GTM Custom HTML: First-touch source capture — Reads UTM/gclid/fbclid from landing URL, writes to dnm_src and dnm_cid via storageHelper (cookie + localStorage dual-write). Overwrites only on new click ID. Requires storageHelper companion gist.

gist-source-capture.js

/**
 * GTM Custom HTML Tag — First-touch source capture
 * File: gtm-source-capture.js
 *
 * What it does:
 *   Reads utm_*, gclid, fbclid parameters from the landing URL
 *   and writes them to two storage keys via storageHelper:
 *   "dnm_src" and "dnm_cid".
 *
 * Requires:
 *   storageHelper (see companion gist) must be loaded BEFORE this tag.
 *   Use GTM Tag Sequencing to ensure correct order.
 *
 * Overwrite logic:
 *   - If no stored data exists → always write
 *   - If data exists → only overwrite when a new click ID
 *     (gclid/fbclid) is present in the URL
 *   - Organic/direct return visits do NOT erase previous paid source
 *
 * Storage TTL: 30 days (cookie) + localStorage fallback (ITP-resilient)
 * Cookie domain: auto-detected by storageHelper (covers subdomains)
 *
 * GDPR / privacy note:
 *   This script does NOT perform consent checks.
 *   In GTM, set the trigger condition to: analytics_storage = granted.
 */

(function () {
  /* --- Constants ------------------------------------------------- */
  var DNM_TTL = 2592000; // 30 days in seconds
  var KEY_SOURCE = "dnm_src"; // source|medium|campaign|content|term
  var KEY_CID = "dnm_cid"; // click ID type + value

  /* --- Helper: read URL parameter -------------------------------- */
  function getParam(name) {
    var match = window.location.search.match(
      new RegExp("[?&]" + name + "=([^&]*)"),
    );
    return match ? decodeURIComponent(match[1].replace(/\+/g, " ")) : "";
  }

  /* --- 1. Collect URL parameters --------------------------------- */
  var source = getParam("utm_source");
  var medium = getParam("utm_medium");
  var campaign = getParam("utm_campaign");
  var content = getParam("utm_content");
  var term = getParam("utm_term");
  var gclid = getParam("gclid");
  var fbclid = getParam("fbclid");

  // Determine click ID type
  var clickId = gclid ? gclid : fbclid ? fbclid : "";
  var clickType = gclid ? "gclid" : fbclid ? "fbclid" : "";

  // Derive source from click ID when UTMs are missing (auto-tagging)
  if (!source && gclid) {
    source = "google";
    medium = "cpc";
  }
  if (!source && fbclid) {
    source = "facebook";
    medium = "paid_social";
  }

  /* --- 2. Overwrite decision ------------------------------------- */
  var existing = storageHelper.get(KEY_SOURCE);

  // Write conditions:
  //   a) No stored data yet
  //   b) A new click ID is present (paid click overrides previous)
  var shouldWrite = !existing || clickId !== "";

  if (!shouldWrite) {
    return;
  } // preserve existing, exit
  if (!source) {
    return;
  } // nothing to save, exit

  /* --- 3. Build values ------------------------------------------- */
  // dnm_src format: "source|medium|campaign|content|term"
  var srcValue = [
    source || "",
    medium || "",
    campaign || "",
    content || "",
    term || "",
  ].join("|");

  // dnm_cid format: "type:value"  e.g. "gclid:Cj0KCQjw..."
  var cidValue = clickType && clickId ? clickType + ":" + clickId : "";

  /* --- 4. Write via storageHelper (cookie + localStorage) -------- */
  var opts = { maxAge: DNM_TTL };
  storageHelper.set(KEY_SOURCE, srcValue, opts);
  if (cidValue) {
    storageHelper.set(KEY_CID, cidValue, opts);
  }

  /* --- 5. Push to dataLayer (for use as GTM variables) ----------- */
  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({
    event: "dnm_source_captured",
    dnm_first_source: source || "",
    dnm_first_medium: medium || "",
    dnm_first_campaign: campaign || "",
    dnm_first_click_id: cidValue || "",
  });
})();