;
    navigator.mozBluetooth.defaultAdapter.startLeScan([]).then(handle => {
      handle.ondevicefound = e => {
        if (e.device.address !== address) return;
        // write some code to fly the drone
    }, err => console.error(err));

    Now that we have a reference to the device address, we can set up a connection. The protocol we use to talk back and forth to the device is called GATT, the Generic Attribute Profile. The idea behind GATT is that a device can have multiple standard services. For example, a heart rate sensor can implement the battery service and the heart rate service. Because these services are standardized, a consuming application only needs to write the implementation logic once, and can talk to any heart rate monitor.

    Characteristics are aspects of a given service. For example, a heart rate service will implement heart rate measurement and heart rate max. Characteristics can be readable and writeable depending on how they are defined. This goes the same with the drone. It has a service for flying the drone and characteristics to let you control the drone from your phone.

    Luckily Martin Dlouhý (as far as I can tell, he was the first) has already decoded the communication protocol for the Rolling Spider drone, so we can use his work and the new Bluetooth API to start flying…

    // Have a way of knowing when the connection drops
    e.device.gatt.onconnectionstatechanged = cse => {
      console.log('connectionStateChanged', cse);
    // Receive events (battery change f.e.) from device
    e.device.gatt.oncharacteristicchanged = cce => {
      console.log('characteristicChanged', cce);
    // Set up the connection
    e.device.gatt.connect().then(() => {
      return e.device.gatt.discoverServices();
    }).then(() => {
      // devices have services, and services have characteristics
      var services =;
      console.log('services', services);
      // find the characteristic that handles flying the drone
      var c = services.reduce((curr, f) => curr.concat(f.characteristics), [])
        .filter(c => c.uuid === '9a66fa0b-0800-9191-11e4-012d1540cb8e')[0];
      // take off instruction!
      var buffer = new Uint8Array(0x04, counter++, 0x02, 0x00, 0x01, 0x00]);
      c.writeValue(buffer).then(() => {
        console.log('take off successful!');

    The Mozilla team in Taipei used this to create a demo application for Firefox OS, demonstrating the capabilities of the new API during the Mozilla Work Week in Whistler last June. With the API now available in the browser, we can take that work, host it as a web page, beef up the graphics a bit, and have a web site flying a drone!

    Such amaze

    Such amaze. Much drone.


    It’s an exciting time for the Web! With more and more devices coming online we need a way to discover and interact with them without much hassle. The combination of Physical Web and WebBluetooth allows us to create frictionless experiences for users willing to interact with real-world appliances and new devices. Although we’re a long way off, we’re heading in the right direction. Google and Mozilla are actively developing the technology; I’ve got high hopes that everything in this blog post will be common knowledge in a year!

    If that’s not fast enough for you, you can play around with an experimental build of Firefox OS which enables everything seen in this post. This build runs on the Flame developer device. First, upgrade to nightly_v3 base image, then flash this build.


    Thanks to Tzu-Lin Huang and Sean Lee for building the initial drone code; the WebBluetooth team in Mozilla Taipei (especially Jocelyn Liu) for quick feedback and patches when I complained about the API; Chris Williams for putting the drone in my gift bag; Scott Jenson for answering my numerous questions about the Physical Web; and Telenor Digital for letting me play with drones for two weeks.

  7. ES6 In Depth: Modules

    ES6 In Depth is a series on new features being added to the JavaScript programming language in the 6th Edition of the ECMAScript standard, ES6 for short.

    When I started on Mozilla’s JavaScript team back in 2007, the joke was that the length of a typical JavaScript program was one line.

    This was two years after Google Maps launched. Not long before that, the predominant use of JavaScript had been form validation, and sure enough, your average <input onchange=> handler would be… one line of code.

    Things have changed. JavaScript projects have grown to jaw-dropping sizes, and the community has developed tools for working at scale. One of the most basic things you need is a module system, a way to spread your work across multiple files and directories—but still make sure all your bits of code can access one another as needed—but also be able to load all that code efficiently. So naturally, JavaScript has a module system. Several, actually. There are also several package managers, tools for installing all that software and coping with high-level dependencies. You might think ES6, with its new module syntax, is a little late to the party.

    Well, today we’ll see whether ES6 adds anything to these existing systems, and whether or not future standards and tools will be able to build on it. But first, let’s just dive in and see what ES6 modules look like.

    Module basics

    An ES6 module is a file containing JS code. There’s no special module keyword; a module mostly reads just like a script. There are two differences.

    • ES6 modules are automatically strict-mode code, even if you don’t write "use strict"; in them.

    • You can use import and export in modules.

