mergeSort.js

function mergeSort(arr) {
  arr = arr || [];
  if (arr.length <= 1) {
    return arr;
  }
  var midpoint = Math.floor(arr.length / 2)
    , arr1 = arr.slice(0, midpoint)
    , arr2 = arr.slice(midpoint, arr.length)
    ;

  return merge(mergeSort(arr1), mergeSort(arr2))
}

function merge(arr1, arr2) {
  var sorted = [];

  do {
    var head1 = arr1.shift()
      , head2 = arr2.shift();

    if (!head1) {
      if (head2) {
        sorted.push(head2)
      }
      break
    }

    if (!head2) {
      if (head1) {
        sorted.push(head1)
      }
      break
    }

    if (head1 < head2) {
      sorted.push(head1);
      arr2.unshift(head2);
    } else {
      sorted.push(head2);
      arr1.unshift(head1);
    }
  } while (true);

  if (arr1.length > 0) {
    arr1.forEach(function (char) {
      sorted.push(char);
    });
  }
  if (arr2.length > 0) {
    arr2.forEach(function(char) {
      sorted.push(char)
    });
  }

  return sorted;
}

var _ = require('lodash')

const N = 26
  ,   A = 'a';

function randArray(arrSize) {
  function randInt(max) {
    return Math.round(Math.random() * max);
  }
  arrSize = arrSize || N;
  return _(_.range(arrSize)).map(() => {
    return String.fromCharCode(A.charCodeAt(0) + randInt(N));
  }).value();
}

const alphabet = ((a, n) => {
  const code = a.charCodeAt('0');
  return _.range(n).map((_, idx) => String.fromCharCode(code + idx) ).join('');
})(A, N);

const alphaArray = alphabet.split('');

var arr = randArray(26);
console.log('rand->', arr.join(', '));
console.log('sort->', mergeSort(arr).join(', '));
var arr = _.reverse(alphaArray)
console.log('rand->', arr.join(', '))
console.log('sort->', mergeSort(arr).join(', '));