判断一个对象是不是数组

const isArray0 = obj => Array.isArray(obj)
const isArray1 = obj => Object.prototype.toString.call(obj) === "[object Array]"
console.log(isArray0([]))
console.log(
  isArray0({
    0: "value",
    length: 1
  })
)
console.log(isArray1([]))
console.log(
  isArray1({
    0: "value",
    length: 1
  })
)
console.log(isArray0(arguments))
console.log(isArray1(arguments))
console.log(isArray0(document.getElementsByClassName("")))
console.log(isArray1(document.getElementsByClassName("")))
console.log(isArray0(document.getElementsByTagName("div")))
console.log(isArray1(document.getElementsByTagName("div")))
console.log(
  Array.from({
    0: "value",
    length: 1
  })
)
console.log(
  isArray0(
    Array.from({
      0: "value",
      length: 1
    })
  )
)
console.log(
  isArray1(
    Array.from({
      0: "value",
      length: 1
    })
  )
)

es5 方法实现拷贝

// for遍历基本类型，注意for of是取数组元素，for in是取对象属性
const clone0 = arr => {
  const result = []
  for (let item of arr) {
    result.push(item)
  }
  return result
}

const arr0 = [1, 2, 3]
const newArr0 = clone0(arr0)
arr0[2] = 233
console.log(newArr0)

/// slice是纯函数，用来截取指定区间的内容
const arr1 = [1, 2, 3]
const newArr1 = arr1.slice()
arr1[2] = 233
console.log(newArr1)

// concat也是纯函数
const arr2 = [1, 2, 3]
const newArr2 = Array.prototype.concat(arr2)
arr2[2] = 233
console.log(newArr2)

es6 方法实现拷贝

const arr3 = [1, 2, 3]
const newArr3 = Object.assign([], arr3)
arr3[2] = 233
console.log(newArr3)

const arr4 = [1, 2, 3]
const newArr4 = [...arr4]
arr4[2] = 233
console.log(newArr4)

const arr5 = [1, 2, 3]
const newArr5 = arr5.map(i => i)
arr5[2] = 233
console.log(newArr5)

const arr6 = [1, 2, 3]
const newArr6 = arr6.reduce((out, cur) => {
  out.push(cur)
  return out
}, [])
arr6[2] = 233
console.log(newArr6)

const arr7 = [1, 2, 3]
const newArr7 = arr7.filter(_ => 1)
arr7[2] = 233
console.log(newArr7)

API 实现

map

Array.prototype.myMap = function(callbackfn, thisArg) {
  const newArr = []
  for (let i = 0; i < this.length; i++) {
    newArr.push(callbackfn.call(thisArg, this[i], i, this))
  }
  return newArr
}
console.log([1, 2, 3].myMap(i => i * i))

reduce

为什么原生的 reduce 没有 thisArg??

Array.prototype.myReduce = function(callbackfn, initialValue = null) {
  for (let i = 0; i < this.length; i++) {
    initialValue = callbackfn(initialValue, this[i], i, this)
    // initialValue = callbackfn.call(thisArg, initialValue, this[i], i, this)
  }
  return initialValue
}
console.log([1, 2, 3].myReduce((pre, cur) => pre + cur))

filter

Array.prototype.myFilter = function(callbackfn, thisArg) {
  const newArr = []
  for (let i = 0; i < this.length; i++) {
    callbackfn.call(thisArg, this[i], i, this) && newArr.push(this[i])
  }
  return newArr
}
console.log([1, 2, 3].myFilter(i => i >= 2))

对象作用域

const obj = {
  value: 2,
  testFilter() {
    // function函数作用域，上下文随调用时的作用域决定
    return [1, 2, 3].myFilter(function(i) {
      return i >= this.value
    }, this)
  },
  testReduce() {
    return [1, 2, 3].myReduce((pre, cur) => pre + this.value + cur, 0)
  }
}
console.log(obj.testFilter())
console.log(obj.testReduce())