Lesson 12 — Transform records

Filter, then total

Backend code often answers questions like "how much money did we actually receive?". That is two steps: filter keeps the paid orders, then reduce adds up their amounts. Chaining them reads left to right like a sentence.

orders.filter((o) => o.paid).reduce((s, o) => s + o.amount, 0);

Why it matters: this filter-then-sum pattern is behind most revenue numbers you will ever compute.