desalasworks presents:

a selection of works by steven de salas

How to Build a Cross-Platform Metal Detector App


Did you know that your smartphone has an in-built magnetometer to power its compass?

The earth’s magnetic field is big but not particularly strong, and that means that the magnetometer in your phone is sensitive enough to detect small ferrous metal objects such as a coin, a large nail or a piece of cutlery which also generate small magnetic fields of their own. In other words YOU DONT HAVE TO GO AND BUY A METAL DETECTOR, you’ve ALREADY GOT ONE INSIDE YOUR PHONE! We just have to unlock its potential.

Do you want to try a bit of hacking?

In this post, I’m going to show you how you can build your own cross-platform metal detector app for BOTH  iPhone and Android phones, this app will interface with the magnetometer hardware in your phone and notify you of any ferrous metal or magnetic objects nearby.

This should take from 30 mins to 2 hrs of your time for either platform and depending on the speed of your internet connection, perhaps longer if you have bad luck and something doesn’t work straight away.

Introducing Cordova (or PhoneGap, for Adobe fans)

The Apache Cordova project is an open source initiative aimed at bundling a HTML page (ie `index.html` – such as the one below) into a native app that will work across just about every smartphone device. This is great news because it means that with you only need to code your app once and it works on multiple devices!

# index.html

<head><title>My First App</title></head>
<button onclick=”alert(‘World’)”>Hello</button>

PhoneGap is the same thing, but with a snazzier name trademarked by Adobe (the Photoshop people) and with an online service that gives you a wizard-like interface for building apps so you dont have to deal with obscure command line errors, but then you have to pay Adobe for the favour.

So what does this ‘Cordova’ thingy do for me?

Using Cordova means that you don’t have to know how to setup a mobile app project because its all done for you! Cordova will basically generate all the folders and files you need to get started with a basic HTML ‘Hello World’ app for each platform.

Cordova also has a powerful ‘plugin’ framework that allows access to the phone’s hardware, in this post we will be downloading 2 different plugins to unlock the Magnetometer and Vibration functionality inside your phone.

To get started with a Cordova app you need to install NodeJS (http://nodejs.org) which is a powerful javascript command line tool that Cordova uses to do most of the donkey-work for setting up your project.

The Code

Go have a look at the following ‘index.html‘ file in Github, it contains all the code you need for a basic magnetometer app:


Save it to your computer:

1. Right-click on ‘Raw‘ button
2. Choose ‘Save link as..’ on the context menu
3. Put it somewhere sensible where you can find it

Hey, this is HTML right?

Correct, that means your browser can open it.

Double click to open it on a browser. You should get something like this:

Err.. Why?

Well, your mobile has a magnetometer in it your PC doesn’t. So all you’ll get out of it in a desktop browser is a ZERO reading.

And thats ok, it just means everything is working as expected.


The Android App

What You’ll need:

1. A PC or Mac with NodeJS, Java (JDK7) and Android Studio installed.
2. An Android (4.0+) device and USB cable for testing.
3. Basic understanding of a command line interface (you can navigate between folders).

CORPORATE FIREWALLS: Note that the steps below may not work behind a corporate firewall, you may need extra steps to set up internet access via a corporate proxy.

You’ve already installed NodeJS right? If its a old version (older than 0.11) you might want to download the latest and re-install it.

Ok, next you will need Java JDK 7 installed and Android Studio, these are needed to create a native app for Android. You can get them on the following links:

JDK7: http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html -> Remember x86=32bit, x64=64bit

Android Studio: https://developer.android.com/sdk/index.html

BEWARE: after installing/running Android Studio, it will want to dowload the latest Android SDKs (the bits of functionality that enable your PC to talk to the phone hardware), this can take anywhere from 10 minutes and up to a couple of hours over a slow internet connection.

Installing Cordova

We are now going to install Cordova and create an Android project.

Open up a command line window and type :

> npm install -g cordova

Creating the Android Project

Next, navigate to your projects folder and create a new cordova project with android support:

> cordova create magnetometer my.magnetometer magnetometer
> cd magnetometer
> cordova platform add android

If all goes well, this should have created a bunch of files and folders (ie a project structure) within your ‘magnetometer’ project folder, you should be able to see them using a file explorer:

Copying our Metal Detector Code

The folder ‘www‘ contains our web code. Here is where we’ll be placing the index.html file that we downloaded before.

Building the App

Go back to the command line and run an extra couple of commands to ‘unlock’ the vibration and magnetometer functionality on your phone.

> cordova plugin add cordova-plugin-vibration
> cordova plugin add cordova-plugin-magnetometer

Next we are going to ‘build’ the app, so you can use it on your android phone. We need one more cordova command to ‘prepare’ the app:

> cordova prepare

Then follow these instructions:

1. Open Android Studio
2. On the Welcome Prompt. Select ‘Import Project’
3. Click on the ‘platforms\android’ folder inside your project.
4. Press ‘OK’.
5. Press ‘OK’ again if you get an extra prompt about Gradle settings.
6. Wait for the progress bar to finish.

This will create an android project, next we will build the APK file that can be used to run the magnetometer app on your phone.