    Let’s talk about export first. Everything declared inside a module is local to the module, by default. If you want something declared in a module to be public, so that other modules can use it, you must export that feature. There are a few ways to do this. The simplest way is to add the export keyword.

    // kittydar.js - Find the locations of all the cats in an image.
    // (Heather Arthur wrote this library for real)
    // (but she didn't use modules, because it was 2013)
    export function detectCats(canvas, options) {
      var kittydar = new Kittydar(options);
      return kittydar.detectCats(canvas);
    export class Kittydar {
      ... several methods doing image processing ...
    // This helper function isn't exported.
    function resizeCanvas() {

    You can export any top-level function, class, var, let, or const.

    And that’s really all you need to know to write a module! You don’t have to put everything in an IIFE or a callback. Just go ahead and declare everything you need. Since the code is a module, not a script, all the declarations will be scoped to that module, not globally visible across all scripts and modules. Export the declarations that make up the module’s public API, and you’re done.

    Apart from exports, the code in a module is pretty much just normal code. It can use globals like Object and Array. If your module runs in a web browser, it can use document and XMLHttpRequest.

    In a separate file, we can import and use the detectCats() function:

    // demo.js - Kittydar demo program
    import {detectCats} from "kittydar.js";
    function go() {
        var canvas = document.getElementById("catpix");
        var cats = detectCats(canvas);
        drawRectangles(canvas, cats);

    To import multiple names from a module, you would write:

    import {detectCats, Kittydar} from "kittydar.js";

    When you run a module containing an import declaration, the modules it imports are loaded first, then each module body is executed in a depth-first traversal of the dependency graph, avoiding cycles by skipping anything already executed.

    And those are the basics of modules. It’s really quite simple. ;-)

    Export lists

    Rather than tagging each exported feature, you can write out a single list of all the names you want to export, wrapped in curly braces:

    export {detectCats, Kittydar};
    // no `export` keyword required here
    function detectCats(canvas, options) { ... }
    class Kittydar { ... }

    An export list doesn’t have to be the first thing in the file; it can appear anywhere in a module file’s top-level scope. You can have multiple export lists, or mix export lists with other export declarations, as long as no name is exported more than once.

    Renaming imports and exports

    Once in a while, an imported name happens to collide with some other name that you also need to use. So ES6 lets you rename things when you import them:

    // suburbia.js
    // Both these modules export something named `flip`.
    // To import them both, we must rename at least one.
    import {flip as flipOmelet} from "eggs.js";
    import {flip as flipHouse} from "real-estate.js";

    Similarly, you can rename things when you export them. This is handy if you want to export the same value under two different names, which occasionally happens:

    // unlicensed_nuclear_accelerator.js - media streaming without drm
    // (not a real library, but maybe it should be)
    function v1() { ... }
    function v2() { ... }
    export {
      v1 as streamV1,
      v2 as streamV2,
      v2 as streamLatestVersion

    Default exports

    The new standard is designed to interoperate with existing CommonJS and AMD modules. So suppose you have a Node project and you’ve done npm install lodash. Your ES6 code can import individual functions from Lodash:

    import {each, map} from "lodash";
    each([3, 2, 1], x => console.log(x));

    But perhaps you’ve gotten used to seeing _.each rather than each and you still want to write things that way. Or maybe you want to use _ as a function, since that’s a useful thing to do in Lodash.

    For that, you can use a slightly different syntax: import the module without curly braces.

    import _ from "lodash";

    This shorthand is equivalent to import {default as _} from "lodash";. All CommonJS and AMD modules are presented to ES6 as having a default export, which is the same thing that you would get if you asked require() for that module—that is, the exports object.

    ES6 modules were designed to let you export multiple things, but for existing CommonJS modules, the default export is all you get. For example, as of this writing, the famous colors package doesn’t have any special ES6 support as far as I can tell. It’s a collection of CommonJS modules, like most packages on npm. But you can import it right into your ES6 code.

    // ES6 equivalent of `var colors = require("colors/safe");`
    import colors from "colors/safe";

    If you’d like your own ES6 module to have a default export, that’s easy to do. There’s nothing magic about a default export; it’s just like any other export, except it’s named "default". You can use the renaming syntax we already talked about:

    let myObject = {
      field1: value1,
      field2: value2
    export {myObject as default};

    Or better yet, use this shorthand:

    export default {
      field1: value1,
      field2: value2

    The keywords export default can be followed by any value: a function, a class, an object literal, you name it.

    Module objects

    Sorry this is so long. But JavaScript is not alone: for some reason, module systems in all languages tend to have a ton of individually small, boring convenience features. Fortunately, there’s just one thing left. Well, two things.

    import * as cows from "cows";

    When you import *, what’s imported is a module namespace object. Its properties are the module’s exports. So if the “cows” module exports a function named moo(), then after importing “cows” this way, you can write: cows.moo().

