GTM recipe: a clean e-commerce data layer specification

Most tracking problems are data layer problems wearing a disguise. This recipe is the specification we hand to client dev teams — steal it.

The events

Implement exactly these, named exactly this way (they map 1:1 to GA4's recommended e-commerce events):

• view_item — product page view
• add_to_cart
• begin_checkout
• add_payment_info
• purchase

The purchase push

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({ ecommerce: null }); // clear the previous object
window.dataLayer.push({
  event: "purchase",
  ecommerce: {
    transaction_id: "T-10345",       // MUST be unique per order
    value: 128.40,                    // order total ex-shipping, number not string
    currency: "GBP",
    items: [{
      item_id: "SKU-2231",
      item_name: "Oak Side Table",
      price: 128.40,
      quantity: 1,
      item_category: "Furniture"
    }]
  }
});

Three rules that prevent 90% of the bugs we get called in to fix:

1. Clear before you push. The { ecommerce: null } reset stops old items leaking into new events.
2. Numbers are numbers. "128.40" as a string breaks revenue in subtle, quiet, expensive ways.
3. transaction_id is sacred. It's how GA4 deduplicates and how server-side events match browser events. Never omit it, never reuse it.

GTM variable setup

Create Data Layer Variables (version 2) for each field: ecommerce.transaction_id, ecommerce.value, ecommerce.currency, ecommerce.items. Then every tag — GA4, Google Ads, Meta — reads from the same variables. One source, no per-tag drift.

QA checklist

• Purchase fires once per order (test a page refresh on the thank-you page — the classic double-count).
• Values match the order management system to the penny for 20 sample orders.
• Events fire with consent granted and are correctly suppressed with consent denied.
• items array is populated on every event, not just purchase.

Run that checklist quarterly. Data layers rot the moment a developer touches the checkout.