Make sure you’ve waited until all the progress bars are finished before the next steps.

1. Android Studio will need to get all required Java libraries that the project depends on, this usually takes some time. Did you really wait until all progress bars and loading messages were finished?
2. Ok, click on the ‘Play’ button at the top menu as per screen below.
3. This will trigger a build using Gradle. You can see the progress on the ‘Gradle Console’ window.
4. When ready, a popup will appear asking if you want to run the emulator. DO NOT press OK here. It will grind your computer to a halt. Just press ‘Cancel’ on the popup.

Installing the App on your Android Phone

Next we are going to email ourselves the completed app.

1. Navigate to your ‘magnetometer’ project folder using the file explorer.
2. Click ‘platforms’ -> ‘android’ -> ‘build’ -> ‘outputs’ -> ‘apk’
3. Find the file ‘android-debug.apk

4. Next. Email it to yourself.
5. Go to your Android phone.
6. Open ‘Settings’ > ‘Security’
7. Turn ON ‘Unknown Sources’ (you can turn it back OFF later)
8. Open your email, click on the attached ‘APK’ file.
9. You should then get an installation screen as per below.

10. Go to your Apps and look for ‘magnetometer’, then open it.
11. READY! Start finding some metal objects!

The iOS App

What You’ll need:

– A newish Mac with NodeJS and XCode installed
– An Apple Developer membership (US$99)
– An iPhone (4.0+) and USB cable for testing.
– Basic understanding of a command line interface (you can navigate between folders).

Installing Cordova

1. Make sure you have NodeJS installed and its at a reasonably new version (0.12+).
2. If you need to update the version of node. Do it following this article.
3. Open a terminal window (Look for the ‘Terminal’ app in Launchpad)
4. Type the following command to install Cordova :

$ sudo npm install -g cordova

5. You’ll be prompted to enter a password for your user account (on the mac).
6. There will probably be some warnings when installing Cordova (“WARN”), you can safely ignore these. If you get any “ERROR”s they are a bit more of a problem and will need attention before continuing. Type the errors into Google for help.

Creating the iOS Project

Once Cordova has been installed, we are going to create the project structure for our magnetometer project. Firstly, navigate to your projects folder. The commands to use are :

$ cordova create magnetometer my.magnetometer magnetometer
$ cd magnetometer
$ cordova platform add ios
$ cordova plugin add cordova-plugin-vibration
$ cordova plugin add cordova-plugin-magnetometer

Copying our Metal Detector App Code

This should have created your project structure as per following screenshot. Make sure you delete the contents of the ‘www’ folder and replace them with the ‘index.html’ file you downloaded earlier.

You will then need to run the following command on your ‘magnetometer’ folder to ‘prepare’ the files (it will copy the ‘index.html’ to the XCode build folder).

$ cordova prepare

Building the iOS App

And now we can continue to XCode by finding the file ‘magnetometer.xcodeproj’ inside our project folder (look inside the sub-folder ‘platforms’ and ‘ios’). Just double-click on it and it will open (if you have XCode installed).

Make sure you are signed in as an apple developer. Joining costs US$99. You cannot drop an app on your iOS device without one of these:

1. Go to ‘XCode’ (on the top menu)
2. Select ‘Preferences…’, then ‘Accounts’
3. Click the plus (+) sign to add an account
4. Enter your Apple Developer email and password

Running the App on your iPhone

Then just go and run the app! This bit is even simpler than for Android!

1. Connect your iPhone to your Mac using the USB cable.
2. Select your iPhone as the device ‘target’ on the top menu
3. Press the ‘Play’ button to run the build.

And VOILA! Now start finding some metal with your iPhone!

And thats IT!

That concludes my post for today, thanks for taking an interest. I hope you didn’t have to bang your head against the keyboard too much to get this working.

Please feel free to write your comments and experiences below.

If you liked it then you are welcome to shout me a beer or help me out by sharing it with your friends and colleagues.


Tasmanian Convict Finder

Browse and search through detailed records of 85,000 convicts transported to Tasmania and those convicted locally through the convict system (1803-1893).


This app uses publicly available data from data.gov.au provided by LINC and the Department of Education in Tasmania, as well as newspaper articles sourced from the National Library of Australia.

It is cross-platform and available in both iPhone and Android version.



Research your Ancestry

The app provides a useful reference for anyone seeking to find out more about the convict history of this beautiful island in Australia, and is a handy pocket guide for people researching their ancestry.


Initially built as an entry into Govhack 2015, this app was completed though unfortunately not in time to join the prize lists.


AutoTrader UK

This was a role working on Interface Development for Trader Media’s online marketplace with over 1 billion page impressions per month.


I mainly focused on creating rich interfaces and user journeys with Event-driven and Object-Oriented JavaScript, DOM, HTML5, jQuery, AJAX, JSON and Backbone.js MVC framework continuously tested with Jasmine/Rhino.


Our team used Agile development approach with Continuous Integration (CI), pair-programming and short development cycles based on Thoughtworks Extreme Programming (XP) model for Java/JUnit TDD projects.