    Aggregating modules

    Sometimes the main module of a package is little more than importing all the package’s other modules and exporting them in a unified way. To simplify this kind of code, there’s an all-in-one import-and-export shorthand:

    // world-foods.js - good stuff from all over
    // import "sri-lanka" and re-export some of its exports
    export {Tea, Cinnamon} from "sri-lanka";
    // import "equatorial-guinea" and re-export some of its exports
    export {Coffee, Cocoa} from "equatorial-guinea";
    // import "singapore" and export ALL of its exports
    export * from "singapore";

    Each one of these export-from statements is similar to an import-from statement followed by an export. Unlike a real import, this doesn’t add the re-exported bindings to your scope. So don’t use this shorthand if you plan to write some code in world-foods.js that makes use of Tea. You’ll find that it’s not there.

    If any name exported by “singapore” happened to collide with the other exports, that would be an error, so use export * with care.

    Whew! We’re done with syntax! On to the interesting parts.

    What does import actually do?

    Would you believe… nothing?

    Oh, you’re not that gullible. Well, would you believe the standard mostly doesn’t say what import does? And that this is a good thing?

    ES6 leaves the details of module loading entirely up to the implementation. The rest of module execution is specified in detail.

    Roughly speaking, when you tell the JS engine to run a module, it has to behave as though these four steps are happening:

    1. Parsing: The implementation reads the source code of the module and checks for syntax errors.

    2. Loading: The implementation loads all imported modules (recursively). This is the part that isn’t standardized yet.

    3. Linking: For each newly loaded module, the implementation creates a module scope and fills it with all the bindings declared in that module, including things imported from other modules.

      This is the part where if you try to import {cake} from "paleo", but the “paleo” module doesn’t actually export anything named cake, you’ll get an error. And that’s too bad, because you were so close to actually running some JS code. And having cake!

    4. Run time: Finally, the implementation runs the statements in the body of each newly-loaded module. By this time, import processing is already finished, so when execution reaches a line of code where there’s an import declaration… nothing happens!

    See? I told you the answer was “nothing”. I don’t lie about programming languages.

    But now we get to the fun part of this system. There’s a cool trick. Because the system doesn’t specify how loading works, and because you can figure out all the dependencies ahead of time by looking at the import declarations in the source code, an implementation of ES6 is free to do all the work at compile time and bundle all your modules into a single file to ship them over the network! And tools like webpack actually do this.

    This is a big deal, because loading scripts over the network takes time, and every time you fetch one, you may find that it contains import declarations that require you to load dozens more. A naive loader would require a lot of network round trips. But with webpack, not only can you use ES6 with modules today, you get all the software engineering benefits with no run-time performance hit.

    A detailed specification of module loading in ES6 was originally planned—and built. One reason it isn’t in the final standard is that there wasn’t consensus on how to achieve this bundling feature. I hope someone figures it out, because as we’ll see, module loading really should be standardized. And bundling is too good to give up.

    Static vs. dynamic, or: rules and how to break them

    For a dynamic language, JavaScript has gotten itself a surprisingly static module system.

    • All flavors of import and export are allowed only at toplevel in a module. There are no conditional imports or exports, and you can’t use import in function scope.

    • All exported identifiers must be explicitly exported by name in the source code. You can’t programmatically loop through an array and export a bunch of names in a data-driven way.

    • Module objects are frozen. There is no way to hack a new feature into a module object, polyfill style.

    • All of a module’s dependencies must be loaded, parsed, and linked eagerly, before any module code runs. There’s no syntax for an import that can be loaded lazily, on demand.

    • There is no error recovery for import errors. An app may have hundreds of modules in it, and if anything fails to load or link, nothing runs. You can’t import in a try/catch block. (The upside here is that because the system is so static, webpack can detect those errors for you at compile time.)

    • There is no hook allowing a module to run some code before its dependencies load. This means that modules have no control over how their dependencies are loaded.

    The system is quite nice as long as your needs are static. But you can imagine needing a little hack sometimes, right?

    That’s why whatever module-loading system you use will have a programmatic API to go alongside ES6’s static import/export syntax. For example, webpack includes an API that you can use for “code splitting”, loading some bundles of modules lazily on demand. The same API can help you break most of the other rules listed above.

    The ES6 module syntax is very static, and that’s good—it’s paying off in the form of powerful compile-time tools. But the static syntax was designed to work alongside a rich dynamic, programmatic loader API.

    When can I use ES6 modules?

