Minor refactor, add detailed cookie handling info

app/assets/javascripts/ee_trial_banner/ee_trial_banner.js

@@ -10,6 +10,45 @@ export default class EETrialBanner {
   }
 
   init() {
+    this.setCookies();
+    this.$trialBanner.on('close.bs.alert', e => this.handleTrialBannerDismiss(e));
+  }
+
+  /**
+   * Trial Expiring/Expired Banner has two stages;
+   * 1. Show banner when user enters last 7 days of trial
+   * 2. Show banner again when last 7 days are over and license has expired
+   *
+   * Stage 1:
+   *    Banner is showed when `trial_license_message` is sent by backend
+   *    for the first time (in `app/views/layouts/header/_default.html.haml`).
+   *    Here, we perform following steps;
+   *
+   *    1. Set cookie `show_ee_trial_banner` with expiry same as license
+   *    2. Set cookie value to `true`
+   *    3. Show banner using `toggleBanner(true)`
+   *
+   *    At this stage, if user dismisses banner, we set cookie value to `false`
+   *    and everytime page is initialized, we check for cookie existence as
+   *    well as its value, and decide show/hide status of banner
+   *
+   * Stage 2:
+   *    At this point, Cookie we had set earlier will be expired and
+   *    backend will now send updated message in `trial_license_message`.
+   *    Here, we perform following steps;
+   *
+   *    1. Check if cookie is defined (it'll not be defined as it is expired now)
+   *    2. If cookie is gone, we re-set `show_ee_trial_banner` cookie but with
+   *       expiry of 20 years
+   *    3. Set cookie value to `true`
+   *    4. Show banner using `toggleBanner(true)`, which now has updated message
+   *
+   *    At this stage, if user dismisses banner, we set cookie value to `false`
+   *    and our existing logic of show/hide banner based on cookie value continues
+   *    to work. And since, cookie is set to expire after 20 years, user won't be
+   *    seeing banner again.
+   */
+  setCookies() {
     const today = new Date();
 
     // Check if Cookie is defined
@@ -31,8 +70,6 @@ export default class EETrialBanner {
       // Cookie was defined, let's read value and show/hide banner
       this.toggleBanner(Cookies.get(this.COOKIE_KEY) === 'true');
     }
-
-    this.$trialBanner.on('close.bs.alert', e => this.handleTrialBannerDismiss(e));
   }
 
   toggleBanner(state) {