MBTA mTicket

A 6 month cross-platform (iOS and Android) mobile app project for Masabi, a specialist mTicketing company based near London Bridge. The MBTA mTicket application was launched in the Android and iPhone app markets in November 2012, and was a complete success grossing $1 million of sold tickets in the Boston Rail network for the first ten weeks of operation.

MBTA mTicket in the Google Play store.

MBTA mTicket in the Google Play store.

Developing the Front End User Experience

The project focused on building the User Interface for the application, this was done as a HTML5 /JavaScript UI due to the inherent advantages in being able to share code across different platforms. During development, we regularly tested against Android, iOS, BlackBerry, Symbian WebOS and Windows Mobile.

MBTA mTicket Payment Walkthrough

The process for purchasing tickets is straight forward, no complicated steps or black magic.

To purchase a round trip ticket to/from Boston the steps are:

1) Click ‘Buy Tickets’
2) Choose a ‘From’ and ‘To’ Station from the list.
3) Choose a ‘Round Trip’ ticket
4) Choose an existing card (the app stores a payment key after the first time).
5) Enter the Card CVV


6) Review your purchase
7) Press ‘Pay Now’
8) DONE!


Once the transaction goes through, the ticket is loaded into the My Tickets section. You now have two round trips to Boston waiting for you.

These last few panels are a shot of your ticket. The rectangle at the top shows the current time, the time block moves back and forth, and the three blocks are multi-colored. The color scheme changes on a daily basis according to a scheme that the conductor knows. Once the ticket is used it changes into a grey scheme. The conductor when he approaches will generally glance at the screen, nod with a “thanks” and put a paper slip in the holder above your head. Done.


MBTA mTicket in the News

These are some of the online articles that were published before and after launch:

Apr 23rd, 2012

MBTA and Masabi team up for first smartphone rail ticketing system in the US, launching in Boston this fall

Jul 12th, 2012

New York MTA announces smartphone-based ticketing trials aboard Metro-North Railroad

July 11, 2012

New Smartphone App Could Replace Railroad Tickets

Nov 12, 2012

MBTA & Masabi Launch US` First Smartphone Commuter Rail Ticketing System

Jul 11, 2012

Digital train tickets to replace paper for some NY commuters

November 27, 2012

mTicket: MBTA releases mobile ticketing app at South Station

December 04, 2012

mTicket: T reports $225,000 in mobile app ticket sales since November launch

January 15, 2013

mTicket sees $1 million in commuter line sales

November 21st, 2012

What Bostonians Are Thankful For

November 27th, 2012

Mobile Ticketing Coming to MBTA Boats & More Commuter Trains, Riders Can Purchase Monthly Passes

Customer Respose

The MBTA mTicketing app went on to become a huge success shortly after launch. Customers reported a ‘seamless experience’ with ‘great usability’, ‘very convenient’, ‘neat & efficient’ etc.

These are some of the responses the MBTA customers tweeted shortly after launch:

25 Techniques for Javascript Performance Optimization

These are some the techniques I use for enhancing the performance of JavaScript, they have mostly been collected over various years of using the language to improve the interactivity of websites and web applications.

My thanks go out to Marco of zingzing.co.uk for reminding me of the importance of optimizing JavaScript, and for teaching me some of the techniques below.

Most of the techniques involve common sense once you have understood the underlying problem. I’ve categorised them into 5 broad categories, each with an underlying problem and solution as follows:

1. Avoid interaction with host objects

Watch out for these guys. Repeated interaction with them will kill your performance.


Native JavaScript is compiled into machine code by most scripting engines offering incredible performance boost, however interaction with host (browser) objects outside the javascript native environment raises unpredictability and considerable performance lag, particularly when dealing with screen-rendered DOM objects or objects which cause Disk I/O (such as WebSQL).


You can’t really get away from them, but keep your interaction with host objects to an absolute minimum.


  1. Use CSS classes instead of JavaScript for DOM animation.

    Its a good habit to try an implement any animation (or DOM interaction) with CSS if you can get away with it. CSS3 Transitions have been around for a while now, so there are few excuses not to use them. You can even use a polyfill if you are worried about older browsers. Think also of hover menus using the :hover pseudo-class, or styling and display of elements using @keyframes, :before and :after, this is because unlike JavaScript, CSS solutions are heavily optimized by the browser, often down to the level of using the GPU for extra processing power.

    I realize this might sound like irony (if you want to optimize JavaScript – avoid using it for animation), but the reality is that this technique is executed from within your JavaScript code, it just involves putting more effort on the CSS classes.

  2. Use fast DOM traversal with document.getElementById().

    Given the availability of jQuery, it is now easier than ever to produce highly specific selectors based on a combination of tag names, classes and CSS3. You need to be aware that this approach involves several iterations while jQuery loops thorough each subset of DOM elements and tries to find a match. You can improve DOM traversal speeds by picking nodes by ID.

    // jQuery will need to iterate many times until it finds the right element
    var button = jQuery('body div.dialog > div.close-button:nth-child(2)')[0];
    // A far more optimized way is to skip jQuery altogether.
    var button = document.getElementById('dialog-close-button');
    // But if you need to use jQuery you can do it this way.
    var button = jQuery('#dialog-close-button')[0];
  3. Store pointer references to in-browser objects.