    To use modules today, you’ll need a compiler such as Traceur or Babel. Earlier in this series, Gastón I. Silva showed how to use Babel and Broccoli to compile ES6 code for the web; building on that article, Gastón has a working example with support for ES6 modules. This post by Axel Rauschmayer contains an example using Babel and webpack.

    The ES6 module system was designed mainly by Dave Herman and Sam Tobin-Hochstadt, who defended the static parts of the system against all comers (including me) through years of controversy. Jon Coppeard is implementing modules in Firefox. Additional work on a JavaScript Loader Standard is underway. Work to add something like <script type=module> to HTML is expected to follow.

    And that’s ES6.

    This has been so much fun that I don’t want it to end. Maybe we should do just one more episode. We could talk about odds and ends in the ES6 spec that weren’t big enough to merit their own article. And maybe a little bit about what the future holds. Please join me next week for the stunning conclusion of ES6 In Depth.

  8. Keyboard events in Firefox OS TV: Part 2

    Implementation details for keyboard events

    In our introductory post, Keyboard events in Firefox OS TV, we described four keyboard event scenarios triggered by the Info key on a Smart TV remote: SYSTEM-ONLY, SYSTEM-FIRST, APP-CANCELLED, AND APP-FIRST. We explained how these keyboard events are activated, described the default sequence of events, and explored the iframe structure in Firefox OS and its embedded browser, in order to understand how developers can implement the four keyboard event scenarios.

    Now we’ll take a closer look at each of the four scenarios, complete with example code for each event-handling scenario.


    If a keyboard event is categorized as SYSTEM-ONLY, then the desired response is defined in mozbrowserbeforekey*’s event handler. Once this key is pressed, the system receives the mozbrowserbeforekey* event before the key* event is dispatched to an app. In addition, the key* events dispatch is cancelled once the system event handler is called. Now, we need to figure out a way to stop the event dispatch. Fig 5 – Keyboard events in Firefox OS TV shows that the keyboard events are dispatched to the system process, then also to the app process. To stop dispatching events to the the embedded page, event.preventDefault() is a straightforward solution. Event.preventDefault() is used to prevent the default action. The defined default action of the mozbrowserbeforekey* event is to dispatch the key* event. For this reason, by calling event.preventDefault() in mozbrowserbeforekey*’s event handler, key* events won’t be dispatched. The final result will be the same as shown in Fig 7.


    [Fig 7]


    This is very similar to the implementation of SYSTEM-ONLY. The only difference — it’s not necessary to call event.preventDefault() in mozbrowserbeforekey*’s event handler. Apps are able to handle the key* event after the system finishes processing it.


    [Fig 8]


    [Fig 9]

    If specific keyboard events are designated for use by apps only, such as those assigned to the four colored keys (shown in Fig 9) on smart TV remotes, then event.preventDefault() will be called in the app’s key* event handler. The event.preventDefault() cannot prevent the mozbrowserafterkey* event from being dispatched to the system, but the property embeddedCancelled of mozbrowserafterkey* will be set to true once the embedded app calls event.preventDefault(). The value of embeddedCancelled lets the system detect whether or not this event has been handled already. If this value is true, the system does nothing.


    [Fig 10]


    The difference between APP-FIRST and APP-CANCELLED is that with APP-FIRST event.preventDefault() will not be called in the app’s event handler. Therefore, the value of embeddedCancelled is false and the system can take over the keyboard event.


    [Fig 11]

