Recursive Reduce

reduce.js

// Recursive Reduce
const reduce = (x, y, z) => {
  const head = z[0]
  return !head
    ? y
    : reduce(x, x(y, head), z.slice(1))
}

// Direct usage of reduce
reduce((acc, x) => acc + x, 0, [1, 2, 3]) // 6

// Create map using reduce
const map = (morph, arr) => 
  reduce((acc, x) => acc.concat(morph(x)), [], arr)

map(x => x * 2, [1, 2, 3]) // [2, 4, 6]