Arrays: map, filter, and reduce

array.map(fn) builds a new array by running fn on every item and keeping whatever it returns - same length in and out, values transformed. array.filter(fn) builds a new array by keeping only the items where fn returns true - same values, fewer of them. Neither touches the original array.

Both take an arrow function: n => n * 2 doubles, n => n % 2 === 0 checks for even. You'll reach for this pair constantly - it replaces most of the loops you'd otherwise write to transform or narrow down a list.

Sometimes you don't want a new array at all - you want one value out of it, like a total. array.reduce((acc, n) => acc + n, 0) walks the array once, carrying an accumulator (acc, starting at 0) forward and adding each item to it - exactly how you'd total up a list without writing your own loop.

Your task: given the array nums, create doubled (every number doubled), evens (only the even numbers), and total (the sum of every number in nums).

You'll practice:

Transforming an array with .map()
Narrowing an array with .filter()
Collapsing an array to one value with .reduce()

const nums = [1, 2, 3, 4, 5, 6];

// Create doubled: each number in nums, doubled.
// Create evens: only the even numbers from nums.
// Create total: the sum of every number in nums.