Add a `client_purchase` event

const script = document.createElement('script');
script.setAttribute('src', 'https://www.googletagmanager.com/gtag/js?id=G-XXXXXXX');
script.setAttribute('async', '');
document.head.appendChild(script);

window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'G-XXXXXXX');

analytics.subscribe("product_added_to_cart", (event) => {

  let totalPrice = event.data.cartLine.merchandise.price.amount * event.data.cartLine.quantity;
  gtag("event", "client_add_to_cart", {
    currency: event.data.cartLine.merchandise.price.currencyCode,
    value: totalPrice.toFixed(2),
    items: [
      {
        item_name: event.data.cartLine.merchandise.product.title,
        item_id: event.data.cartLine.merchandise.sku,
        item_variant: event.data.cartLine.merchandise.title,
        currency: event.data.cartLine.merchandise.price.currencyCode,
        item_brand: event.data.cartLine.merchandise.product.vendor,
        price: event.data.cartLine.merchandise.price.amount,
        quantity: event.data.cartLine.quantity
      }
    ]
  });
});

function ga4CheckoutEvents(event) {
  let checkout = event.data.checkout;
  let lineItems = [];

  for (const checkoutLineItem of event.data.checkout.lineItems){
    lineItems.push({
      item_id: checkoutLineItem.variant.sku,
      item_name: checkoutLineItem.title,
      item_variant: checkoutLineItem?.variant.title,
      currency: checkoutLineItem.variant.price.currencyCode,
      item_brand: checkoutLineItem.variant.product.vendor,
      price: checkoutLineItem.variant.price.amount,
      quantity: checkoutLineItem.quantity
    });
  }

  payload = {
    currency: checkout.totalPrice.currencyCode,
    value: checkout.totalPrice.amount,
    items: lineItems
  };
  
  return payload;
}

analytics.subscribe("checkout_completed", (event) => {

  let payload = ga4CheckoutEvents(event);
  let checkout = event.data.checkout;

  payload.transaction_id = checkout.order?.id || checkout.token;
  payload.shipping = checkout.shippingLine?.price.amount || checkout.shipping_line?.price.amount || 0;
  payload.tax = checkout.totalTax?.amount || 0;

  gtag("event", "client_purchase", payload);
});