Apa sing ditindakake map, filter lan reduce?

Question

IT Interview Practice · Accepted Answer

Kaé minangka **immutable** array iterators — kaé ngasilake nilai anyar tinimbang ngowahi sing asli, sing ndadekake kode luwih resik lan luwih deklaratif tinimbang manual `for` loops.

```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
```

## Carane reduce makarya (sing biasane angel)

Reduce mlaku ing array kanthi ngasilake "accumulator." Saben langkah ngasilake accumulator anyar:

```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
```

Tidak mung kanggo jumlah — reduce bisa gawe obyek, nglompokake item, utawa ngerata-rata:

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

## Napa luwih seneng nggunakake

Kaé **chain** kanthi resik lan ora ngowahi, sing penting ing UI sing didorong keadaan. Gunakake `forEach` mung kanggo efek samping (iku ora ngasilake apa-apa). Kanggo array gedhe ngendi sampeyan bakal nglintasi akeh kali, `reduce` tunggal utawa loop biasa bisa luwih efisien.