merge_sort.js

const mergeSort = arr => {
  if (arr.length < 2) return arr
  const middleIdx = Math.floor(arr.length / 2)

  let left = arr.splice(0, middleIdx)
  let right = arr.splice(0)

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

const merge = (arr1, arr2) => {
  const res = []

  while (arr1.length && arr2.length) {
    if (arr1[0] < arr2[0]) {
      res.push(arr1.splice(0, 1)[0])
      continue
    } 
    res.push(arr2.splice(0, 1)[0])
  }

  return res.concat(arr1, arr2)  
}


const arr = [4,1,6,-9,3,2,8,-7,7]

const res = mergeSort(arr);

console.time('test')
console.log('res', res) // [ -9, -7, 1, 2, 3, 4, 6, 7, 8 ] test: 0.607ms
console.timeEnd('test')