Arrays, Objects & Loops

GA4 ecommerce is just arrays and objects, so these are the most important structures you will touch. An object is a labelled bag of values; an array is an ordered list.

const item = { item_id: "AUR-WH-09", item_name: "Aurora Headphones", price: 129 };
item.price;        // 129  (dot access)
item["item_name"]; // "Aurora Headphones"  (bracket access)

const cart = [item, { item_id: "CBL-USB", price: 9 }]; // array of objects
cart.length;       // 2
cart[0].item_id;   // "AUR-WH-09"

Looping to transform a list

You will constantly turn one list into another shape. .map() is the workhorse: it returns a new array with each item transformed.

// Site cart → GA4 items array
const items = cart.map(function (line) {
  return {
    item_id: line.sku,
    item_name: line.title,
    price: Number(line.price),
    quantity: line.qty,
  };
});

.filter() keeps a subset, and .reduce() collapses a list to one value (like a cart total), but .map() covers the majority of tagging work.