    Use this technique to reduce DOM traversal trips by storing references to browser objects during instantiation for later usage. For example, if you are not expecting your DOM to change you should store a reference to DOM or jQuery objects you are going to use when your page is created; if you are building a DOM structure such as a dialog window, make sure you store a few handy reference to DOM objects inside it during instantiation, so you dont need to find the same DOM object over an over again when a user clicks on something or drags the dialog window.

    If you haven’t stored a reference to a DOM object, and you need to iterate inside a function, you can create a local variable containing a reference to that DOM object, this will considerably speed up the iteration as the local variable is stored in the most accessible part of the stack.

  4. Keep your HTML super-lean (get rid of all those useless DIV and SPAN tags)

    This is extremely important, the time needed to query and modify DOM is directly proportional the the amount and complexity of HTML that needs to be rendered.

    Using half the amount of HTML will roughly double the DOM speed, and since DOM creates the greatest performance drag on any complex JavaScript app, this can produce a considerable improvement. See ‘Reduce Number of DOM Elements’ guidance in Yahoo YSlow.

  5. Batch your DOM changes, especially when updating styles.

    When making calls to modify DOM make sure you batch them up so as to avoid repeated screen rendering, for example when applying styling changes. The ideal approach here is to make many styling changes in one go by adding or removing a class, rather than apply each individual style separately. This is because every DOM change prompts the browser to re-render the whole UI using the boxing model. If you need to move an item across the page using X+Y coordinates, make sure that these two are applied at the same time rather than separately. See these examples in jQuery:

    // This will incurr 5 screen refreshes
    jQuery('#dialog-window').width(600).height(400).css('position': 'absolute')
                           .css('top', '200px').css('left', '200px');
    // Let jQuery handle the batching
         width: '600px',
         height: '400px',
         position: 'absolute',
         top: '200px',
         left: '200px'
    // Or even better use a CSS class.
  6. Build DOM separately before adding it to the page.

    As per the last item, every DOM update requires the whole screen to be refreshed, you can minimize the impact here by building DOM for your widget ‘off-line’ and then appending your DOM structure in one go.

  7. Use buffered DOM inside scrollable DIVs.

    This is an extension of the fourth point above (Keep HTML super-lean), you can use this technique to remove items from DOM that are not being visually rendered on screen, such as the area outside the viewport of a scrollable DIV, and append the nodes again when they are needed. This will reduce memory usage and DOM traversal speeds. Using this technique the guys at ExtJS have managed to produce an infinitely scrollable grid that doesn’t grind the browser down to a halt.

2. Manage and Actively reduce your Dependencies

Poorly managed JavaScript dependencies degrade user experience.


On-screen visual rendering and user experience is usually delayed while waiting for script dependencies load onto the browser. This is particularly bad for mobile users who have limited bandwidth capacity.


Actively manage and reduce dependency payload in your code.


  1. Write code that reduces library dependencies to an absolute minimum.

    Use this approach to reduce the number of libraries your code requires to a minimum, ideally to none, thus creating an incredible boost to the loading times required for your page.

    You can reduce dependency on external libraries by making use of as much in-browser technology as you can, for example you can use document.getElementById('nodeId') instead of jQuery('#nodeId'), or document.getElementsByTagName('INPUT') instead of jQuery('INPUT') which will allow you to get rid of jQuery library dependency.

    If you need complex CSS selectors use Sizzle.js instead of jQuery, which is far more lightweight (4kb instead of 80kb+).

    Also, before adding any new library to the codebase, evaluate whether or you really need it. Perhaps you are just after 1 single feature in the whole library? If that’s the case then take the code apart and add the feature separately (but don’t forget to check the license and acknowledge author if necessary).

  2. Minimize and combine your code into modules.

    You can bundle distinct components of your application into combined *.js files and pass them through a javascript minimizer tool such as Google Closures or JsMin that gets rid of comments and whitespacing.

    The logic here is that a single minimized request for a 10Kb .js file completes faster than 10 requests for files that are 1-2kb each due to lower bandwidth usage and network latency.

  3. Use a post-load dependency manager for your libraries and modules.

    Much of your functionality will not need to be implemented until after the page loads. By using a dependency manager (such as RequireJS or Webpack) to load your scripts after the page has completed rendering you are giving the user a few extra seconds to familiarise themselves with the layout and options before them.

    Make sure that your dependency manager can ‘remember’ which dependencies have been loaded so you dont end up loading the same libraries twice for each module. See guidance for Pre-Loading and Post-loading in Yahoo YSLow, and be mindful about loading only what is necessary at each stage of the user journey.

  4. Maximise use of caching (eTags, .js files, etc).

    Cache is your best friend when it comes to loading pages faster. Try to maximise the use of cache by applying ETags liberally and putting all your javascript into files ending in *.js found in static URI locations (avoid dynamic Java/C# bundle generations ending with *.jsp and *.ashx) . This will tell the browser to use the locally cached copy of your scripts for any pages loaded after the initial one.

