Quicksort in Javascript

function quickSort(array, startIndex, endIndex) {
  // verify that the start and end index have not overlapped
  if (startIndex < endIndex) {
    // calculate the pivotIndex
    let pivotIndex = partition(array, startIndex, endIndex)
    // sort the left sub-array
    quickSort(array, startIndex, pivotIndex)
    // sort the right sub-array
    quickSort(array, pivotIndex + 1, endIndex)
  }
}

function partition(array, startIndex, endIndex) {
  let pivotIndex = Math.floor((startIndex + endIndex) / 2)
  let pivotValue = array[pivotIndex]

  while (true) {
    // start at the FIRST index of the sub-array and increment
    // FORWARD until we find a value that is > pivotValue
    while (array[startIndex] < pivotValue) {
      startIndex++
    }

    // start at the LAST index of the sub-array and increment
    // BACKWARD until we find a value that is < pivotValue
    while (array[endIndex] > pivotValue) {
      endIndex--
    }

    if (startIndex >= endIndex) return endIndex

    // swap values at the startIndex and endIndex
    let temp = array[startIndex]
    array[startIndex] = array[endIndex]
    array[endIndex] = temp
  }
}

let array = [12, 11, 15, 10, 9, 1, 2, 3, 13, 14, 4, 5, 6, 7, 8]
quickSort(array, 0, array.length - 1)
alert(array)