    Sample code

    Event handlers
    function handleEvent(event) {
      dump("Receive event '" + event.type + "'.");
      // Handle event here.....
    function handleEventAndPreventDefault(event) {
      dump("Receive event '" + event.type + "'.");
      // Handle event here.....
      // Call preventDefault() to stop the default action.
      // It means that the event is already handled.
    function checkAttrAndHandleEvent(event) {
      dump("Receive event '" + event.type + 
           "' with embeddedCancelled equals to '" +
           event.embeddedCancelled + "'.");
      if (!event.embeddedCancelled) {
        // Do something if the event wasn't being handled before!
        // The following code should be executed in APP-FIRST scenario only!
    • mozbrowser iframe host page
    window.addEventListener('mozbrowserbeforekeydown', handleEventAndPreventDefault);
    window.addEventListener('mozbrowserbeforekeyup', handleEventAndPreventDefault);
    window.addEventListener('mozbrowserafterkeydown', function() { }); // no use
    window.addEventListener('mozbrowserafterkeyup', function() { }); // no use
    • the embedded page
    // This function will never be triggered because the preventDefault() is called in mozbrowserbeforekeyXXX's handler.
    window.addEventListener('keydown', handleEvent);
    window.addEventListener('keyup', handleEvent);
    • Results of keydown related events
    Order the embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeydown Receive event ‘mozbrowserbeforekeydown’.
    2 mozbrowserafterkeydown
    • Results of keyup related events
    Order the embedded page the host page Output
    1 mozbrowserbeforekeyup Receive event ‘mozbrowserbeforekeyup’.
    2 mozbrowserafterkeyup
    • mozbrowser iframe host page
    window.addEventListener('mozbrowserbeforekeydown', handleEvent);
    window.addEventListener('mozbrowserbeforekeyup', handleEvent);
    window.addEventListener('mozbrowserafterkeydown', function() { }); // no use
    window.addEventListener('mozbrowserafterkeyup', function() { }); // no use
    • the embedded page
    window.addEventListener('keydown', handleEvent);
    window.addEventListener('keyup', handleEvent);
    • Received results of keydown related events
    Order mozbrowser-embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeydown Receive event ‘mozbrowserbeforekeydown’.
    2 keydown Receive event ‘keydown’.
    3 mozbrowserafterkeydown
    • Received results of keyup related events
    Order the embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeyup Receive event ‘mozbrowserbeforekeyup’.
    2 keyup Receive event ‘keyup’.
    3 mozbrowserafterkeyup Receive event ‘mozbrowserafterkeyup’ with embeddedCancelled equals to ‘true’.
    • mozbrowser iframe host page
    window.addEventListener('mozbrowserbeforekeydown', function() { }); // no use
    window.addEventListener('mozbrowserbeforekeyup', function() { }); // no use
    window.addEventListener('mozbrowserafterkeydown', checkAttrAndHandleEvent);
    window.addEventListener('mozbrowserafterkeyup', checkAttrAndHandleEvent);
    • mozbrowser iframe embedded page
    window.addEventListener('keydown', handleEventAndPreventDefault);
    window.addEventListener('keyup', handleEventAndPreventDefault);
    • Received results of keydown related events
    Order the embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeydown
    2 keydown Receive event ‘keydown’.
    3 mozbrowserafterkeydown Receive event ‘mozbrowserafterkeydown’ with embeddedCancelled equals to ‘true’.
    • Received results of keyup related events
    Order mozbrowser-embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeyup
    2 keyup Receive event ‘keyup’.
    3 mozbrowserafterkeyup Receive event ‘mozbrowserafterkeyup’ with embeddedCancelled equals to ‘true’.
    • mozbrowser iframe host page
    window.addEventListener('mozbrowserbeforekeydown', function() { }); // no use
    window.addEventListener('mozbrowserbeforekeyup', function() { }); // no use
    // This will be trigger after keydown event is
    // dispatched to mozbrowser iframe embedded page
    window.addEventListener('mozbrowserafterkeydown', checkAttrAndHandleEvent);
    window.addEventListener('mozbrowserafterkeyup', checkAttrAndHandleEvent);
    • mozbrowser iframe embedded page
    window.addEventListener('keydown', handleEvent);
    window.addEventListener('keyup', handleEvent);
    • Received results of keydown related events
    Order mozbrowser-embedded page mozbrowser-iframe host page Output
    1 mozbrowserbeforekeydown
    2 keydown Receive event ‘keydown’.
    3 mozbrowserafterkeydown Receive event ‘mozbrowserafterkeydown’ with embeddedCancelled equals to ‘false’.
    • Received results of keyup related events
    Order mozbrowser-embedded page mozbrowser iframe host page Output
    1 mozbrowserbeforekeyup
    2 keyup Receive event ‘keyup’.
    3 mozbrowserafterkeyup Receive event ‘mozbrowserafterkeyup’ with embeddedCancelled equals to ‘false’.




  9. Trainspotting: Firefox 40

    Trainspotting is a series of articles highlighting features in the lastest version of Firefox. A new version of Firefox is shipped every six weeks – we at Mozilla call this pattern “release trains.”

    Firefox keeps on shippin' shippin' shippin' /
    Into the future…
    —Steve Miller Band, probably

    Like a big ol’ jet airliner, a new version of Firefox has been cleared for takeoff! Let’s take a look at some of the snazzy new things in store for both users and developers.

    For a full list of changes and additions, take a look at the Firefox 40 release notes.

    Developer Tools

    Find what you’re looking for in the Inspector, but don’t know where it is on the page? You can now scroll an element into view via the Markup View in the Inspector:


    Sift through complex stylesheets more easily by filtering CSS rules:

    You can now toggle how colors are represented by Shift+clicking on them in the Rules view:


    The Web Console will now warn of code that is unreachable because it comes after a return statement:


    The Developer Tools have also gained a powerful new set of Performance analysis tools, which are demonstrated along with all the other Firefox 40 Developer Tools changes in this in-depth blog post.

