What do map, filter and reduce do?

Question

Accepted Answer

They are **immutable** array iterators — they return a new value instead of mutating the original, which leads to cleaner, more declarative code than 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
```

## How reduce works (the one people find tricky)

Reduce walks the array carrying an "accumulator." Each step returns the new accumulator:

```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
```

It's not just for sums — reduce can build objects, group items, or flatten:

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

## Why prefer them

They **chain** cleanly and don't mutate, which matters in state-driven UIs. Use `forEach` only for side effects (it returns nothing). For huge arrays where you'd otherwise chain many passes, a single `reduce` or a plain loop can be more efficient.