Write & document public JS API for actions & events

the attached patch could be a starting point for this issue:

• creates a global namespace which can be shared between modules who wish to extend it
• defer the scipt to allow other scipts to set hanlders first
• creates global variables for storing the current status and categories
• replaced the use of getter callback, thus cookies are read at init, global vars are used afterwards
• triggers events and defienes observers pre- & post- load & save status & category (8 in total)
• shows withdraw button after accept all and save preferences without reload

done it for both D7 and D8

D8 sample:

(function ($, Drupal, drupalSettings) {

  "use strict";

  // Sep up the namespace as a function to store list of arguments in a queue.
  Drupal.eu_cookie_compliance = Drupal.eu_cookie_compliance || function() {
    (Drupal.eu_cookie_compliance.queue = Drupal.eu_cookie_compliance.queue || []).push(arguments)
  };

  // Initialize the object with some data.
  Drupal.eu_cookie_compliance.a = +new Date;

  // A shorter name to use when accessing the namespace.
  var self = Drupal.eu_cookie_compliance;

  // Define a new handler.
  var postPreferencesLoadHandler = function(response) {

    // Do your magic based on the current state:
    // response = {
    //   currentStatus: 0 / 1 / 2,
    //   currentCategories: ['functional', 'marketing', ...]
    // }

  };

  // Attach the handler to the event.
  Drupal.eu_cookie_compliance('postPreferencesLoad', postPreferencesLoadHandler);

})(jQuery, Drupal, drupalSettings);

example usage (sending category data to Google Tag Manager) for both D7 and D8 can be found in my sandbox module:
https://www.drupal.org/sandbox/reszli/3138494

Tested the last patch, and with it I'm getting a console error.

Uncaught TypeError: Cannot read property 'length' of undefined
    at Function.self.handleEvent (eu_cookie_compliance.js?v=1:781)
    at Function.Drupal.eu_cookie_compliance.getCurrentStatus (eu_cookie_compliance.js?v=1:433)
    at HTMLBodyElement.<anonymous> (eu_cookie_compliance.js?v=1:25)
    at Function.each (jquery.min.js?v=3.4.1:2)
    at k.fn.init.each (jquery.min.js?v=3.4.1:2)
    at Object.attach (eu_cookie_compliance.js?v=1:22)
    at drupal.js?v=8.8.5:25
    at Array.forEach (<anonymous>)
    at Object.Drupal.attachBehaviors (drupal.js?v=8.8.5:22)
    at drupal.init.js?v=8.8.5:28

  /**
   * Handle event by finding and executing handlers pushed to the queue.
   */
  self.handleEvent = function(eventName, observer) {
    var handlers = filterQueue(eventName);

    for (var i = 0; i < handlers.length; i++) {
      if (typeof handlers[i] !== 'undefined') {
        observer.subscribe(handlers[i]);
        observer.fire({
          currentStatus: _euccCurrentStatus,
          currentCategories: _euccSelectedCategories
        });
        observer.unsubscribe(handlers[i]);
      }
    }
  };

handlers has no length. My guess is filterQueue's if statement isn't met so it returns nothing. Moving the "var handlers = []" and "return handlers" outside the if statement should fix it.

thanks Grayle! indeed, when there are no handlers attached the code would fail
attached patch fixes this by always returning at least an empty array when filtering the queue

NOTE: the patches here apply only if you also have the latest patch from #3068670: Improve the UX for cookie categories

@reszli: Do you have any steps or sample code that can be used to review this issue?

@svenryen example usage (sending category data to Google Tag Manager) for both D7 and D8 can be found in my sandbox module
https://www.drupal.org/sandbox/reszli/3138494

> we should definitely use this chance for a refactoring of the JS code.

I disagree with this. I think we should get these new features out the door in 1.x. Refactoring all the JS code (and removing jQuery) is probably a 2.x task.

I tried the EUCC GTM module the other day and it seems to work.

Is that sufficient testing, or do we need more testing of this patch?

We've been using this patch successfully in several projects, so for us I think we can call that RTBC.

I trust you on that (will still give it some scrutiny as well) so bumping the status.

@grayle, I tried the d8 and maybe I'm missing something, but it doesn't cleanly apply. We've added quite a bit to the module since June. Would you be able to do a re-roll of the patches?

Sure thing

Rerolled D8 patch against current head of dev (instead of against patched dev as old patch).

Committed the D8 version just now.

Talked with Sven on slack to review/commit this patch