JavaScript for persisting dataLayer array and data model composition across pages

dataLayerHistory.js

(function() {
  // Set the timeout for when the dataLayer history should be purged. The default is 30 minutes.
  // The timeout needs to be in milliseconds.
  var timeout = 30*60*1000;

  // Change dataLayerName only if you've defined another named for the dataLayer array in your
  // GTM container snippet.
  var dataLayerName = 'dataLayer';

  // Don't change anything below.
  // Initial settings
  var oldPush     = window[dataLayerName].push,
      containerId = {{Container ID}};

  // Method to copy items from dataLayer from before the GTM container snippet was loaded.
  var backfillHistory = function() {
    var tempHistory = [],
        i           = 0,
        len         = window[dataLayerName].length;
    for (; i < len; i++) {
      tempHistory.push(window[dataLayerName][i]);
    }
    return tempHistory;
  };

  // Method to check if object is a plain object.
  // From https://bit.ly/2A3Fuqe
  var isPlainObject = function(value) {
    if (!value || typeof value !== 'object' ||    // Nulls, dates, etc.
        value.nodeType ||                             // DOM nodes.
        value === value.window) {                      // Window objects.
      return false;
    }
    try {
      if (value.constructor && !value.hasOwnProperty('constructor') &&
          !value.constructor.prototype.hasOwnProperty('isPrototypeOf')) {
        return false;
      }
    } catch (e) {
      return false;
    }
    var key;
    for (key in value) {}
    return key === undefined || value.hasOwnProperty(key);
  };

  // Method to merge the stored data model and the history model together.
  // From https://bit.ly/2FrPQWL
  var mergeStates = function(storedModel, historyModel) {
    for (var property in storedModel) {
      if (storedModel.hasOwnProperty(property)) {
        var storedProperty = storedModel[property];
        if (Array.isArray(storedProperty)) {
          if (!Array.isArray(historyModel[property])) historyModel[property] = [];
          mergeStates(storedProperty, historyModel[property]);
        } else if (isPlainObject(storedProperty)) {
          if (!isPlainObject(historyModel[property])) historyModel[property] = {};
          mergeStates(storedProperty, historyModel[property]);
        } else {
          historyModel[property] = storedProperty;
        }
      }
    }
  };

  var getODataModel = function() {
    return window.google_tag_manager[containerId].dataLayer.get({split: function() { return []; }});
  };

  // **Initialize upon first load**
  // Build the history array from local storage
  var dHistory = window._dataLayerHistory = JSON.parse(
    window.localStorage.getItem('_dataLayerHistory') || '{}'
  );

  // Method to reset the history array to the current page state only
  dHistory.reset = function() {
    dHistory.timeout = new Date().getTime() + timeout;
    dHistory.history = backfillHistory();
    for (var prop in dHistory.model) {
      if (dHistory.model.hasOwnProperty(prop) && prop !== 'get') {
        delete dHistory.model[prop];
      }
    }
    mergeStates(getODataModel(), dHistory.model);
    window.localStorage.setItem('_dataLayerHistory', JSON.stringify(dHistory));
  };

  // If initial load
  if (!dHistory.timeout) {
    dHistory.timeout = new Date().getTime() + timeout;
    dHistory.history = [];
    dHistory.model = {};
  }

  dHistory.history = dHistory.history.concat(backfillHistory());
  mergeStates(getODataModel(), dHistory.model);

  // If timeout is reached, reset the history array
  if (dHistory.hasOwnProperty('timeout') && dHistory.timeout < (new Date().getTime())) {
    dHistory.reset();
  }

  // From https://bit.ly/2A2ZcCG
  dHistory.model.get = function(key) {
    var target = dHistory.model;
    var split = key.split('.');
    for (var i = 0; i < split.length; i++) {
      if (target[split[i]] === undefined) return undefined;
      target = target[split[i]];
    }
    return target;
  };

  // Write the new history into localStorage
  window.localStorage.setItem('_dataLayerHistory', JSON.stringify(dHistory));

  window[dataLayerName].push = function() {
    try {

      // Initial settings
      var states     = [].slice.call(arguments, 0),
          timeNow    = new Date().getTime(),
          results    = oldPush.apply(window[dataLayerName], states),
          oDataLayer = window[dataLayerName],
          oDataModel = getODataModel();

      // If timeout is reached, reset the history array
      if (dHistory.hasOwnProperty('timeout') && dHistory.timeout < (new Date().getTime())) {
        dHistory.reset();
      }
      // Push latest item from dataLayer into the history array
      dHistory.history.push(oDataLayer[oDataLayer.length-1]);

      // Merge GTM's data model with the history model
      mergeStates(oDataModel, dHistory.model);

      // Update the timeout
      dHistory.timeout = timeNow + timeout;

      // Write the new history into localStorage
      window.localStorage.setItem('_dataLayerHistory', JSON.stringify(dHistory));
      return results;
    } catch(e) {
      console.log('Problem interacting with dataLayer history: ' + e);
      var states  = [].slice.call(arguments, 0),
          results = oldPush.apply(window[dataLayerName], states);
      return results;
    }
  };
})();

Thank you very much, but how to use it?

Thank you very much, but how to use it?

There is an article explaining this step-by-step here: https://www.simoahava.com/analytics/persist-datalayer-across-pages/

I am not able to retrieve data of a previous page if I push the data to the dataLayer via form submit event.

This is what I am getting:
VM585:4 Problem interacting with dataLayer history: TypeError: Converting circular structure to JSON
--> starting at object with constructor 'HTMLFormElement'
| property 'jQuery351089137954889473742' -> object with constructor 'Object'
| property '.wForm' -> object with constructor 'Object'
| property 'form' -> object with constructor 'S.fn.init'
--- property '0' closes the circle