From d3131020b30ef48413a527e4d4843c949b69bc98 Mon Sep 17 00:00:00 2001
From: Ikenna Okpala <me@ikennaokpala.com>
Date: Wed, 13 Jul 2016 15:26:28 +0100
Subject: [PATCH] Set duration (in days) for AB Test cookie.

Here, I have added a new option (cookieDuration), this option help to set the cookie expiration duration in days.

When this option isn't supplied the the default cookie duration is set to expire in 30 days.

The motivation here is, by reducing the cookie expiration will allow returning users to be assigned
to different cohorts during A/B tests.

According to the performance analyst, A/B tests performance is measured using sessions and with a default 30 day expiration,
they will only see the one variant for that period.  Being able to reduce this increases the likelihood of users seeing different variants
across their sessions.
---
 docs/javascript.md                     | 14 ++++++++++++++
 javascripts/govuk/multivariate-test.js |  3 ++-
 spec/unit/multivariate-test.spec.js    | 23 +++++++++++++++++++++++
 3 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/docs/javascript.md b/docs/javascript.md
index 895c6c51..50c831d3 100644
--- a/docs/javascript.md
+++ b/docs/javascript.md
@@ -178,6 +178,20 @@ var test = new GOVUK.MultivariateTest({
 });
 ```
 
+If you want to set the cookie expiration then you can optionally set cookieDuration as follows:
+
+```javascript
+var test = new GOVUK.MultivariateTest({
+  name: 'car_tax_button_text',
+  cookieDuration: 14,
+  cohorts: {
+    pay_your_car_tax: {weight: 25, callback: function() { ... }}, // 25%
+    give_us_money:    {weight: 75, callback: function() { ... }}  // 75%
+  }
+});
+```
+Here, it is set to expire in 14 days time. if this option is not set the default cookie expiration (30 days) take effect.
+
 If you have a complex test, it may be worth extending MultivariateTest with
 your own. Callbacks can be strings which will call a method of that name
 on the current object.
diff --git a/javascripts/govuk/multivariate-test.js b/javascripts/govuk/multivariate-test.js
index 3d0858be..57e18661 100644
--- a/javascripts/govuk/multivariate-test.js
+++ b/javascripts/govuk/multivariate-test.js
@@ -18,6 +18,7 @@
     this._loadOption(options, 'runImmediately', true);
     this._loadOption(options, 'defaultWeight', 1);
     this._loadOption(options, 'contentExperimentId', null);
+    this._loadOption(options, 'cookieDuration', 30);
 
     if (this.runImmediately) {
       this.run();
@@ -69,7 +70,7 @@
     var cohort = GOVUK.cookie(this.cookieName());
     if (!cohort || !this.cohorts[cohort]) {
       cohort = this.chooseRandomCohort();
-      GOVUK.cookie(this.cookieName(), cohort, {days: 30});
+      GOVUK.cookie(this.cookieName(), cohort, {days: this.cookieDuration});
     }
     return cohort;
   };
diff --git a/spec/unit/multivariate-test.spec.js b/spec/unit/multivariate-test.spec.js
index d211bacf..4b024ddf 100644
--- a/spec/unit/multivariate-test.spec.js
+++ b/spec/unit/multivariate-test.spec.js
@@ -124,6 +124,29 @@ describe("MultivariateTest", function() {
       expect(test.fooCallback).toHaveBeenCalled();
     });
 
+    it("should assign 30 if cookieDuration isn't defined", function() {
+      GOVUK.cookie.and.returnValue('foo');
+      var test = new GOVUK.MultivariateTest({
+        name: 'cookie_duration_test',
+        cohorts: {
+          foo: {callback: function(){}}
+        }
+      });
+      expect(test.cookieDuration).toEqual(30);
+    });
+
+    it("should assign the user's cookie duration, when cookieDuration is defined", function() {
+      GOVUK.cookie.and.returnValue('foo');
+      var test = new GOVUK.MultivariateTest({
+        name: 'cookie_duration_test',
+        cookieDuration: 14,
+        cohorts: {
+          foo: {callback: function(){}}
+        }
+      });
+      expect(test.cookieDuration).toEqual(14);
+    });
+
     it("should assign a new random cohort if the assigned cohort does not exist", function() {
       var fooSpy = jasmine.createSpy('fooSpy');
       var barSpy = jasmine.createSpy('barSpy');