Vanilla A/B testing with Abo

In a previous post I wrote about how unhandy it is to develop A/B tests in Optimizely’s web interface. In this post I want to get one step further. If we don’t develop our experiments in Optimizely, why do we need to copy paste the code to Optimizely? Eventually, what’s so hard about front-end A/B testing?

What do we need for front-end A/B testing?

The principle of front-end A/B testing is actually fairly simple.

  1. there should be a trojan that mutates the initial page
  2. a traffic allocator that divides the traffic between different experiments
  3. some sort of tracking to compare the performance of a B variation to a control A variation

I will not write about the 3rd item in this article. I’ll just say that it is really easy to track different events and goals with Google Analytics and segments can be used for comparing the performance of different variations.

The first 2 problems are easily solvable with JavaScript on the front-end.

Abo. The solution

Abo (або) means ‘or’ in Ukrainian. Abo is a micro front-end testing library.

Abo randomly assigns a variation to a user and keeps that user on the variation providing him a consistent user experience. Lets see the algorithm behind Abo:

Although the algorithm seems very simple, it can achieve anything required for an A/B experiment. Lets suppose you need an experiment that will change the size of the page header for 10% of mobile traffic. You can achive it by writing an assignemt condition that will return true only on cell phones:

var abo = require('abo');

var expt = {
  id: '1',
  name: 'Mobile experiment',

   * The experiment will get 10% of the traffic for
   * which the assignment condition will be satisfied
  traffic: .1,

   * Assignment Condition
   * will return true only if executed on a cell phone
  ac: function() {
    if (typeof matchMedia !== 'function') {
      return false;
    return matchMedia('(max-width: 470px)').matches;

   * This method will be executed only if the experiment
   * will be assigned to the user
  setup: function() {
      font-size: '24px'

 * abo receives an array of experiments and kicks off immediately
 * after receving it

How to structure an Abo project?

Big projects can have dozens of A/B tests running or under development. Hence, it is important to have a good structure for your A/B testing project. Fortunately, there’s a Yeoman generator for generating Abo projects and experiments. In order to use it, you’ll have to install Yeoman first.

npm install -g yo

Yeoman is an amazing tool that helps kick-start new projects! If you haven’t used it so far, check out the list of their generators. You’ll probably find a lot of usefull ones for your needs.

Now that you have Yeoman installed, you can install Yeoman generator for Abo:

npm install -g generator-abo

Great! Now you can create a folder for your A/B testing project and call yo abo to kikstart your project.

mkdir ab
cd ab
yo abo

Yeoman will ask a few questions about your project.

Once you answer all the questions, your project will be created and all the dependencies installed.

How to add Abo to your website?

Once you have the standard Abo project generated, you can bundle the Abo scripts by running foso build -m or self-host them by running foso serve. Foso is a static server that bundles resources using some simple conventions. If you don’t have it already, you can install it by running npm install -g foso.

Foso will bundle your files into the dist directory. A simple project will have only one file bundled into that folder: index.js. That file will have to be added to every single page of your website. If you run foso serve then foso will run a static server hosting the snippets from the dist directory.

The standard address of the static server is http://localhost:1769/, so you can just add a reference to http://localhost:1769/index.js to your website in order to test Abo locally.

How to develop an experiment?

To create your first experiment, you can run yo abo:expt. Yeoman will create a directory for your experiment in the root directory of your A/B project.

Of course, during developing a new experiment you wil want it to be active on the page, no matter what. Hence you won’t need the Abo library and all the other active experiments loaded. Luckily, your yet empty experiment, is already configured to allow independent usage! Go to the experiment’s directory and run foso serve. Foso will bundle the file called _bundle.js and rename it to index.js. This file will be available under http://localhost:1769/index.js.

Have you noticed that the file URL is the same as the one you already added to the page? You can enter different modes by running the foso server in different directories of your A/B testing project. If you run foso serve in the root directory, you can test the overall traffic allocation, experiment assignment, execution, etc. But if you run foso serve inside an experiment’s folder, you’ll enter to that experiment’s demo mode.

The great thing about foso is that it automatically adds a livereload link to the page, so when you update your experiment, the page will automatically reload.

Abo gives you complete freedom to implement your experiments the way you want. If you’d like to see some examples and best practices, check out the TodoMVC A/B testing project.

How to release A/B tests to production?

Everything that you’ll have to export to production will be bundled in the dist directory. You can manually copy/paste those files to your production server.

Although, there is a better solution of delivering your changes to production. The Abo generator creates your project with a ung.json config file. Ung is a static resources manager. In a nutshell, it is a simple NodeJS server that hosts static resources. What makes it a manager? The fact that it can update resources on the fly.

If you create an Ung server in production and update your ung.json file to point to the right domain, you’ll be able to update your experiments in production by running two commands in the root directory of your project:

ung pack prod
ung publish prod

The first command bundles your resources and packs them to a gzip and the second one posts them to production.


Of course, it is harder to maintain your own A/B testing solution, when there are great tools already in the web. But it gives you many advantages:


Currently you are on the .

Abo is working on this page. It is changing the theme of Hyde. Try to open this page in incognito and you might see a different theme applied to the page. Or try to remove the abo.x cookie and reload the page.

As you can see, after you’ve been assigned to a particular theme, you’ll be staying on that theme because the experiment ID is saved to the cookies.

This is the script that is changing the page.

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