  5. Move scripts to the end of the page (not recommended).

    This is the lazy way of handling post-load dependencies, ideally you should implement a post-load dependency manager, but if you only have one or two scripts to load into the page you can add them at the very end of the HTML document where the browser will start loading them after the page is rendered, giving the user a few extra seconds of interaction.

3. Be disciplined with event binding

Be a ninja when using event handling.


Browser and custom event handlers are an incredible tool for improving user experience and reducing the depth of the call stack (so you avoid having a function calling a function which calls another function etc), but since they are hard to track due to their ‘hidden’ execution they can fire many times repeatedly and quickly get out of hand, causing performance degradation.


Be mindful and disciplined when creating event handlers. Get to know your weapons too, if you are using a framework then find out what’s going on underneath the hood.


  1. Use event binding but do it carefully.

    Event binding is great for creating responsive applications. However, it is important that you walk through the execution and various user journeys to make sure they are not firing multiple times or using up unnecessary resources behind the scenes. Comment your code well so they next guy (which may be you a few months down the line) can follow what’s going on and avoid this issue as well.

    If you are using AngularJS make sure you are getting rid of unnecessary ‘watchers’. These are background events that involve heavy processing and will slow down your app, particularly on mobile devices.

  2. Pay special attention event handlers that fire in quick succession (ie, ‘mousemove’).

    Browser events such as ‘mousemove’ and ‘resize’ are executed in quick succession up to several hundred times each second, this means that you need to ensure that an event handler bound to either of these events is coded optimally and can complete in less than 2-3 milliseconds.

    Any overhead greater than that will create a patchy user experience, specially in browsers such as IE that have poor rendering capabilities.

  3. Remember to unbind events when they are not needed.

    Unbinding events is almost as important as binding them. When you add a new event handler to your code make sure that you provide for it to stop firing when it is no longer needed, ideally using once-off execution constructs like jQuery.one() or coding in the unbind behaviour at that point. This will avoid you having the same handler bound multiple times degrading performance.

    If you are using jQuery to bind and unbind events, make sure your selector points to a unique node, as a loose selector can create or remove more handlers than you intend to.

  4. Learn about event bubbling.

    If you are going to use event handlers, it is important that you understand how event bubbling propagates an event up the DOM tree to every ancestor node. You can use this knowledge to limit your dependency on event bubbling with approaches such as jQuery.live() and jQuery.delegate() that require full DOM traversal upon handling each event, or to  stop event bubbling for improved performance. See this great post on the subject.

  5. Use ‘mouseup’ instead of ‘click’.

    Remember that user interaction via the mouse or keyboard fires several events in a specific order. It is useful to remember the order in which these events fire so you can squeeze in your functionality before anything else gets handled, including native browser event handlers.

    A good example of this is to bind your functionality to the ‘mouseup’ event which fires before the ‘click’ event, this can produce a surprising performance boost in older browsers such as IE, making the difference between handling every interaction or missing some of the action if the user triggers clicks many times in succession.

4. Maximise the efficiency of your iterations

Performance becomes critical during long iterations.


Due to the processing time used, iterations are usually the first places where you can address performance flaws in an application.


Get rid of unnecessary loops and calls made inside loops.


  1. Harness the indexing power of JavaScript objects.

    Native JavaScript objects {} can be used as powerful Hashtable data structures with quick-lookup indexes to store references to other objects, acting similarly to the way database indexes work for speeding up search operations by preventing needless looping. 

    So why bother iterating to find something? You can simply use a plain object as an index (think of a phone-book) to get to your desired item quickly and efficiently. Here is an example as follows:

    var data = {
      index: {
                "joeb": {name: "joe", surname: "bloggs", age: 29 },
                "marys": {name: "mary", surname: "smith", age: 25 }
                // another 1000 records
      get: function(username) {
                return this.index[username];
  2. Harness the power of array structures with push() and pop() and shift().

    Array push() pop() and shift() instructions have minimal processing overhead (20x that of object manipulation) due to being language constructs closely related to their low-level assembly language counterparts. In addition, using queue and stack data structures can help simplify your code logic and get rid of unnecessarily loops. See more on the topic in this article.

  3. Take advantage of reference types.

    JavaScript, much like other C-based languages, has both primitive and reference value types. Primitive types such as strings, booleans and integers are copied whenever they are passed into a new function, however reference types such as arrays, objects and dates are passed only as a light-weight reference.You can use this to get the most performance out of recursive functions, such as by passing a DOM node reference recursively to minimise DOM traversal, or by passing a reference parameter into a function that executes within an iteration. Also, remember that comparing object references is far more efficient than comparing strings.

  4. Use Array.prototype.join() for string concatenation.

    PLEASE NOTE: This article was first written in 2012 when string concatenation was a hazard to be aware of. However, these days most JavaScript engines have compilation tricks that have made this issue obsolete. The wording below is only really relevant for historical purposes.

    Joining strings using the plus sign (ie var ab = 'a' + 'b';) creates performance issues in IE when used within an iteration. This is because, like Java and C#, JavaScript uses unmutable strings.

    Basically, when you concatenate two strings, a third string is constructed for gathering the results with its own object instantiation logic and memory allocation. While other browsers have various compilation tricks around this, IE is particularly bad at it.A far better approach is to use an array for carrying out the donkey work, creating an array outside the loop, using push() to add items into to the array and then a join() to output the results. See this link for a more in-depth article on the subject.

