The Ecommerce DataLayer

GA4 ecommerce is entirely dataLayer-driven. A small set of standard events describes the shopping journey, each carrying an items array with a consistent shape.

dataLayer.push({ ecommerce: null });   // clear the previous object
dataLayer.push({
  event: "purchase",
  ecommerce: {
    transaction_id: "T-1042",
    value: 49.0,
    currency: "USD",
    items: [
      { item_id: "SKU_1", item_name: "Cap", price: 49.0, quantity: 1 }
    ]
  }
});

Two habits matter: clear the ecommerce object before each push so values do not leak between events, and keep the item fields (item_id, item_name, price, quantity) identical across every event.