merge-sort.js

function merge(arr, l, m, r) {
  var i, j, k;
  var n1 = m - l + 1;
  var n2 = r - m;

  var L = new Array(n1);
  var R = new Array(n2);

  for (i = 0; i < n1; i++) L[i] = arr[l + i];
  for (j = 0; j < n2; j++) R[j] = arr[m + 1 + j];

  i = 0;
  j = 0;
  k = l;

  while (i < n1 && j < n2) {
    if (L[i] <= R[j]) {
      arr[k] = L[i];
      i++;
    } else {
      arr[k] = R[j];
      j++;
    }
    k++;
  }

  while (i < n1) {
    arr[k] = L[i];
    i++;
    k++;
  }

  while (j < n2) {
    arr[k] = R[j];
    j++;
    k++;
  }
}

function mergeSort(arr, l, r) {
  if (l < r) {
    let m = Math.floor(l + (r - l) / 2);
    mergeSort(arr, l, m);
    mergeSort(arr, m + 1, r);
    merge(arr, l, m, r);
  }
}