quicksort.js

let quicksort = (arr) => {
  quicksortHelper(arr, 0, arr.length - 1)
}

let quicksortHelper = (arr, first, last) => {
  if(first < last){
    let p = partition(arr, first, last)
    quicksortHelper(arr, first, p - 1)
    quicksortHelper(arr, p + 1, last)
  }
  
}

let partition = (arr, first, last) => {
  
    let pivot = arr[first]
    let lMark = first + 1
    let rMark = last
    let done = false

    while (!done) {
      while (arr[lMark] <= pivot && lMark <= rMark) {
        lMark++
      }

      while (arr[rMark] >= pivot && lMark <= rMark) {
        rMark--
      }

      if (lMark > rMark) {
        done = true
      } else {
        let temp = arr[lMark]
        arr[lMark] = arr[rMark]
        arr[rMark] = temp
      }
    }
  
    let temp = arr[first]
    arr[first] = arr[rMark]
    arr[rMark] = temp

    return rMark
}

let arr = [9,8,7]
quicksort(arr)
console.log(arr)