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reduce is the most powerful of the functional array methods: it walks through the collection keeping an accumulator that is updated at every step. When the loop is over, it returns the accumulator.

Signature

arr.reduce((acc, elem) => nuovoAcc, valoreIniziale);

• acc starts at valoreIniziale.
• For each element a new acc is computed.
• The final result is the last acc.

Basic examples: sum and product

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

const somma = numeri.reduce((a, n) => a + n, 0);
// 14

const prodotto = numeri.reduce((a, n) => a * n, 1);
// 60

Without an initial value, reduce starts from the first element (risky on potentially empty arrays → error). Always pass an initial value when you can.

Building an object: counting

const parole = ['mela', 'pera', 'mela', 'kiwi', 'mela'];

const conteggio = parole.reduce((acc, p) => {
  acc[p] = (acc[p] ?? 0) + 1;
  return acc;
}, {});
// { mela: 3, pera: 1, kiwi: 1 }

Finding max / min

const nums = [3, 7, 1, 9, 4];

const max = nums.reduce((a, n) => (n > a ? n : a), -Infinity);
// 9

(In practice for max/min prefer Math.max(...nums); this is a didactic example.)

Try it

Review exercise