    Signed Add-ons


    Malicious extensions are a growing problem in all browsers. Because Firefox Add-ons have tremendous power, there needs to be a better way to protect users from malicious code running wild. Starting in Firefox 42, all Firefox add-ons will be required to be signed in order to be able to be installed by end-users. In Firefox 40, users will be warned about un-signed extensions, but can opt to install them anyway. You can read more about why extension signing is needed, and also check out the overall plan for the roll-out of signed extensions.

    Event offsetX and offsetY

    Sometimes a good idea is a good idea, even if it takes 14 years! Firefox now supports the offsetX and offsetY properties for MouseEvents. This makes it much easier for code to track mouse events on an element within a page, without needing to know where in the page the element is. As always, perform capability checks to ensure that your code works across browsers:

    el.addEventListener(function (e) {
      var x, y;
      if ('offsetX' in e) {
         x = e.offsetX;
         y = e.offsetY;
      } else {
        // addition needed for every offsetParent up the chain
        x = e.clientX + /* ... */;
        y = e.clientY + /* ... */;
      addGlitterMouseTrails(x, y);

    But Wait, There’s More!

    Every new version of Firefox has dozens of bug fixes and changes to make browsing and web development better- I’ve only touched upon a few. Finally, it’s well worth noting that 55 developers contributed their first code change to Firefox in this release, and 49 of them were brand new volunteers. Shipping would not be the same without these awesome contributions! Thank you!

    For all the rest of the details, check out the Developer Release Notes or even the full list of fixed bugs. Happy Browsing!

  10. ES6 In Depth: Subclassing

    ES6 In Depth is a series on new features being added to the JavaScript programming language in the 6th Edition of the ECMAScript standard, ES6 for short.

    Two weeks ago, we described the new classes system added in ES6 for handling trivial cases of object constructor creation. We showed how you can use it to write code that looks like this:

    class Circle {
        constructor(radius) {
            this.radius = radius;
        static draw(circle, canvas) {
            // Canvas drawing code
        static get circlesMade() {
            return !this._count ? 0 : this._count;
        static set circlesMade(val) {
            this._count = val;
        area() {
            return Math.pow(this.radius, 2) * Math.PI;
        get radius() {
            return this._radius;
        set radius(radius) {
            if (!Number.isInteger(radius))
                throw new Error("Circle radius must be an integer.");
            this._radius = radius;

    Unfortunately, as some people pointed out, there wasn’t time to talk then about the rest of the power of classes in ES6. Like traditional class systems (C++ or Java, for example), ES6 allows for inheritance, where one class uses another as a base, and then extends it by adding more features of its own. Let’s take a closer look at the possibilities of this new feature.

    Before we get started talking about subclassing, it will be useful to spend a moment reviewing property inheritance and the dynamic prototype chain.

    JavaScript Inheritance

    When we create an object, we get the chance to put properties on it, but it also inherits the properties of its prototype objects. JavaScript programmers will be familiar with the existing Object.create API which allows us to do this easily:

    var proto = {
        value: 4,
        method() { return 14; }
    var obj = Object.create(proto);
    obj.value; // 4
    obj.method(); // 14

    Further, when we add properties to obj with the same name as ones on proto, the properties on obj shadow those on proto.

    obj.value = 5;
    obj.value; // 5
    proto.value; // 4

    Basic Subclassing

    With this in mind, we can now see how we should hook up the prototype chains of the objects created by a class. Recall that when we create a class, we make a new function corresponding to the constructor method in the class definition which holds all the static methods. We also create an object to be the prototype property of that created function, which will hold all the instance methods. To create a new class which inherits all the static properties, we will have to make the new function object inherit from the function object of the superclass. Similarly, we will have to make the prototype object of the new function inherit from the prototype object of the superclass, for the instance methods.

    That description is very dense. Let’s try an example, showing how we could hook this up without new syntax, and then adding a trivial extension to make things more aesthetically pleasing.

