map, filter na reduce hufanya nini?

Question

Accepted Answer

Ni vipita-safu (iterators) **visivyobadilisha (immutable)** — hurudisha thamani mpya badala ya kubadilisha safu ya asili, jambo linalopelekea msimbo safi zaidi na wa kitamko (declarative) kuliko vitanzi vya `for` vya kufanya kwa mkono.

```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
```

## Jinsi reduce inavyofanya kazi (kile watu wanachokiona kigumu)

Reduce hupitia safu ikibeba "accumulator." Kila hatua hurudisha accumulator mpya:

```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
```

Si kwa ajili ya jumla pekee — reduce inaweza kujenga objects, kupanga vitu makundi, au kusawazisha (flatten):

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

## Kwa nini uzipendelee

Zina**unganishwa (chain)** kwa usafi na hazibadilishi data, jambo lenye umuhimu katika UIs zinazoendeshwa na state. Tumia `forEach` kwa madhara ya kando (side effects) tu (haurudishi chochote). Kwa safu kubwa mno ambapo ungeunganisha mapito mengi, `reduce` moja au kitanzi cha kawaida kinaweza kuwa na ufanisi zaidi.