# Iteration: map, filter, reduce, forEach — JavaScript

Source: https://www.geekswithgeeks.com/en/javascript/js-array-iteration

> The functional methods that replace most loops.

## forEach & map

`forEach` just runs code per item; `map` transforms every item into a new array.

```js
const nums = [1, 2, 3];
nums.forEach(n => console.log(n * 2));

const doubled = nums.map(n => n * 2);
console.log(doubled);   // [2, 4, 6]
```

## filter & reduce

`filter` keeps items that pass a test; `reduce` folds the whole array down to one value.

```js
const nums = [1, 2, 3, 4, 5];
const evens = nums.filter(n => n % 2 === 0);
const total = nums.reduce((sum, n) => sum + n, 0);
console.log(evens);  // [2, 4]
console.log(total);  // 15
```

## map vs forEach

`forEach` returns `undefined` — use it for side effects. `map` returns a **new array** — use it to transform data.

**Quiz:** Which method returns a new array shorter than or equal to the original?

- [ ] map
- [x] filter
- [ ] forEach

*Answer:* filter. `filter` only keeps items that pass the test, so the result can never be longer than the input.