Custom JS - Prevent Duplicate Transaction

function() {
  // customTask Builder by Simo Ahava
  //
  // More information about customTask: https://www.simoahava.com/analytics/customtask-the-guide/
  //
  // Change the default values for the settings below.

  // transactionDeduper: Configuration object for preventing duplicate transactions from being recorded.
  // https://bit.ly/2AvSZ2Y
  var transactionDeduper = {
    keyName: '_transaction_ids',
    cookieExpiresDays: 365
  };

  // DO NOT EDIT ANYTHING BELOW THIS LINE
  var readFromStorage = function(key) {
    if (!window.Storage) {
      // From: https://stackoverflow.com/a/15724300/2367037
      var value = '; ' + document.cookie;
      var parts = value.split('; ' + key + '=');
      if (parts.length === 2) return parts.pop().split(';').shift();
    } else {
      return window.localStorage.getItem(key);
    }
  };

  var writeToStorage = function(key, value, expireDays) {
    if (!window.Storage) {
      var expiresDate = new Date();
      expiresDate.setDate(expiresDate.getDate() + expireDays);
      document.cookie = key + '=' + value + ';expires=' + expiresDate.toUTCString();
    } else {
      window.localStorage.setItem(key, value);
    }
  };

  var globalSendHitTaskName   = '_ga_originalSendHitTask';

  return function(customTaskModel) {

    window[globalSendHitTaskName] = window[globalSendHitTaskName] || customTaskModel.get('sendHitTask');
    var tempFieldObject, dimensionIndex, count, ga, tracker, decorateTimer, decorateIframe, iframe;

    customTaskModel.set('sendHitTask', function(sendHitTaskModel) {

      var originalSendHitTaskModel = sendHitTaskModel,
          originalSendHitTask      = window[globalSendHitTaskName],
          canSendHit               = true;

      var hitPayload, hitPayloadParts, param, val, regexI, trackingId, snowplowVendor, snowplowVersion, snowplowPath, request, originalTrackingId, hitType, nonInteraction, d, transactionId, storedIds;

      try {

        // transactionDeduper
        if (typeof transactionDeduper === 'object' && transactionDeduper.hasOwnProperty('keyName') && transactionDeduper.hasOwnProperty('cookieExpiresDays') && typeof sendHitTaskModel.get('&ti') !== 'undefined') {
          transactionId = sendHitTaskModel.get('&ti');
          storedIds = JSON.parse(readFromStorage(transactionDeduper.keyName) || '[]');
          if (storedIds.indexOf(transactionId) > -1 && ['transaction', 'item'].indexOf(sendHitTaskModel.get('hitType')) === -1) {
            canSendHit = false;
          } else if (storedIds.indexOf(transactionId) === -1) {
            storedIds.push(transactionId);
            writeToStorage(transactionDeduper.keyName, JSON.stringify(storedIds), transactionDeduper.cookieExpiresDays);
          }
        }
        // /transactionDeduper

        if (canSendHit) {
          originalSendHitTask(sendHitTaskModel);
        }

      } catch(e) {
        originalSendHitTask(originalSendHitTaskModel);
      }

    });

  };
}