JS Bin// source http://jsbin.com/dukekol

mergesort.js

'use strict';

var seq = [1, 4, 6, 3, 1, 4, 6, 3, 25, 593, 0, 53, 24, 352, 53905, 234];

function merge(array) {
  if (array.length < 2) {
    console.log('returns ' + array);
    return array;
  }
  var mid = array.length / 2;
  var l = array.slice(0, mid);
  var r = array.slice(mid, array.length);
  console.log('done');
  return mergeSort(merge(l), merge(r));
}

function mergeSort(left, right) {
  var acc = [];
  while (left.length && right.length) {
    if (left[0] <= right[0]) {
      acc.push(left.shift());
    } else {
      acc.push(right.shift());
    }
  }
  while (left.length) {
    acc.push(left.shift());
  }
  while (right.length) {
    acc.push(right.shift());
  }
  return acc;
}

console.log(merge(seq));