    Continuing with our previous example, suppose we have a class Shape that we want to subclass:

    class Shape {
        get color() {
            return this._color;
        set color(c) {
            this._color = parseColorAsRGB(c);
            this.markChanged();  // repaint the canvas later

    When we try to write code that does this, we have the same problem we had in the previous post with static properties: there’s no syntactic way to change the prototype of a function as you define it. While you can get around this with Object.setPrototypeOf, the approach is generally less performant and less optimizable for engines than having a way to create a function with the intended prototype.

    class Circle {
        // As above
    // Hook up the instance properties
    Object.setPrototypeOf(Circle.prototype, Shape.prototype);
    // Hook up the static properties
    Object.setPrototypeOf(Circle, Shape);

    This is pretty ugly. We added the classes syntax so that we could encapsulate all of the logic about how the final object would look in one place, rather than having other “hooking things up” logic afterwards. Java, Ruby, and other object-oriented languages have a way of declaring that a class declaration is a subclass of another, and we should too. We use the keyword extends, so we can write:

    class Circle extends Shape {
        // As above

    You can put any expression you want after extends, as long as it’s a valid constructor with a prototype property. For example:

    • Another class
    • Class-like functions from existing inheritance frameworks
    • A normal function
    • A variable that contains a function or class
    • A property access on an object
    • A function call

    You can even use null, if you don’t want instances to inherit from Object.prototype.

    Super Properties

    So we can make subclasses, and we can inherit properties, and sometimes our methods will even shadow (think override) the methods we inherit. But what if you want to circumvent this shadowing mechanic?

    Suppose we want to write a subclass of our Circle class that handles scaling the circle by some factor. To do this, we could write the following somewhat contrived class:

    class ScalableCircle extends Circle {
        get radius() {
            return this.scalingFactor * super.radius;
        set radius() {
            throw new Error("ScalableCircle radius is constant." +
                            "Set scaling factor instead.");
        // Code to handle scalingFactor

    Notice that the radius getter uses super.radius. This new super keyword allows us to bypass our own properties, and look for the property starting with our prototype, thus bypassing any shadowing we may have done.

    Super property accesses (super[expr] works too, by the way) can be used in any function defined with method definition syntax. While these functions can be pulled off of the original object, the accesses are tied to the object on which the method was first defined. This means that pulling the method off into a local variable will not change the behavior of the super access.

    var obj = {
        toString() {
            return "MyObject: " + super.toString();
    obj.toString(); // MyObject: [object Object]
    var a = obj.toString;
    a(); // MyObject: [object Object]

    Subclassing Builtins

    Another thing you might want to do is write extensions to the JavaScript language builtins. The builtin data structures add a huge amount of power to the language, and being able to create new types that leverage that power is amazingly useful, and was a foundational part of the design of subclassing. Suppose you want to write a versioned array. (I know. Trust me, I know.) You should be able to make changes and then commit them, or roll back to previously committed changes. One way to write a quick version of this is by subclassing Array.

    class VersionedArray extends Array {
        constructor() {
            this.history = [[]];
        commit() {
            // Save changes to history.
        revert() {
            this.splice(0, this.length, this.history[this.history.length - 1]);

    Instances of VersionedArray retain a few important properties. They’re bonafide instances of Array, complete with map, filter, and sort. Array.isArray() will treat them like arrays, and they will even get the auto-updating array length property. Even further, functions that would return a new array (like Array.prototype.slice()) will return a VersionedArray!

    Derived Class Constructors

    You may have noticed the super() in the constructor method of that last example. What gives?

    In traditional class models, constructors are used to initalize any internal state for instances of the class. Each consecutive subclass is responsible for initializing the state associated with that specific subclass. We want to chain these calls, so that subclasses share the same initialization code with the class they are extending.

    To call a super constructor, we use the super keyword again, this time as if it were a function. This syntax is only valid inside constructor methods of classes that use extends. With super, we can rewrite our Shape class.

    class Shape {
        constructor(color) {
            this._color = color;
    class Circle extends Shape {
        constructor(color, radius) {
            this.radius = radius;
        // As from above

    In JavaScript, we tend to write constructors that operate on the this object, installing properties and initializing internal state. Normally, the this object is created when we invoke the constructor with new, as if with Object.create() on the constructor’s prototype property. However, some builtins have different internal object layouts. Arrays, for example, are laid out differently than regular objects in memory. Because we want to be able to subclass builtins, we let the basemost constructor allocate the this object. If it’s a builtin, we will get the object layout we want, and if it’s a normal constructor, we will get the default this object we expect.

    Probably the strangest consequence is the way this is bound in subclass constructors. Until we run the base constructor, and allow it to allocate the this object, we don’t have a this value. Consequently, all accesses to this in subclass constructors that occur before the call to the super constructor will result in a ReferenceError.

    As we saw in the last post, where you could omit the constructor method, derived class constructors can be omitted, and it is as if you had written:

    constructor(...args) {

    Sometimes, constructors do not interact with the this object. Instead, they create an object some other way, initalize it, and return it directly. If this is the case, it is not necessary to use super. Any constructor may directly return an object, independent of whether super constructors were ever invoked.

