mergeSortIterative.js

function mergeSort(array) {
  let len = array.length
  let b = 1
  while (b < len) {

    for (let i = 0; i + b < len; i += b * 2) {
      let l = i
      let r = i + b

      let left = array.slice(l, l+b)
      let right = array.slice(r, r+b)

      let j = i
      for (let item of merge(left, right)) // merge is the function we defined earlier
        array[j++] = item
    }

    b *= 2
  }

  return array
}