X'tagħmel map, filter u reduce?

Question

IT Interview Practice · Accepted Answer

Huma huma **immutable** array iterators — irrigguardaw valur ġdid minflok li jbiddlu l-oriġinali, li jirriżulta f'kodiċi aktar ċċar u aktar dichjarattiv milli loops `for` manuali.

```js
const nums = [1, 2, 3, 4];

// map: transform each item → array of the SAME length
nums.map(n => n * 2);            // [2, 4, 6, 8]

// filter: keep items passing a test → SHORTER array
nums.filter(n => n % 2 === 0);  // [2, 4]

// reduce: fold the array into ONE value (accumulator + current)
nums.reduce((sum, n) => sum + n, 0); // 10
//          ^acc  ^item            ^initial value
```

## Kif jaħdem reduce (dak li ħafna drabi jsibuha komplessa)

Reduce jimxi mill-array biddata "accumulator". Kull pass jirrigguarda l-accumulator il-ġdid:

```js
// step 0: acc=0,  n=1 → 1
// step 1: acc=1,  n=2 → 3
// step 2: acc=3,  n=3 → 6
// step 3: acc=6,  n=4 → 10
```

Mhux biss għall-ammonti — reduce tista' tibni oġġetti, tiġbor oġġetti, jew titkeppi:

```js
users.reduce((acc, u) => { acc[u.id] = u; return acc; }, {}); // index by id
```

## Għaliex huma preferiti

Huma **chain** bir-reqqa u ma jbiddlux, li huwa importanti fit-UIs mifqugħa mill-istat. Uża `forEach` biss għal effetti sekondji (ma triddx xejn). Għal arrays kbar fejn għandek inkella tkun qed tirkib bosta passes, `reduce` wieħed jew loop normali tista' tkun aktar effiċjenti.