3.0.0 • Public • Published

Traffic Cop

Simple, lightweight, developer-focused front-end A/B testing

(If you want to skip the spiel and get straight to business, check out the docs.)

How to install and use

Install via npm: npm install @mozmeao/trafficcop

Import the library at your applications entrypoint via require, import or by using a global variable in your script tag:

  • import TrafficCop from '@mozmeao/trafficcop';
  • const TrafficCop = require('@mozmeao/trafficcop');
  • const TrafficCop = window.TrafficCop;

What does it do?

Traffic Cop is a small bit of JavaScript that decides if a visitor should see a variation of the current page. If so, one of two things happens:

  1. A developer-specified JavaScript function is passed the chosen variation and executed.
  2. The user is redirected to the current URL with a developer-specified query string parameter appended.

Example flow

  1. Visitor lands on www.example.com/product
  2. Visitor is chosen for variation 2
  3. Visitor is then either: a. redirected to www.example.com/product?v=2 or b. a developer-specified JavaScript function is passed the value of 2 and executed

What happens on www.example.com/product?v=2, or in the JavaScript function, is completely up to the developer (possibly you, dear reader).

Why did we build it?

Most of the content experiments on mozilla.org simply direct (or police, if you will) targeted visitors into pre-set variation cohorts. We weren't aware of any developer-focused (simple, light, flexible) solutions, so we wrote one.

In contrast to third-party options (e.g. Optimizely), Traffic Cop offers:

  1. Security — Many third-party options require loading JS from their site, which is a potential XSS vector. Traffic Cop can (and should) be served from your site/CDN.
  2. Privacy - Traffic Cop does not use cookies of any kind (unlike most third-party solutions), nor does it store or send any experiment data itself (that part is up to you and your consent management solution).
  3. Performance — Traffic Cop is light and has zero dependencies, resulting in less than 2KB of JS when minified. (In our experience, Optimizely's JS bundle was regularly above 200KB.)
  4. Your workflow — Traffic Cop offers great flexibility in when and how you write and load variation code. No need to type jQuery in a text box on a third-party web page.
  5. Savings — No need to pay for a third-party service.

How does it work?

A visitor hits a URL running an experiment (meaning the appropriate JS is loaded). Traffic Cop picks a random number, and, if that random number falls within the range specified by a variation (see below), either redirects the visitor to that variation or executes an arbitrary, developer-specified callback function.

For redirects, Traffic Cop assumes all variations are loaded through a query string parameter appended to the original URL. This keeps things simple, as no new URL patterns need to be defined (and later removed) for each experiment. First, check for the query string parameter (wherever your application might do that sort of thing) and then load different content accordingly.

The configuration file has two steps. Instantiate a new Traffic Cop with your experiment configuration, and then initialize it.

// example configuration for a redirect experiment
var wiggum = new TrafficCop({
  variations: {
    ‘v=1: 10.5,
    ‘v=2: 0.25


In the above example, a visitor would have a 10.5% chance of being chosen for v=1, and a 0.25% chance for v=2. If the visitor is selected for a variation, the user will be redirected to the current URL with either ?v=1 or ?v=2 appended.

Note that Traffic Cop supports percentages into the hundredths, but no smaller.

// example configuration for a callback function experiment
function myCallback(variation) {
    console.log('The chosen variation was ' + variation);
    // and then change button color based on variation chosen...

var lou = new TrafficCop({
  customCallback: myCallback,
  variations: {
    ‘a’: 25,
    ‘b’: 25,
    'c': 25


In the above example, a visitor would have a 25% chance of being chosen for a, b, or c. The chosen variation will be passed to the myCallback function (which can do whatever it likes).

Check out the docs for more complete information.

Further Documentation


This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.



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