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A JavaScript AB testing framework, ported from

AB testing's goal is to identify changes to web pages that increase or maximize an outcome of interest.


  • 100% client-side
  • Audience segmentation:-
    • Basic - audience is segmented in to random buckets.
    • Deterministic - allows predictable allocation of users in to tests based on some external key (Eg, username)
  • Fixed duration tests - that automatically close and delete their footprint.
  • Isolation of each test audience - so a user can not accidentally be in several tests at once.
  • Agnostic of where the data is logged - most companies have their own customer data repositories.
  • Minimal payload - ~2kb (minified + gzip) with no additional cookie overhead created.

Browser Support

Experiments works in Chrome 15, Firefox 11, IE 9, and Safari 535.2.

Experiments makes use of CustomEvent, which will need polyfilling in IE.


Let's say we want to test the effectiveness of a button that says 'help' versus one that says 'stuck?'. Our hypothesis is that, when a user sees an error message, 2% more people will click 'help' than 'stuck?'.

To represent the test in code form we create a profile - a plain JavaScript object with a unique test identifier.

var profile = {
    id: 'help'

Next we need to create our two variants - 'help' and 'stuck'.

Each variant is represented by a JavaScript function, so we add them to the profile. In this experiment we simply need to update the copy of two buttons in the page, but tests can largely do whatever is needed - change the layout, load in extra data, truncate bodies of text etc.

var profile = {
    id: 'help',
    variants: [ 
			id: 'help',
			test: function () {
			id: 'stuck',
			test: function () {

Now we need an audience for the test. Again, we define a property in our profile. The profile defines the audience as a % of your total visitors, so if we want 10% of people to participate in the test we can state an audience of '0.1', or for 5%, an audience value of '0.05'.

var profile = {
    id: 'help',
    audience: 0.1,   // ie. 0.1 = 10%
    variants: [ 

Often we want to run several tests simultaneously and be confident that variations in one test aren't influencing a second. To avoid this overlap we use the idea of a audience offset.

Each user in a test is assigned a persistent integer, evenly distributed from 1 to 1,000,000, and we allocate blocks of these users to individual tests.

For example, a profile with an audience of 10% and an offset of 0.3 will allocate all the users with an identifier in the range 300,000 to 400,000 to the test and split the variants evenly between this group of people.

var profile = {
    id: 'help',
    audience: 0.1,
    audienceOffset: 0.3, 
    variants: [ 

In the profile described above we would expect,

  • 5% of people to be allocated to the 'help' group
  • 5% of people to be allocated to the 'stuck' group
  • 90% of people to not be in the test.

The later group can obviously be selected for other tests.

A good AB test runs for a fixed period of time. For this reason each test profile must have an expiry date, represented as JavaScript Date object.

If the date is the past the test framework will refuse to execute the test.

var profile = {
    id: 'help',
    expiry: new Date(2015, 1, 1)    // ie. runs until 1 Jan, 2015

Lastly we might want to scope the test to an particular context. For example, our help test should not be executed on pages that do not produce error messages.

The canRun function is evaluated each time the test is loaded and decides if the test should be run.

var profile = {
    id: 'help',
	canRun: function () {
        return (page.config.type === 'article');

With our profile we can now instantiate a test, segment the audience and run it.

var help = new Experiments(profile).segment().run();


Each experiment will at least need to track the number of participants and number of successful conversions.

The framework itself does not do this, but provides events for each experiment to listen out for and hook their tracking code to.

The events are fired on document.body and contain the following event.detail.


  • variant - the segment the user has been assigned to.



Where "{}" is the name of the experiment.


Often the goal of an AB test is to track a users behaviour over a period of time - typically this will mean collecting data over several sessions across several devices.

Purely client-side test frameworks make this hard to do as cookies and localStorage are somewhat volatile and, more crtically, local to a web browser on a single device.

One approach to solving this problem is using psuedorandom number generators.

These are deterministic algorithms that can, given the same input, generate an evenly distributed set of numbers, which we can fairly segment on.

For example, lets say we have user ID of 'UID=314146' stashed in a cookie. We can extract that and pass it to the framework to repeatedly bucket the user in to the same ID and same test variant, Eg.

var k = getCookie('UID').split('=')[1];  // k = 314146
new Experiments(profile, { seed: key }).segment().run();

This is very useful when testing people across different devices over long periods of time and, helpfully, independent of needing a persistance layer beyond any current sign-in system.


Compile the code and open the example.html file in ./demos

Running a test

With developer tools, we can feed the above profile in to the AB test framework, force our variant to 'pink', then run the test.

var a = new Experiments(p, { variant: 'pink' }).run()

You should see the page background turn pink, and running the test on every subsequent visit will turn the page pink until the test has expired.

Allocate yourself in to the control group and re-run the test and the background should turn white.

var a = new Experiments(p, { variant: 'control' }).run();

For the duration of the test we can track the data of that user (say, pages per visit or scroll depth) and compare with the control group to see if that variant had the positive impact we thought it would have.


You can inspect data the tests create in local storage.

Firstly, each test subject is allocated a persistant id (an integer) that is shared across tests.


Next, each test remembers the variant the user is in over mulitple sessions,


In the real world we want the test subjects allocated randomly in to a variants (or excluded from the test), so we don't specify the variant in the Experiments constructor and invoke segment() instead, before running the experiment,

var a = new Experiments(profile).segment().run();

The segment() function decides if a user should be in the test, and, if they are, splits the audience between a 'control' group and a number of 'variants'.


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