GTM Custom HTML: Purchase source enrichment — Reads dnm_src/dnm_cid via storageHelper (cookie-first, localStorage fallback) at purchase, pushes first_touch_source/medium/campaign/click_id to dataLayer as GA4 event-scope custom dimensions. Does NOT affect attribution model. Requires storageHelper companion gist.

gist-purchase-enrichment.js

/**
 * GTM Custom HTML Tag — Purchase source enrichment
 * File: gtm-purchase-enrichment.js
 *
 * What it does:
 *   When a purchase event fires, reads dnm_src / dnm_cid via storageHelper
 *   and pushes custom event parameters to dataLayer for the GA4
 *   purchase tag to pick up.
 *
 * Requires:
 *   storageHelper (see companion gist) must be loaded BEFORE this tag.
 *   Use GTM Tag Sequencing to ensure correct order.
 *
 * Parameters sent to GA4:
 *   first_touch_source    → e.g. "google"
 *   first_touch_medium    → e.g. "cpc"
 *   first_touch_campaign  → e.g. "TR_branded_max"
 *   first_touch_click_id  → e.g. "gclid:Cj0KCQjw..."
 *
 * IMPORTANT:
 *   These parameters are sent as event-scoped custom dimensions.
 *   They must be defined in GA4 Admin → Custom definitions (Event scope).
 *   They do NOT affect the attribution model — used for reporting only.
 *
 * Trigger:
 *   In GTM, fire this tag BEFORE your GA4 purchase tag
 *   (use Tag Sequencing or bind to your purchase trigger).
 */

(function () {
  /* --- 1. Read from storageHelper (cookie-first, localStorage fallback) */
  var rawSource = storageHelper.get("dnm_src"); // "source|medium|campaign|content|term"
  var rawCid = storageHelper.get("dnm_cid"); // "gclid:Cj0K..." or "fbclid:Ab12..."

  /* --- 2. Parse -------------------------------------------------- */
  var parts = rawSource ? String(rawSource).split("|") : [];
  var source = parts[0] || "(not set)";
  var medium = parts[1] || "(not set)";
  var campaign = parts[2] || "(not set)";
  // parts[3] = content, parts[4] = term — extend if needed

  var clickId = rawCid ? String(rawCid) : "(not set)";

  /* --- 3. DataLayer push ----------------------------------------- */
  // GA4 purchase tag reads these values.
  // Add them as "Event Parameters" in your GA4 purchase tag in GTM.
  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({
    event: "dnm_purchase_enriched",
    first_touch_source: source,
    first_touch_medium: medium,
    first_touch_campaign: campaign,
    first_touch_click_id: clickId,
  });

  /* --- 4. Debug log ---------------------------------------------- */
  if (window.location.search.indexOf("dnm_debug=1") !== -1) {
    console.group("[DNM] Purchase source enrichment");
    console.log("source   :", source);
    console.log("medium   :", medium);
    console.log("campaign :", campaign);
    console.log("click_id :", clickId);
    console.groupEnd();
  }
})();