Get the current google optimize test index from google tag manager

getOptimizeTestIndexFromTageManager.js

function getOptimizeTestIndexFromTageManager() {
  // returns null if not a test, 0 for the control, or 1 [2,3,4...] for
  // the variant index
  var optimizeContainerID = 'GTM-XXXXXXX'

  if (!window.google_tag_manager) {
    console.warn('Google Tag Manager object not found');
    return;
  }

  var experimentID = window.google_tag_manager[optimizeContainerID] ?
    window.google_tag_manager[optimizeContainerID].experiment : null;
  if (
    !window.google_tag_manager[optimizeContainerID] || 
    !window.google_tag_manager[optimizeContainerID].experiment
  ) {
    return null;
  }

  return parseInt(
    window.google_tag_manager[optimizeContainerID].experiment.split('$')[1],
    10
  );
}