    Another strange side effect of having the basemost class allocate the this object is that sometimes the basemost class doesn’t know what kind of object to allocate. Suppose you were writing an object framework library, and you wanted a base class Collection, some subclasses of which were arrays, and some of which were maps. Then, by the time you ran the Collection constructor, you wouldn’t be able to tell which kind of object to make!

    Since we’re able to subclass builtins, when we run the builtin constructor, internally we already have to know about the prototype of the original class. Without it, we wouldn’t be able to create an object with the proper instance methods. To combat this strange Collection case, we’ve added syntax to expose that information to JavaScript code. We’ve added a new Meta Property, which corresponds to the constructor that was directly invoked with new. Calling a function with new sets to be the called function, and calling super within that function forwards the value.

    This is hard to understand, so I’ll just show you what I mean:

    class foo {
        constructor() {
    class bar extends foo {
        // This is included explicitly for clarity. It is not necessary
        // to get these results.
        constructor() {
    // foo directly invoked, so is foo
    new foo(); // foo
    // 1) bar directly invoked, so is bar
    // 2) bar invokes foo via super(), so is still bar
    new bar(); // bar

    We’ve solved the problem with Collection described above, because the Collection constructor can just check and use it to derive the class lineage, and determine which builtin to use. is valid inside any function, and if the function is not invoked with new, it will be set to undefined.

    The Best of Both Worlds

    Hope you’ve survived this brain dump of new features. Thanks for hanging on. Let’s now take a moment to talk about whether they solve problems well. Many people have been quite outspoken about whether inheritance is even a good thing to codify in a language feature. You may believe that inheritance is never as good as composition for making objects, or that the cleanliness of new syntax isn’t worth the resulting lack of design flexibility, as compared with the old prototypal model. It’s undeniable that mixins have become a dominant idiom for creating objects that share code in an extensible way, and for good reason: They provide an easy way to share unrelated code to the same object without having to understand how those two unrelated pieces should fit into the same inheritance structure.

    There are many vehemently held beliefs on this topic, but I think there are a few things worth noting. First, the addition of classes as a language feature does not make their use mandatory. Second, and equally important, the addition of classes as a language feature doesn’t mean they are always the best way to solve inheritance problems! In fact, some problems are better suited to modeling with prototypal inheritance. At the end of the day, classes are just another tool that you can use; not the only tool nor necessarily the best.

    If you want to continue to use mixins, you may wish that you could reach for classes that inherit from several things, so that you could just inherit from each mixin, and have everything be great. Unfortunately, it would be quite jarring to change the inheritance model now, so JavaScript does not implement multiple inheritance for classes. That being said, there is a hybrid solution to allow mixins inside a class-based framework. Consider the following functions, based on the well-known extend mixin idiom.

    function mix(...mixins) {
        class Mix {}
        // Programmatically add all the methods and accessors
        // of the mixins to class Mix.
        for (let mixin of mixins) {
            copyProperties(Mix, mixin);
            copyProperties(Mix.prototype, mixin.prototype);
        return Mix;
    function copyProperties(target, source) {
        for (let key of Reflect.ownKeys(source)) {
            if (key !== "constructor" && key !== "prototype" && key !== "name") {
                let desc = Object.getOwnPropertyDescriptor(source, key);
                Object.defineProperty(target, key, desc);

    We can now use this function mix to create a composed superclass, without ever having to create an explicit inheritance relationship between the various mixins. Imagine writing a collaborative editing tool in which editing actions are logged, and their content needs to be serialized. You can use the mix function to write a class DistributedEdit:

    class DistributedEdit extends mix(Loggable, Serializable) {
        // Event methods

    It’s the best of both worlds. It’s also easy to see how to extend this model to handle mixin classes that themselves have superclasses: we can simply pass the superclass to mix and have the return class extend it.

    Current Availability

    OK, we’ve talked a lot about subclassing builtins and all these new things, but can you use any of it now?

    Well, sort of. Of the major browser vendors, Chrome has shipped the most of what we’ve talked about today. When in strict mode, you should be able to do just about everything we discussed, except subclass Array. Other builtin types will work, but Array poses some extra challenges, so it’s not surprising that it’s not finished yet. I am writing the implementation for Firefox, and aim to hit the same target (everything but Array) very soon. Check out bug 1141863 for more information, but it should land in the Nightly version of Firefox in a few weeks.

    Further off, Edge has support for super, but not for subclassing builtins, and Safari does not support any of this functionality.

    Transpilers are at a disadvantage here. While they are able to create classes, and to do super, there’s basically no way to fake subclassing builtins, because you need engine support to get instances of the base class back from builtin methods (think Array.prototype.splice).

    Phew! That was a long one. Next week, Jason Orendorff will be back to discuss the ES6 module system.