mergeSort.js

'use strict';

var arr = [6, 5, 3, 1, 8, 7, 2, 4];

function merge(left, right) {
  var li = 0,
      ri = 0,
     arr = [];

  while (li < left.length && ri < right.length) {
    if (left[li] < right[ri])
      arr.push(left[li++]);
    else
      arr.push(right[ri++]);
  }
  
  return arr.concat(left.splice(li)).concat(right.splice(ri));
}

function mergeSort(arr) {
  if (arr.length < 2) return arr;

  var middle = Math.floor(arr.length / 2),
        left = arr.splice(0, middle),
       right = arr;

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

arr = mergeSort(arr);

console.log(arr);