Stores a copy of tealium_visitor_id in local storage

tealium_visitor_id_in_local_storage.js

/* 
 *
 * Summary: Copies Tealium visitor id cookie value to localStorage to create a more 
 *   durable anonymous visitor id.
 * 
 * Requirements:
 *   - This localStorage value will not be updated until the 1 year point
 *   - This JS Code below is copy/pasted to a JavaScript Extension in Tealium iQ 
 *     with an "All Tags" scope
 * 
 * Note:
 *   - Many vendors support mapping of visitor id, use the tealium_visitor_id for
 *   mapping to override each vendor's visitor id (configure mapping where needed).
 *   - This solution does not attempt to write back over the current cookie value that
 *   may contain a different tealium_visitor_id.  Code shows how to update the 
 *   tealium_visitor_id to default to the value retrieved from localStorage.  The data 
 *   layer would then contain the most-likely-to-be-consistent value.
 *   - While this technique can be used for more than just an anonymous id, remember
 *   that any PII information is best kept out of client-side cookies and localStorage
 *  
 */

// This string used for localStorage key as well as attribute name in data layer object
var localStorageKey = "teal_vid2";

// Update '365' number below to persist visitor id shorter/longer than 1 year
var durationInMS = 365*24*60*60*1000;

function getVisitorIdFromLocalStorage(){
  var val = localStorage.getItem(localStorageKey);
  return val || "";
}

/*
 * Format of value in localStorage is 1551462917775_1551a323423465325235aafbe29177fe75
 * (the timestamp of when it was written, followed by "_", followed by visitor id)
 */
function copyVisitorIdToLocalStorage(visitorId) {
   var currentTime = new Date().getTime(),
     timeWritten,
     temp;

   /*
    * If the time elapsed is greater than duration, then overwrite with new id
    * OR, if it is empty, then set for the first time
    */
   temp =  getVisitorIdFromLocalStorage();
   timeWritten = parseInt(temp.split("_")[0] || 0);
   if (temp === "" || (currentTime - timeWritten) > durationInMS) {
     localStorage.setItem(localStorageKey, currentTime + "_" + visitorId);
   }
}

// The b object is a reference to current data layer
copyVisitorIdToLocalStorage(b.tealium_visitor_id);
b[localStorageKey] = getVisitorIdFromLocalStorage();

/* 
 * If you want to always default to use the value in localStorage for visitor id, then
 * keep the following line below:
 */

b.tealium_visitor_id = b[localStorageKey] || b.tealium_visitor_id;

It appears localStorage is now limited similar to Cookies in Safari. Reference: https://webkit.org/blog/9521/intelligent-tracking-prevention-2-3/