Applicative abstraction in vanilla.js

applicative.js

const arr = {
  ap(fab, fa) {
    return fab.reduce((acc, f) => acc.concat(fa.map(f)), [])
  }
}

const f = (n) => n * 2
const g = (n) => n + 1

const a1 = [f, g]
const a2 = [1, 2, 3]

arr.ap(a1, a2) // => [2, 4, 6, 2, 3, 4]