map and filter

Two essential methods: they transform an array into a new array, without mutating the original.

map: transform each element

It takes a function, applies it to each element, and returns an array of the same length with the results.

const numeri = [1, 2, 3, 4];

const doppi = numeri.map((n) => n * 2);
// [2, 4, 6, 8]

const stringhe = numeri.map((n) => `n=${n}`);
// ['n=1', 'n=2', 'n=3', 'n=4']

The original stays untouched:

numeri; // [1, 2, 3, 4]  ← unchanged

filter: select a subset

It takes a predicate (a function that returns true/false). It keeps only the elements for which the predicate is true.

const numeri = [1, 2, 3, 4, 5, 6];

const pari = numeri.filter((n) => n % 2 === 0);
// [2, 4, 6]

const grandi = numeri.filter((n) => n > 3);
// [4, 5, 6]

Combining the two

Classic pipeline: filter, then transform.

const prezzi = [12, 5, 100, 30, 7];

const grandiDoppi = prezzi.filter((p) => p > 10).map((p) => p * 2);
// [24, 200, 60]

Try it

Review exercise