merge.js

var a = [
  34, 203, 3, 746, 200, 984, 198, 764, 9, 4, 17, 200, 242, 23, 12, 2, 98,
  49, 32, 64, 23, 45, 95, 75, 34, 507, 123, 454, 76, 34, 12, 5, 23, 55, 11,
  95, 45, 33, 63, 66, 234, 76, 76, 45, 44, 77, 22, 12, 90, 9001, 34, 23, 1
];

var merge_sort = (function () {

  var merge = function (left, right) {
    var result = [];

    while (left.length && right.length) {
      if (left[0] <= right[0]) {
        result.push(left.shift());
      } else {
        result.push(right.shift());
      }
    }

    while (left.length) {
      result.push(left.shift());
    }

    while (right.length) {
      result.push(right.shift());
    }

    return result;
  };

  var merge_sort = function (arr) {
    if (arr.length < 2) {
      return arr;
    }

    var middle = parseInt(arr.length / 2);
    var left = arr.slice(0, middle);
    var right = arr.slice(middle, arr.length);

    return merge(merge_sort(left), merge_sort(right));
  };

  return merge_sort;
}());

console.log(merge_sort(a));