The merge sorting algorithm (in place version)

mergesort-in-place.js

function mergeSort (arr) {
  function merge (l, m, r) {
    let i = 0, j = 0, k = l
  
    const leftL = m - l + 1
    const rightL = r - m

    const tempL = []
    const tempR = []

    for (; i < leftL; i++) 
      tempL[i] = arr[l + i]

    for(; j < rightL; j++) 
      tempR[j] = arr[m + 1 + j]

    i = j = 0

    while (i < leftL && j < rightL) {
      if (tempL[i] < tempR[j])
        arr[k++] = tempL[i++]
      else
        arr[k++] = tempR[j++]
    }

    while (i < leftL) 
      arr[k++] = tempL[i++]	

    while (j < rightL) 
      arr[k++] = tempR[j++]
  }

  function sort (l, r) {
    if (l < r) {
      const m = Math.floor((r + l) / 2)

      sort(l, m)
      sort(m + 1, r)

      merge(l, m, r)
    }
  }
  
  sort(0, arr.length - 1)
}