recursion.js

const sum = (numbers = []) => {
  if (numbers.length === 0) {
    return 0
  }
  return numbers[0] + sum(numbers.slice(1))
}

const max = (numbers = []) => {
  if (numbers.length === 0) {
    return 0
  }

  if (numbers.length === 2) {
    return numbers[0] > numbers[1]
      ? numbers[0]
      : numbers[1]
  }

  const subMax = max(numbers.slice(1))

  return numbers[0] > subMax
    ? numbers[0]
    : subMax
}

const quickSort = (numbers = []) => {
  if (numbers.length < 2) {
    return numbers
  }
  
  const pivot = numbers[0]

  const rest = numbers.slice(1)

  const left = rest.filter((value) => value <= pivot)

  const right = rest.filter((value) => value > pivot)

  return [ ...quickSort(left), pivot, ...quickSort(right) ]
}