Lesson 15 — map & filter

Keep some, change all

filter keeps only the items that pass a test; map turns every item into a new one. Both return a new array and leave the original alone, so you can chain them.

[1, 2, 3, 4].filter((x) => x % 2 === 0);   // [2, 4]
[2, 4].map((x) => x * 10);                  // [20, 40]

x % 2 === 0 means "x divided by 2 leaves no remainder" — x is even.

Why it matters: this is how modern JavaScript reshapes data, and it is everywhere in React.