map and filter

Loops you do not have to write

In lesson 4-3 you wrote loops to transform and select. Arrays carry built-in methods that do those jobs in one line. Each takes a function (usually an arrow) and applies it to every item.

map transforms each item and returns a new array of the same length:

const nums = [1, 2, 3, 4, 5];
const doubled = nums.map(n => n * 2);   // [2, 4, 6, 8, 10]

filter keeps the items where your arrow answers true, returning a new, possibly shorter array:

const evens = nums.filter(n => n % 2 === 0);   // [2, 4]

Neither method touches the original array. And because each returns an array, you can chain them:

prices.filter(p => p >= 10).map(p => p / 2)

Python's list comprehensions covered both jobs: [n * 2 for n in nums] is a map, and [n for n in nums if ...] is a filter.