Add block of Google Analytics dimensions for A/B tests

The GA dimensions from 40-49 have been allocated to A/B tests. These may
be reserved for any A/B test for the duration of the test.

This commit identifies which dimension to use for the A/B test based on
an extra `data` attribute added to the `meta` tag. Only dimensions in
the allowed range are permitted. Any others are ignored and are not
reported to GA.

app/assets/javascripts/analytics/static-analytics.js

@@ -13,6 +13,7 @@
     setPixelDensityDimension();
     setHTTPStatusCodeDimension();
     this.setDimensionsFromMetaTags();
+    this.setAbTestDimensionsFromMetaTags();
     this.callMethodRequestedByPreviousPage();
 
     // Track initial pageview
@@ -94,9 +95,26 @@
     this.setOrganisationsDimension(dimensions['analytics:organisations']);
     this.setWorldLocationsDimension(dimensions['analytics:world-locations']);
     this.setRenderingApplicationDimension(dimensions['rendering-application']);
-    this.setAbTestDimension(dimensions['ab-test']);
   };
 
+  StaticAnalytics.prototype.setAbTestDimensionsFromMetaTags = function() {
+      var $abMetas = $('meta[name^="govuk:ab-test"]'),
+          staticAnalytics = this,
+          // This is the block of dimensions assigned to A/B tests
+          minAbTestDimension = 40,
+          maxAbTestDimension = 49;
+
+      $abMetas.each(function() {
+        var $meta = $(this),
+        dimension = parseInt($meta.data('analytics-dimension')),
+        testNameAndBucket = $meta.attr('content');
+
+        if (dimension >= minAbTestDimension && dimension <= maxAbTestDimension) {
+          staticAnalytics.setDimension(dimension, testNameAndBucket);
+        }
+      });
+  }
+
   StaticAnalytics.prototype.trackPageview = function(path, title, options) {
     this.analytics.trackPageview(path, title, options);
   };
@@ -157,9 +175,5 @@
     this.setDimension(21, position);
   };
 
-  StaticAnalytics.prototype.setAbTestDimension = function(testNameAndBucket) {
-    this.setDimension(40, testNameAndBucket);
-  };
-
   GOVUK.StaticAnalytics = StaticAnalytics;
 })();

spec/javascripts/analytics/static-analytics-spec.js

@@ -62,7 +62,6 @@ describe("GOVUK.StaticAnalytics", function() {
           <meta name="govuk:political-status" content="historic">\
           <meta name="govuk:analytics:organisations" content="<D10>">\
           <meta name="govuk:analytics:world-locations" content="<W1>">\
-          <meta name="govuk:ab-test" content="name-of-test:name-of-ab-bucket">\
         ');
 
         analytics = new GOVUK.StaticAnalytics({universalId: 'universal-id'});
@@ -75,7 +74,6 @@ describe("GOVUK.StaticAnalytics", function() {
         expect(universalSetupArguments[8]).toEqual(['set', 'dimension7', 'historic']);
         expect(universalSetupArguments[9]).toEqual(['set', 'dimension9', '<D10>']);
         expect(universalSetupArguments[10]).toEqual(['set', 'dimension10', '<W1>']);
-        expect(universalSetupArguments[11]).toEqual(['set', 'dimension40', 'name-of-test:name-of-ab-bucket']);
       });
 
       it('ignores meta tags not set', function() {
@@ -86,8 +84,53 @@ describe("GOVUK.StaticAnalytics", function() {
         universalSetupArguments = window.ga.calls.allArgs();
 
         expect(universalSetupArguments[4]).toEqual(['set', 'dimension1', 'section']);
-        expect(universalSetupArguments[5]).toEqual(['send', 'pageview']);
+        assertClosingArgumentIsAtIndex(universalSetupArguments, 5);
       });
+
+      it('sets A/B meta tags as dimensions', function() {
+        $('head').append('\
+          <meta name="govuk:ab-test" content="name-of-test:name-of-ab-bucket" data-analytics-dimension="42">\
+          <meta name="govuk:ab-test" content="name-of-other-test:name-of-other-ab-bucket" data-analytics-dimension="48">\
+        ');
+
+        analytics = new GOVUK.StaticAnalytics({universalId: 'universal-id'});
+        universalSetupArguments = window.ga.calls.allArgs();
+
+        expect(universalSetupArguments[4]).toEqual(['set', 'dimension42', 'name-of-test:name-of-ab-bucket']);
+        expect(universalSetupArguments[5]).toEqual(['set', 'dimension48', 'name-of-other-test:name-of-other-ab-bucket']);
+      });
+
+      it('ignores dimensions outside of the A/B test range', function () {
+        $('head').append('\
+          <meta name="govuk:ab-test" content="name-of-test-dimension-too-low:some-bucket" data-analytics-dimension="39">\
+          <meta name="govuk:ab-test" content="name-of-valid-test:some-bucket" data-analytics-dimension="40">\
+          <meta name="govuk:ab-test" content="name-of-other-valid-test:some-bucket" data-analytics-dimension="49">\
+          <meta name="govuk:ab-test" content="name-of-test-dimension-too-high:some-bucket" data-analytics-dimension="50">\
+        ');
+
+        analytics = new GOVUK.StaticAnalytics({universalId: 'universal-id'});
+        universalSetupArguments = window.ga.calls.allArgs();
+
+        expect(universalSetupArguments[4]).toEqual(['set', 'dimension40', 'name-of-valid-test:some-bucket']);
+        expect(universalSetupArguments[5]).toEqual(['set', 'dimension49', 'name-of-other-valid-test:some-bucket']);
+        assertClosingArgumentIsAtIndex(universalSetupArguments, 6);
+      });
+
+      it('ignores A/B meta tags with invalid dimensions', function () {
+        $('head').append('\
+          <meta name="govuk:ab-test" content="name-of-test:some-bucket">\
+          <meta name="govuk:ab-test" content="name-of-test:some-bucket" data-analytics-dimension="not a number">\
+        ');
+
+        analytics = new GOVUK.StaticAnalytics({universalId: 'universal-id'});
+        universalSetupArguments = window.ga.calls.allArgs();
+
+        assertClosingArgumentIsAtIndex(universalSetupArguments, 4);
+      });
+
+      function assertClosingArgumentIsAtIndex(setupArgs, argIndex) {
+        expect(setupArgs[argIndex]).toEqual(['send', 'pageview']);
+      }
     });
   });