mergeSort.js

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


mergeSort(arr, 0, arr.length - 1)

function mergeSort (arr, low, high) {
  
  	if (low == high) {
    	return [arr[low]]
    }
	var middle = Math.floor((low + high) / 2)
    var leftArr = mergeSort(arr, low, middle)
    var rightArr = mergeSort(arr, middle+1, high)
    
    var newArr = merge(leftArr, rightArr)
    return newArr
}


function merge (arr1, arr2) {
	var newArr = []
    var start1 = 0
    var start2 = 0
	while(start1 < arr1.length && start2 < arr2.length) {
      if (arr1[start1] <= arr2[start2]) {
          newArr.push(arr1[start1])
          start1 += 1
      } else {
          newArr.push(arr2[start2])
          start2 += 1
      }
	}
  
    while(start1 < arr1.length) {
    	newArr.push(arr1[start1])
        start1 += 1
    }
   while(start2 < arr2.length) {
    	newArr.push(arr2[start2])
        start2 += 1
    }
  return newArr
}
(9) [1, 2, 2, 3, 3, 4, 5, 6, 7]