Lesson 15 — map & filter

Transforming data

filter keeps only the items that pass a test, and map turns every item into a new one. Both return brand-new arrays and can be chained.

Why it matters: this is how modern JavaScript reshapes data cleanly — no manual loops — and it is everywhere in React and real apps.

Tip: order matters: filter first to shrink the list, then map to transform what is left.