18-merge-sort.js

function mergeSort(arr) {
  if (arr.length < 2) return arr;
  var middleIndex = Math.floor(arr.length / 2);
  var firstHalf = arr.slice(0, middleIndex);
  var secondHalf = arr.slice(middleIndex);
  return merge(mergeSort(firstHalf), mergeSort(secondHalf));
}

function merge(array1, array2) {
  var result = [];
  while(array1.length && array2.length) {
    var minElm;
    if(array1[0] < array2[0]) minElm = array1.shift();
    else minElm = array2.shift();
    result.push(minElm);
  }
  if(array1.length) result = result.concat(array1);
  else if(array2.length) result = result.concat(array2);
  return result;
}

// console.log(mergeSort([3,5,8,20], [1,2,12,17]));
console.log(mergeSort([11, 7, 4, 1, 15, 12, 3]));