analytics_controller.js

import { Controller } from 'stimulus'

export default class extends Controller {

  connect() {
    if ( typeof window.ga != 'undefined' ) {
      const currentExperiments = this.experiments()

      Object.keys(currentExperiments).forEach((experimentId) => {
        ga('set', 'expId', experimentId)
        ga('set', 'expVar', currentExperiments[experimentId])
      })

      ga('set',  'location', location.href.split('#')[0])
      ga('send', 'pageview', { "title": document.title })

      if ( this.event() ) {
        const event = this.event()
        ga('send', 'event', event['category'], event['action'], event['label'])
      }
    }
  }

  experiments() {
    return JSON.parse(this.element.getAttribute("data-analytics-experiments"))
  }

  event() {
    return JSON.parse(this.element.getAttribute("data-analytics-event"))
  }
}

helper.rb

  def analytics_controller_data
    data = {}
    
    data["controller"] = "analytics"
    data["analytics-experiments"] = current_experiments.to_json

    if flash[:analytics_event].present?
      data["analytics-event"] = flash[:analytics_event].to_json
    end

    data
  end