Array.prototype.flatMap

flatMap.js

const array = [1, 3]
const f = x => [x, x + 1]

array.map(f) // > [[1, 2], [3, 4]]
array.flatMap(f) // > [1, 2, 3, 4]

// Ahora un ejemplo un poco más curioso...
const heroes = ["Eren Jaeger", "Mikasa Ackerman"]
const splitBySpace = xs => xs.split(" ")

heroes.map(splitBySpace) // > [["Eren", "Jaeger"], ["Mikasa", "Ackerman"]]
heroes.flatMap(splitBySpace) // > ["Eren", "Jaeger", "Mikasa", "Ackerman"]