5. Become friends with the JavaScript lexicon

Become a friend of the ECMA Standard and it make your code faster.


Due to its loosely-typed and free-for-all nature, JavaScript can be written using a very limited subset of lexical constructs with no discipline or controls applied to its use. Using simple function patterns repetitively often leads to poorly thought-out ‘spaghetti’ code that is inefficient in terms of resource use.


Learn when and how to apply the constructs of the ECMAScript language standard to maximise performance.


  1. Shorten the scope chain

    In JavaScript, whenever a function is executed, a set of first order variables are instantiated as part of that function. These include the immediate scope of a function (the this variable) with its own scope chain, the arguments of the function and all locally-declared variables.

    If you try and access a globally-declared variable or a closure further up the scope chain, it will take extra effort to traverse up the chain every level util the compiler can wire up the variable you are after. You can thus improve execution by reducing the depth of the call stack, and by only using the local scope (this), the arguments of the function, as well as locally declared variables. This article explains the matter further.

  2. Make use of ‘this’, by passing correct scope using ‘call’ and ‘apply’.

    This is particularly useful for writing asynchronous code using callbacks, however it also improves performance because you are not relying on global or closure variables held further up the scope chain. You can get the most out of the scope variable (this) by rewiring it using the special call() and apply() methods that are built into each function. See the example below:

    var Person = Object.create({
      init: function(name) {
         this.name = name;
      do: function(callback) {
    var john = new Person('john');
    john.do(function() {
        alert(this.name); // 'john' gets alerted because we rewired 'this'.
  3. Learn and use native functions and constructs.

    ECMAScript provides a whole host of native constructs that save you having to write your own algorithms or rely on host objects. Some examples include Math.floor(), Math.round(), (new Date()).getTime() for timestamps, String.prototype.match() and String.prototype.replace() for regexes, parseInt(n, radix) for changing numeral systems, === instead of == for faster type-based comparsion, instanceof for checking type up the hierarchy, & and | for bitwise comparisons. And the list goes on and on.

    Make sure you use all these instead of trying to work out your own algorithms as you will not only be reinventing the wheel but affecting performance.

  4. Use ‘switch’ instead of lengthy ‘if-then-else’ statements.

    This is because  ‘switch’ statements can be optimized more easily during compilation. There is an interesting article in O’Reily about using this approach with JavaScript.


Object-Oriented JavaScript Inheritance

This is simple but crucial stuff in JavaScript. Its easy to forget how to do object-oriented inheritance from scratch when you are dealing with several JS frameworks and each of them has pre-built methods that support this functionality in a slightly different way.

JavaScript is a functional language that uses prototypal inheritance. That means you can use classical inheritance such as that supported by Java and C#, however you need to be disciplined about the way you write code and need do a couple of things every time you add a class to your type hierarchy.

When creating a new class, we are essentially creating a function and assigning a new instance of another function as the prototype. This is the basis of ‘prototypal’ inheritance, ie. if you are dealing with ‘Dog’ type that inherits from an ‘Animal’ class, the template for your dog is a newly created instance of animal.

For Example:

// Create a new class called 'Animal'
var Animal = function() {};
// Create an instance of the 'Animal'
var anAnimal = new Animal();
// Create a class called 'Dog'
var Dog = function() {};
// Assign the template
Dog.prototype = anAnimal;
// Instantiate the dog
var lassie = new Dog();

Abstracting inheritance

I’ve created a simple static method called ‘extend’ on the Object class. Most frameworks do this in one shape or another (ie ExtJS uses Ext.extend(), John Resig likes to use Class.extend() method, Mootools uses new Class(properties) approach etc).

A ‘static’ method means that only the ‘Object’ class template will have it, so we do not pass it over to our children through inheritance. See the following code:

// Create a static 'extends' method on the Object class
// This allows us to extend existing classes
// for classical object-oriented inheritance
Object.extend = function(superClass, definition) {
    var subClass = function() {};
    subClass.prototype = new superClass();
    for (var prop in definition) {
        subClass.prototype[prop] = definition[prop];
    return subClass;

This allows us to simplify the process of using object-oriented inheritance by abstracting the prototype assignment into this separate function call.

// Create an 'Animal' class by extending
// the 'Object' class with our magic method
var Animal = Object.extend(Object, {
    move : function() {alert('moving...');}
// Create a 'Dog' class that extends 'Animal'
var Dog = Object.extend(Animal, {
    bark : function() {alert('woof');}
// Instantiate Lassie
var lassie = new Dog();
// She can move AND bark!

Adding Constructors

But, we seem to have forgotten one thing. What about constructors?

