Javascript Quicksort

quicksort.js

// Quicksort
// Time: O(n log(n))
// Space: O(log(n))

const test = require('assert').deepEqual

const quickSort = (arr) => {
  if (arr.length <= 1) return arr
  
  const pivotIndex = arr.length - 1
  const pivotVal = arr[pivotIndex]
  const left = []
  const right = []
  
  arr.slice(0, pivotIndex).forEach(val => {
    val < pivotVal ? left.push(val) : right.push(val) 
  })
  
  return quickSort(left).concat([pivotVal], quickSort(right))
  
}

test(quickSort([1]), [1])
test(quickSort([2, 1]), [1, 2])
test(quickSort([2, 1, -1]), [-1, 1, 2])
test(quickSort([2, 1, -1, 0.23]), [-1, 0.23, 1, 2])

Note that this creates new arrays, should reorder in-place for improved space complexity