数组API：map、filter、reduce实现

map：

Array.prototype.myMap = function (fn) {
  const arr = this;

  const result = [];
  for (let i = 0; i < arr.length; i++) {
    result.push(fn.call(arr, arr[i], i, arr))
  }

  return result
}

filter：

Array.prototype.myFilter = function (fn) {
  const arr = this;

  const result = [];
  for (let i = 0; i < arr.length; i++) {
    const boolean = fn.call(arr, arr[i], i, arr)
    boolean && result.push(arr[i])
  }

  return result
}

reduce：

Array.prototype.myReduce = function (fn, value) {
  const arr = this
  if (arr.length === 0) return

  let result = value || arr[0]
  let startIndex = value ? 0 : 1
  for (let i = startIndex; i < arr.length; i++) {
    result = fn.call(arr, result, arr[i], i, arr)
  }

  return result
}

使用：

const arr = [1, 2, 3, 4, 5]

const arr1 = arr.myMap((item, idx, arr) => item + idx + arr[1])
const arr2 = arr.myFilter(item => item % 2 === 1)
const arr3 = arr.myReduce((acc, cur) => {
  acc = acc + '' + cur
  return acc
}, '')