JavaScript uses the function itself as the constructor for a new object. Thus, when we created the Animal object above we could have simply added some sample code to its constructor as follows:

// Assign the name of the animal when it gets instantiated
var Animal = new function(name) {
  this.name = name;
var lassie = new Animal('Lassie');
alert('My pets name is: ' + lassie.name);

Thus we can augment our class creation methodology using the same idea:

// Create a 'Subclass' with a constructor
var SubClass = Object.extend(SuperClass, {
    constructor: function(parameter) {
        this.x = parameter;
    method1: function() {
        // ...
// Instantiate it
var subClass = new SubClass('value');

In order to allow this kind of notation, we can modify our ‘extend’ method to take care of this special ‘constructor’ function. This is done as follows:

// Create a static 'extends' method on the Object class
// This allows us to extend existing classes
// for classical object-oriented inheritance
Object.extend = function(superClass, definition) {
    var subClass = function() {};
    // Our constructor becomes the 'subclass'
    if (definition.constructor !== Object)
        subClass = definition.constructor;
    subClass.prototype = new superClass();
    for (var prop in definition) {
    	if (prop != 'constructor')
            subClass.prototype[prop] = definition[prop];
    return subClass;

And put it to use by writing minimal code that is meaningful to read and leaves the object-creation abstraction sitting behind the scenes:

// Create the 'Animal' class by extending
// the 'Object' class with our magic method
// this time using a constructor
var Animal = Object.extend(Object, {
    constructor: function(name) {
        this.name = name;
    move: function() {
// Instantiate Lassie (as an animal)
var lassie = new Animal('Lassie');
// Now lassie has a name which is
// defined inside the object constructor
alert('My pets name is: ' + lassie.name);

Calling constructors through the inheritance chain.

Now we have come up against a small problem, our ‘Animal’ class can have a constructor, but its child class ‘Dog’ has no way to call this constructor so it can take advantage of the logic for instantiating all animals.

In order to do this we are going to add a special ‘superClass’ property to all of our classes automatically, then use the magic ‘call’ function to call this using the context of the dog instance (to put it in plain english, this will make each ‘Dog’ run the logic for an ‘Animal’).

Lets see, all together now:

// Create a static 'extends' method on the Object class
// This allows us to extend existing classes
// for classical object-oriented inheritance
Object.extend = function(superClass, definition) {
    var subClass = function() {};
    // Our constructor becomes the 'subclass'
    if (definition.constructor !== Object)
        subClass = definition.constructor;
    subClass.prototype = new superClass();
    for (var prop in definition) {
    	if (prop != 'constructor')
            subClass.prototype[prop] = definition[prop];
    // Keep track of the parent class
    // so we can call its constructor too
    subClass.superClass = superClass;
    return subClass;
// Create the 'Animal' class by extending
// the 'Object' class with our magic method
// this time using a constructor
var Animal = Object.extend(Object, {
    constructor: function(name) {
        this.name = name;
    move: function() {
// Create a 'Dog' class that inherits from it
var Dog = Object.extend(Animal, {
    constructor: function(name) {
        // Remember to call the super class constructor
        Dog.superClass.call(this, name);
    bark: function() {
// Instantiate Lassie
var lassie = new Dog('Lassie');
// She can move AND bark AND has a name!
alert('My pets name is: ' + lassie.name);

Multiple inheritance using Interfaces

Now its possible to add support for multiple inheritance to our object creation syntax in Javascript. In single-inheritance object-oriented languages such as Java and C#, multiple inheritance (inheriting properties and methods from more than one class chain) is done using Interfaces.

The implementation is a bit more crude than in Java or C#, the reason why is because Javascript, unlike these languages, is not compiled so we cant throw compilation-time errors. This means that when we implement an interface we are limited to checking that the appropriate members are there at runtime and if not we through an error.

So, lets start first with our intended usage for Interfaces:

var SubClass = Object.extend(SuperClass, {
     method1: function() {
}).implement(Inteface1, Interface2 ...);

Going by this approach, we need to create a method ‘implement’ for every class, since in Javascript our classes are instances of the ‘Function’ object, this means adding this method to Function.prototype.

The code does start to be a bit hairy from this point, its important to point out that the context of this function call (ie this) is the class that we are testing for implementation of a particular interface.

Function.prototype.implement = function() {
    // Loop through each interface passed in and then check
    // that its members are implemented in the context object (this)
    for(var i = 0; i &lt; arguments.length; i++) {
         var interf = arguments[i];
         // Is the interface a class type?
         if (interf.constructor === Object) {
             for (prop in interf) {
                 // Check methods and fields vs context object (this)
                 if (interf[prop].constructor === Function) {
                     if (!this.prototype[prop] ||
                          this.prototype[prop].constructor !== Function) {
                          throw new Error('Method [' + prop
                               + '] missing from class definition.');
                 } else {
                     if (this.prototype[prop] === undefined) {
                          throw new Error('Field [' + prop
                               + '] missing from class definition.');
    // Remember to return the class being tested
    return this;

And there you have it! Its important to point out that this is one of many ways to abstract object-orientation in Javascript. There are different approaches, and as a programmer it helps to pick one you are more comfortable with. Personally, I like this approach because of its simplicity, readability, and ease of use:

// Create a 'Mammal' interface
var Mammal = {
    nurse: function() {}
// Create the 'Pet' interface
var Pet = {
    do: function(trick) {}
// Create a 'Dog' class that inherits from 'Animal'
// and implements the 'Mammal' and 'Pet' interfaces
var Dog = Object.extend(Animal, {
     constructor: function(name) {
          Dog.superClass.call(this, name);
     bark: function() {
     nurse: function(baby) {
          baby.food = 100;
     do: function(trick) {
          alert(trick + 'ing...');
}).implement(Mammal, Pet);
// Instantiate it
var lassie = new Dog('Lassie');
lassie.nurse(new Dog('Baby'));
alert('My pets name is: ' + lassie.name);

Victory Motorcycle Competition

This was a design of a mobile-friendly competition for BWS and Victory Motorcycles. The competition was hosted by RedJelly and ran over a period of 30 days.


