Array juggling in JavaScript with functional programming approach

functional.js

// precondition: xs is always an array

function head(xs) {
  if (xs.length === 0) return null
  return xs[0]
}

function tail(xs) {
  return xs.slice(1)
}

function map(xs, f) {
  const h = head(xs)
  if (h !== null) return [f(h), ...map(tail(xs), f)] // tail recursion
  return xs
}

function filter(xs, p) {
  const h = head(xs)
  if (h !== null) {
    if (p(h)) return [h, ...filter(tail(xs), p)]
    return [...filter(tail(xs), p)]
  }
  return xs
}

console.log(map([1,2], x => x * 2)) // [2,4]
console.log(filter([-1,0,1,2], x => x > 0)) // [1,2]