quicksort in ES6 adapted from https://www.nczonline.net/blog/2012/11/27/computer-science-in-javascript-quicksort/

quicksort.js

#!/usr/bin/env node
"use strict"

class Quicksort extends Array {
  // swap the value in the leftIdx with the value in the rightIdx
  swap(leftIdx, rightIdx) {
    let old = this[leftIdx]
    this[leftIdx] = this[rightIdx]
    this[rightIdx] = old
  }

  // Find the pivot point of which we will compare the left and right values with and
  // then either move the pointer towards the pivot or swap the left and right values
  partition(leftIdx, rightIdx) {
    let pivot = this[Math.floor((rightIdx + leftIdx) / 2)] // middle number
    let left = leftIdx
    let right = rightIdx
    // left point has not crossed the right pointer
    while (left <= right) {
      // the value at the left pointer is less than the pivot then increase the index
      // moving it towards the pivot point
      while (this[left] < pivot) {
        left++
      }

      // the value at the right pointer is greater than the pivot then decrease the index
      // moving it towards the pivot point
      while (this[right] > pivot) {
        right--
      }

      // if the left index is less than the right as it should be then
      // then swap the values
      if (left <= right) {
        this.swap(left, right)
        left++
        right--
      }
    }

    return left;
  }

  // default the sort command to use the length of the array
  sort(left = 0, right = this.length - 1) {
    let index

    // this will recursively call until the partition is less than the index - 1
    // and the index is greater than right
    if (this.length > 1) {
      index = this.partition(left, right);
      if (left < index - 1) {
        this.sort(left, index - 1);
      }

      if (index < right) {
        this.sort(index, right);
      }
    }
    return this
  }
}

var items = [4, 2, 6, 5, 3, 9, 10, 99, 8, 42, 36, 22, 21, 7, 12, 85, 87]

var q = new Quicksort(...items)
console.log(q.sort())
// Quicksort [ 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 21, 22, 36, 42, 85, 87, 99 ]