Markit Environmental Registry

The Markit Environmental Registry is a central repository for tracing the ownership of carbon credits anywhere in the world and is the 1st ranked registry in 2012 Environmental Finance’s survey.

Rich Application for Carbon Market Transactions

Development of a fully Object-Oriented Javascript interface using the latest version of the EXT JS framework, a fully featured AJAX and GUI widget library based on Yahoo UI.

This was a 6 month project going live in September 2010 as part of the Markit suite of environmental products. The Markit Environmental Registry is a central repository for tracing the ownership of carbon credits anywhere in the world and has helped Markit gain 1st place for ‘Best Registry Provider’ in 2012 Environmental Finance’s survey.

While the work was primarily focused on individual paid-for access, there is also a complimentary public view interface that allows non-users to query database directly (written in plain HTML for SEO optimization).

Registry Landing Page

Delivering Features with Optimal Usability

Deliverables include a fully-operating Graphic User Interface compatible with IE (6,7 and 8), Firefox (3.0 upwards), Chrome and Safari in the form of an 8,000 line JavaScript library with 43 compiled javascript classes using object-oriented inheritance for minimal code-repetition and Java nomenclature and namespace breakdown wherever possible (with a view to find a replacement Java programmer) .

AJAX Architecture

The end result is a rich interface for a data-driven web application, with look-and-feel similar to that of a desktop application like Excel or Outlook. The users of the registry can access the following features:

Advanced Dockable Components

Standardised components such as grids (for record lists) allow for easily building functionality with components that can be docked in tab panels or within a popup window. Base components provide key functionality, such as layout, ordering, paging and searching and can be extended depending on the requirements for each section of the application.

Advanced Dockable Components

Simplified Search Criteria

Search criteria is simplified and prioritised to hide items that are less relevant to the user. Searching requires minimal input from the user (usually mouse-scroll and click).

Simplified Search Criteria

Predictive Search

An AJAX favourite. The implementation within the Registry is done using DOM buffering (only the items displayed are being rendered in the browser) which allows for thousands of records to be quickly filtered down to those that fit relevant criteria in milliseconds.

Predictive Search

Context Menus (right mouse button)

Context menus are used for right-clicking on a piece of information to find out relevant actions that can be performed. This is ported from functionality found in desktop application such as Outlook or Excel.

Context Menus

Windowed Interface

Window “lightboxes” are used to display information about individual records. These are pop-ups rendered within the page as modal windows, which provide a high degree of responsiveness in the application and the look-and-feel of traditional desktop applications.

Windowed Interface

Server-Side Validation Messages

Intuitive and appealing server side validation messages (without needing to refresh the page). The fields a user has entered incorrectly are clearly highlighted and provide additional information when hovering above them.

Server Validation

Hierarchical Data Representation (expanding trees)

Data is presented hierarchically wherever relevant, for example a syndication “Basket” used for risk management, can be constructed to contain other items within in. The User Interface displays this as an expandable item, with other items within it.

Hierarchical Data Representation

Object-Oriented Inheritance

The interface is designed as a self-contained JavaScript class library with components (Classes) that inherit from each other. This is done to reduce code-duplication and make the software more manageable.

Object Inheritance

Environmental Registry in the News

Markit Environmental Registry named ‘Best Registry Provider
April 2012


Feedback on the Registry

Some of the feedback received on this project:

UserEli Goncalves (End User)

13 Sep 2010 – Your new platform really became more practical. Congrats!

UserHelen Robinson (Founder)

20 Sep 2010 – You guys have done a wonderful job, thank you very much!

Value Trader

Value Trader is a tool that calculates stock prices based on value fundamentals from Balance Sheet and Profit and Loss statements. Feel free to use it to determine the financial health and estimated value of your stocks.

Live Address


The Landing Page

The landing page provides concise information on a few selected companies based on a “Watchlist”. These are chosen by investors so that they feature every time they come and visit.

Company Details Page

When drilling down into an individual company. The system provides information by looking at financial data over the past few years to determine general health of the company, as well as provide recommendations based on price (by comparing it to the book value and earnings of the company).

Designed for iPhone

The design has been further enhanced to look good on mobile devices, using a mix of small and large type so the important information is clearly displayed even on a small screen.

IP Based Security

The website makes use of IP addresses to authenticate users. The following advantages:

  • Only need to authenticate once (per location).
  • User locations are traced by a collection of IP addresses associated to an email.
  • There are measures in place to detect SQL Injection, Denial of Service and Dictionary Attacks. If a single location tries to hack the site all associated IP addresses are automatically denied access.