function mergeSort(arr){
  // arr 반으로 나누고 나누고
  if(arr.length<2) return arr
  const mid = Math.floor(arr.length/2)
  const first = arr.slice(0, mid)
  const second = arr.slice(mid)
  return merge(mergeSort(first), mergeSort(second))

  function merge(arr1, arr2){
    const mergeArr = []
    while(arr1.length&& arr2.length){
      if(arr1[0]<arr2[0]) mergeArr.push(arr1.shift())
      else mergeArr.push(arr2.shift())
    }
    if(arr1.length) return mergeArr.concat(arr1)
    else return mergeArr.concat(arr2)
  }
}

console.log(mergeSort([5,3,1,4,2,6,8,1,2]))