归并排序的原理与实现？和快排的区别？

function mergeSort(arr) {
  if (arr.length <= 1) return arr
  const mid = arr.length >> 1
  const left = mergeSort(arr.slice(0, mid))
  const right = mergeSort(arr.slice(mid))
  return merge(left, right)
}
function merge(a, b) {
  const out = []
  let i = 0, j = 0
  while (i < a.length && j < b.length)
    out.push(a[i] < b[j] ? a[i++] : b[j++])
  return out.concat(a.slice(i), b.slice(j))
}