# Array 原型方法源码实现大解密

Array.prototype.map = function(callbackfn, thisArg) {
  // 异常处理
  if (this == null) {
  	throw new TypeError("Cannot read property 'map' of null or undefined");
  }
  // Step 1. 转成数组对象，有 length 属性和 K-V 键值对
  let O = Object(this)
  // Step 2. 无符号右移 0 位，左侧用 0 填充，结果非负
  let len = O.length >>> 0
  // Step 3. callbackfn 不是函数时抛出异常
  if (typeof callbackfn !== 'function') {
    throw new TypeError(callbackfn + ' is not a function')
  }
  // Step 4.
  let T = thisArg
  // Step 5.
  let A = new Array(len)
  // Step 6.
  let k = 0
  // Step 7.
  while(k < len) {
    // Step 7.1、7.2、7.3
    // 检查 O 及其原型链是否包含属性 k
    if (k in O) {
      // Step 7.3.1
      let kValue = O[k]
      // Step 7.3.2 执行 callbackfn 函数
      // 传入 this, 当前元素 element, 索引 index, 原数组对象 O
      let mappedValue = callbackfn.call(T, kValue, k, O)
    	// Step 7.3.3 返回结果赋值给新生成数组
      A[k] = mappedValue
    }
    // Step 7.4
    k++
  }
  // Step 8. 返回新数组
  return A
}

// 代码亲测已通过

let arr = [1, , 3, , 5]
console.log(0 in arr) // true
delete arr[0]
console.log(0 in arr) // false
console.log(arr) // [empty × 2, 3, empty, 5]
arr.map(ele => {
  console.log(ele) // 3, 5
})

// 1、原数组新增元素，不受影响
let arr = [1, 2, 3]
let result = arr.map((ele, index, array) => {
  array.push(4);
  return ele * 2
})
console.log(result) 
// 2, 4, 6
// ----------- 完美分割线 -----------


// 2、原数组修改当前索引之前的元素，不受影响
let arr = [1, 2, 3]
let result = arr.map((ele, index, array) => {
  if (index === 1) {
    array[0] = 4
  }
  return ele * 2
})
console.log(result) 
// 2, 4, 6
// ----------- 完美分割线 -----------


// 3、原数组修改当前索引之后的元素，受影响
let arr = [1, 2, 3]
let result = arr.map((ele, index, array) => {
  if (index === 1) {
    array[2] = 4
  }
  return ele * 2
})
console.log(result) 
// 2, 4, 8

// 1、传入 thisArg 但使用箭头函数
let name = 'Muyiy'
let obj = {
    name: 'Hello',
    callback: (ele) => {
        return this.name + ele
    }
}
let arr = [1, 2, 3]
let result = arr.map(obj.callback, obj);
console.log(result) 
// ["1", "2", "3"]，此时 this 指向 window
// 那为啥不是 "Muyiy1" 这样呢，不急，第 3 步介绍
// ----------- 完美分割线 -----------


// 2、传入 thisArg，使用普通函数
let name = 'Muyiy'
let obj = {
    name: 'Hello',
    callback: function (ele) {
        return this.name + ele
    }
}
let arr = [1, 2, 3]
let result = arr.map(obj.callback, obj);
console.log(result) 
// ["Hello1", "Hello2", "Hello3"]，完美
// ----------- 完美分割线 -----------

// 3、不传入 thisArg，name 使用 let 声明
let name = 'Muyiy'
let obj = {
    name: 'Hello',
    callback: function (ele) {
        return this.name + ele
    }
}
let arr = [1, 2, 3]
let result = arr.map(obj.callback);
console.log(result)
// ["1", "2", "3"]
// 为什么呢，因为 let 和 const 声明的变量不会挂载到 window 上
// ----------- 完美分割线 -----------

// 4、不传入 thisArg，name 使用 var 声明
var name = 'Muyiy'
let obj = {
    name: 'Hello',
    callback: function (ele) {
        return this.name + ele
    }
}
let arr = [1, 2, 3]
let result = arr.map(obj.callback);
console.log(result)
// ["Muyiy1", "Muyiy2", "Muyiy3"]
// 看看，改成 var 就好了
// ----------- 完美分割线 -----------

// 5、严格模式
'use strict'
var name = 'Muyiy'
let obj = {
    name: 'Hello',
    callback: function (ele) {
        return this.name + ele
    }
}
let arr = [1, 2, 3]
let result = arr.map(obj.callback);
console.log(result)
// TypeError: Cannot read property 'name' of undefined
// 因为严格模式下 this 指向 undefined

Array.prototype.filter = function(callbackfn, thisArg) {
  // 异常处理
  if (this == null) {
  	throw new TypeError("Cannot read property 'map' of null or undefined");
  }
  if (typeof callbackfn !== 'function') {
    throw new TypeError(callbackfn + ' is not a function')
  }

  let O = Object(this), len = O.length >>> 0,
      T = thisArg, A = new Array(len), k = 0
  // 新增，返回数组的索引
  let to = 0
  
  while(k < len) {
    if (k in O) {
      let kValue = O[k]
      // 新增
      if (callbackfn.call(T, kValue, k, O)) {
        A[to++] = kValue;
      }
    }
    k++
  }
  
  // 新增，修改 length，初始值为 len
  A.length = to;
  return A
}

// 代码亲测已通过

Array.prototype.reduce = function(callbackfn, initialValue) {
  // 异常处理
  if (this == null) {
  	throw new TypeError("Cannot read property 'map' of null or undefined");
  }
  if (typeof callbackfn !== 'function') {
    throw new TypeError(callbackfn + ' is not a function')
  }
  let O = Object(this)
  let len = O.length >>> 0
  let k = 0, accumulator
  
  // 新增
  if (initialValue) {
    accumulator = initialValue
  } else {
    // Step 4.
    if (len === 0) {
      throw new TypeError('Reduce of empty array with no initial value');
    }
    // Step 8.
    let kPresent = false
    while(!kPresent && (k < len)) {
      kPresent = k in O
      if (kPresent) {
        accumulator = O[k] 
      }
      k++
    }
  }
  
  while(k < len) {
    if (k in O) {
      let kValue = O[k]
      accumulator = callbackfn.call(undefined, accumulator, kValue, k, O)
    }
    k++
  }
  return accumulator
}

// 代码亲测已通过

Array.prototype.forEach = function(callbackfn, thisArg) {
  // 相同
  ...
  while(k < len) {
    if (k in O) {
      let kValue = O[k]
 
      // 这部分是 map
      // let mappedValue = callbackfn.call(T, kValue, k, O)
      // A[k] = mappedValue
      
      // 这部分是 forEach
      callbackfn.call(T, kValue, k, O)
    }
    k++
  }
  // 返回 undefined
  // return undefined
}

let arr = [1, , 3, , 5]
console.log(arr) // [1, empty, 3, empty, 5]

let result = arr.map(ele => {
  console.log(ele) // 1, 3, 5
  return ele
})
console.log(result) // [1, empty, 3, empty, 5]