recursive implementation of map and reduce using only lambdas and Array.prototype.pop()

map_reduce.js

let map = (func, arr) =>
  arr.length == 0 ? 
    [] : ((x, y) => [...y, x])(func(arr.pop()), map(func, arr))

// if you want to pull an one-liner:
// let map = (func, arr) => arr.length == 0 ? [] : ((x, y) => [...y, x])(func(arr.pop()), map(func, arr))

let reduce = (func, initial_value, arr) =>
  arr.length == 0 ? 
    initial_value : ((x, y) => func(y, x))(arr.pop(), reduce(func, initial_value, arr))

// similarly, one-liner:
// let reduce = (func, initial_value, arr) => arr.length == 0 ? initial_value : ((x, y) => func(y, x))(arr.pop(), reduce(func, initial_value, arr))

// Usage of map:
let a = [1, 2, 3]
let b = map(x => x * x, a)
console.log(b) // [1, 4, 9]

// Usage of reduce:
let aa = [1, 2, 3]
let bb = reduce((x, y) => x + y, 0, aa)
console.log(bb) // 6