map, filter, reduce on a Real List of Orders

Here's a list of orders and five questions a manager would actually ask about it. Each answer is a few lines of map, filter or reduce. Work through them in order. By the end you'll know which method fits which question, and where a plain loop is the better answer.

The data

Every example starts with this list. Paste it at the top of a file called orders.js and add each snippet below it. Run it with node orders.js.

const orders = [
  { id: 101, customer: "Ana", status: "paid", items: [{ sku: "COF", price: 180, qty: 2 }, { sku: "BRD", price: 65, qty: 1 }] },
  { id: 102, customer: "Ben", status: "pending", items: [{ sku: "COF", price: 180, qty: 1 }] },
  { id: 103, customer: "Ana", status: "paid", items: [{ sku: "TEA", price: 120, qty: 3 }] },
  { id: 104, customer: "Cris", status: "cancelled", items: [{ sku: "BRD", price: 65, qty: 4 }] },
  { id: 105, customer: "Dina", status: "paid", items: [] },
];

Notice order 105: paid, but no items. Real data has records like that. Your code should survive it.

Question 1: What's each order's total?

One number for each order, so map. Inside, the total of the items is a reduce:

const orderTotal = order => order.items.reduce((sum, item) => sum + item.price * item.qty, 0);
const withTotals = orders.map(order => ({ id: order.id, customer: order.customer, total: orderTotal(order) }));
console.log(withTotals);

[
  { id: 101, customer: 'Ana', total: 425 },
  { id: 102, customer: 'Ben', total: 180 },
  { id: 103, customer: 'Ana', total: 360 },
  { id: 104, customer: 'Cris', total: 260 },
  { id: 105, customer: 'Dina', total: 0 }
]

The 0 at the end of reduce is why order 105 gives 0 instead of crashing. Also note the brackets in order => ({ ... }). Without them, the braces are read as a function body and you get undefined.

Question 2: How much did we actually collect?

Only paid orders, then one number: filter, then reduce.

const orderTotal = order => order.items.reduce((sum, item) => sum + item.price * item.qty, 0);
const collected = orders
  .filter(order => order.status === "paid")
  .reduce((sum, order) => sum + orderTotal(order), 0);
console.log(collected);

785

Question 3: Which paid orders are empty?

Probably a bug somewhere upstream, so someone should look. Two conditions in one filter, then map to just the ids:

const suspicious = orders
  .filter(order => order.status === "paid" && order.items.length === 0)
  .map(order => order.id);
console.log(suspicious);

[ 105 ]

Question 4: How many of each product did we sell (paid only)?

Now the result is an object keyed by SKU, and each order has several items. You can do it with reduce, but a loop reads better:

const unitsBySku = {};
for (const order of orders.filter(o => o.status === "paid")) {
  for (const item of order.items) {
    unitsBySku[item.sku] = (unitsBySku[item.sku] ?? 0) + item.qty;
  }
}
console.log(unitsBySku);

{ COF: 2, BRD: 1, TEA: 3 }

filter for the "paid only" part, a loop for the counting. Mixing them is fine. Use whichever makes each step obvious.

Question 5: Who are our paying customers, top spender first?

const orderTotal = order => order.items.reduce((sum, item) => sum + item.price * item.qty, 0);
const spent = {};
for (const order of orders) {
  if (order.status !== "paid") continue;
  spent[order.customer] = (spent[order.customer] ?? 0) + orderTotal(order);
}
const ranking = Object.entries(spent)
  .toSorted(([, a], [, b]) => b - a)
  .map(([customer, total]) => `${customer}: ${total}`);
console.log(ranking);

[ 'Ana: 785', 'Dina: 0' ]

Object.entries turns the object into [name, total] pairs so you can sort and map them. toSorted returns a sorted copy. The older sort would work here too, but it sorts in place, which surprises people when the array is used elsewhere.

Quick reference

• One output per input: map
• Keep some: filter
• One value from many: reduce, with a starting value
• First match: find
• Any or all: some, every
• Building an object, nested lists, or stopping early: a for...of loop

Before you write the next report, say the question out loud. Does it start with "for each", "which ones" or "how much in total"?