手写 map 与 reduce，如何保留空位并处理未提供初始值？

'use strict'

function arrayLike(input) {
  if (input === null || input === undefined) throw new TypeError('输入不能是 null 或 undefined')
  const object = Object(input)
  const number = +object.length
  const length = Number.isNaN(number) || number <= 0
    ? 0
    : Math.min(Math.floor(number), Number.MAX_SAFE_INTEGER)
  return { object, length }
}

function arrayMap(input, callback, thisArg) {
  const { object, length } = arrayLike(input)
  if (typeof callback !== 'function') throw new TypeError('callback 必须是函数')
  const result = new Array(length)
  for (let index = 0; index < length; index++) {
    if (index in object) {
      const value = Reflect.apply(callback, thisArg, [object[index], index, object])
      Object.defineProperty(result, index, {
        value, writable: true, enumerable: true, configurable: true
      })
    }
  }
  return result
}

function arrayReduce(input, callback, initialValue) {
  const { object, length } = arrayLike(input)
  if (typeof callback !== 'function') throw new TypeError('callback 必须是函数')
  let index = 0
  let accumulator
  if (arguments.length >= 3) accumulator = initialValue
  else {
    while (index < length && !(index in object)) index++
    if (index === length) throw new TypeError('空数组 reduce 必须提供初始值')
    accumulator = object[index++]
  }
  for (; index < length; index++) {
    if (index in object) accumulator = Reflect.apply(callback, undefined, [accumulator, object[index], index, object])
  }
  return accumulator
}