Write documentation

- document new analytics init, code structure for analytics modules, integration of new analytics into existing Modules

docs/analytics-gtm/analytics.md

@@ -18,6 +18,42 @@ Events happen when a user interacts with certain things, for example clicking on
 
 Search data is gathered when users perform a search.
 
+## Cookie consent
+
+The analytics code is only loaded if users consent to cookies. This is managed by the `init-ga4.js` script.
+
+If the page loads and cookie consent has already been given, the analytics code is initialised. This includes sending a page view and creating any event listeners for analytics code such as link tracking.
+
+If the page loads and cookie consent has not been given, an event listener is created for the `cookie-consent` event, which is dispatched by the [cookie banner component](https://github.com/alphagov/govuk_publishing_components/pull/2041/commits/777a381d2ccb67f0a7e78ebf659be806d8d6442d). If triggered, the event listener will initialise the analytics code as described above. This allows analytics to begin on the page where the user consents to cookies.
+
+## Code structure
+
+It is important that no analytics code runs until cookie consent is given. Code to be initialised as part of cookie consent should be attached to the `window.GOVUK.analyticsGA4.analyticsModules` object and include an `init` function, using the structure shown below.
+
+```JavaScript
+window.GOVUK = window.GOVUK || {}
+window.GOVUK.analyticsGA4 = window.GOVUK.analyticsGA4 || {}
+window.GOVUK.analyticsGA4.analyticsModules = window.GOVUK.analyticsGA4.analyticsModules || {};
+
+(function (analyticsModules) {
+  'use strict'
+
+  var ExampleCode = {
+    init: function () {
+      // do analytics stuff, like send a page view
+    }
+  }
+
+  analyticsModules.ExampleCode = ExampleCode
+})(window.GOVUK.analyticsGA4.analyticsModules)
+```
+
+When cookie consent is given, `init-ga4.js` looks through the `analyticsModules` object for anything with an `init` function, and executes them if found. This means that analytics code will not be executed unless consent is given, and gives a standard way to add more analytics code without additional initialisation.
+
+### Code structure for Modules
+
+Where analytics code is required as a [GOV.UK JavaScript Module](https://github.com/alphagov/govuk_publishing_components/blob/main/docs/javascript-modules.md), the code structure for the [existing model for deferred loading](https://github.com/alphagov/govuk_publishing_components/blob/main/docs/javascript-modules.md#modules-and-cookie-consent) should be used.
+
 ## Data schemas
 
 All of the data sent to GTM is based on a common schema.