Small and simple A/B testing tool. - Only supports two option 50/50 tests. On purpose. - Open-ended backend tracking. - Uses localStorage instead of cookies to cut down on bandwidth waste.

ab_tracker.js

// Licensed [CC-BY Shane Holloway](http://creativecommons.org/licenses/by/3.0/)
function ab_tracker(track) {
  "use strict";
  ab_test.store = window.localStorage || null;
  ab_test.prefix = 'ab_test::'
  ab_test.track = track
  if (typeof track !== 'function')
    throw new Error('`track` must be a function');

  var truth = {'true':true, 'false':false, null:null};
  return ab_test

  function ab_test(key, setupTestFn) {
    var obj, s=ab_test.store,
      pk=ab_test.prefix+key,
      a=s && truth[s.getItem(pk)];
    if (a===null)
        try{ s.setItem(pk, a=0.5<Math.random()) } catch(err) {}
    ab_test.track(pk, obj={ab:(a?'A':'B'), a:a, b:!a, key:key, pk:pk})
    if (setupTestFn) setupTestFn(!a, obj)
    return obj }
}

example.js

var ab_test = ab_tracker(function(pk, obj) {
  // connect to your test metric backend collector
  analytics.track(pk, obj)
})

ab_test('20130421 test idea', function(test, obj) {
  // enable & perform setup…
  if (test) // for *Experiment*
    document.body.className+=' test_idea';
  else ; // for *Control*
})