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Reset pattern (mandatory)

The rule​

Before every ecommerce event, push { ecommerce: null } to clear state.

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({ ecommerce: null });   // ← reset
window.dataLayer.push({
  event: 'view_item',
  ecommerce: { /* ... */ }
});

Why​

GTM does a recursive merge of dataLayer data. If you push add_to_cart with one item, then view_item with a different item, GTM sees both items in view_item unless you reset.

This corrupts:

  • Revenue attribution per event.
  • Google Ads conversions (potential double-counting).
  • Meta Catalog matching.
function pushEcommerce(eventName, ecommerce, userData) {
  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({ ecommerce: null });
  window.dataLayer.push({
    event: eventName,
    ecommerce,
    ...(userData && { user_data: userData })
  });
}

// Usage
pushEcommerce('add_to_cart', {
  currency: 'EUR',
  value: 84.00,
  items: [{ item_id: 'BIO-CRM-001', item_name: 'Day Cream', price: 42.00, quantity: 2 }]
});

Anti-pattern​

// ❌ BAD: no reset, previous items leak
window.dataLayer.push({ event: 'view_item', ecommerce: { items: [a] } });
window.dataLayer.push({ event: 'add_to_cart', ecommerce: { items: [b] } });
// GTM sees items = [a, b] in add_to_cart 🔥
// ✅ GOOD
window.dataLayer.push({ ecommerce: null });
window.dataLayer.push({ event: 'view_item', ecommerce: { items: [a] } });
window.dataLayer.push({ ecommerce: null });
window.dataLayer.push({ event: 'add_to_cart', ecommerce: { items: [b] } });