Merge Sort in JavaScript

mergeSort.js

var mergeSort = function(array) {
  if (array.length === 1) {
    return array
  } else {
    var split = Math.floor(array.length/2)
    var left = array.slice(0, split)
    var right = array.slice(split)

    left = mergeSort(left)
    right = mergeSort(right)

    var sorted = []
    while (left.length > 0 || right.length > 0) {
      if (right.length === 0 || left[0] <= right[0]) {
        sorted.push(left.shift())
      } else {
        sorted.push(right.shift())
      }
    }

    return sorted
  }
};

// ### Tests:
// console.log(mergeSort([3,6,9,2]))
// console.log(mergeSort([1000,12,124,12,45,578,11,3,6,9,100,50,9,12,1,34]))
// console.log(mergeSort([3,6,9,11,123,111]))