我想你们一定对JavaScript中的数组非常熟悉，我们每天都会用到它的各种方法，比如push、pop、forEach、map……等等。

但是仅仅使用它就足够了吗？太棒了，你一定不想在这里停下来。想和你一起挑战，实现20+个数组方法的功能。

forEach是我们工作中使用频率非常高的一个数组方法，实现起来也比较简单。这是我们需要完成的第一个功能。

(资料图)

代码

Array.prototype.forEach2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  while (i < length) {    // Deleted, the newly added element index i is not in the array, so it will not be accessed    if (this.hasOwnProperty(i)) {      callback.call(thisCtx, this[ i ], i, this)    }    i++  }}

测试一下

let demoArr = [ 1, 2, 3, 4, , 5 ]demoArr.forEach2((it, i) => {  if (i === 1) {    // 5 will not be printed out    demoArr.push(5)  } else if (i === 2) {    // 4 will not be printed out, but "4-4" will be printed out    demoArr.splice(3, 1, "4-4")  }  console.log(it)})/* 1 2 3 4-4 5*/

哇，恭喜！我们已经实现了 forEach 的特性。

您通常使用map做什么？大多数时候是将一个数组转换成另一个数组。

代码

Array.prototype.map2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  // The return value of the map method is a new array  let newArray = []  while (i < length) {    // Deleted and uninitialized values will not be accessed    if (this.hasOwnProperty(i)) {      newArray.push(callback.call(thisCtx, this[ i ], i, this))    }    i++  }  // Return new array  return newArray}

测试一下

let arr = [ 0, 1, 2, 3, 4,, 5 ]let arr2 = arr.map2(function (it, i, array) {  console.log(it, i, array, this)  return it * it}, { name: "fatfish" })console.log(arr2) // [0, 1, 4, 9, 16, 25]

朋友们，你们觉得不难吗？那是因为你太优秀了。

every() 方法测试数组中的所有元素是否通过提供的函数实现的测试。它返回一个布尔值。

every方法有3个你以前可能没有注意到的点，它们是什么？

let emptyArr = []// Calling every method on an empty array returns trueconsole.log(emptyArr.every((it) => it > 0)) // true// The `callback` method will only be called by an index that has already been assigned a value.let arr = [ 0, 1, 2, 3, 4,, 5, -1 ]// The `callback` method will not be called when an array value is deleted or an index that has never been assigned a value.delete arr[7]console.log(arr.every((it) => it >= 0)) // true

代码

Array.prototype.every2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  // If the length of the array is 0, the while loop will not be entered  while (i < length) {    // False will be returned as long as a value does not conform to the judgment of callback    if (this.hasOwnProperty(i) && !callback.call(thisCtx, this[ i ], i, this)) {      return false    }    i++  }  return true}

测试一下

let emptyArr = []// Calling every method on an empty array returns trueconsole.log(emptyArr.every2((it) => it > 0)) // true// The `callback` method will only be called by an index that has already been assigned a value.let arr = [ 0, 1, 2, 3, 4,, 5, -1 ]// The `callback` method will not be called when an array value is deleted or an index that has never been assigned a value.delete arr[7]console.log(arr.every2((it) => it >= 0)) // true

some() 方法测试数组中的至少一个元素是否通过了由提供的函数实现的测试。

代码

Array.prototype.some2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  while (i < length) {    // Returns true if any element meets the callback condition    if (this.hasOwnProperty(i) && callback.call(thisCtx, this[ i ], i, this)) {      return true    }    i++  }  return false}

测试一下

let emptyArr = []// An empty array will return falseconsole.log(emptyArr.some2((it) => it > 0)) // falselet arr = [ 0, 1, 2, 3, 4,, 5, -1 ]delete arr[7]console.log(arr.some2((it) => it < 0)) // falseconsole.log(arr.some2((it) => it > 0)) // true

filter() 方法创建一个新数组，其中包含通过提供的函数实现的测试的所有元素。

Array.prototype.filter2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  // The return value will be a new array  let newArray = []  let i = 0  while (i < length) {    if (this.hasOwnProperty(i) && callback.call(thisCtx, this[ i ], i, this)) {      newArray.push(this[ i ])    }    i++  }  return newArray}

测试一下

// The position with index 5 will not be traversed because it has no initialization valuelet arr = [ 0, 1, 2, -3, 4,, 5 ]// we try to remove the last elementdelete arr[6]// filter out values greater than 0let filterArr = arr.filter2((it) => it > 0)console.log(filterArr) // [ 1, 2, 4 ]

这个函数有点复杂。让我们用一个例子来看看它是如何使用的。

const sum = [1, 2, 3, 4].reduce((prev, cur) => {  return prev + cur;})console.log(sum) // 10// initializationprev = initialValue = 1, cur = 2// step 1prev = (1 + 2) =  3, cur = 3// step 2prev = (3 + 3) =  6, cur = 4// step 3prev = (6 + 4) =  10, cur = undefined (quit)

代码

Array.prototype.reduce2 = function (callback, initValue) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  let pre = initValue  let i = 0  const length = this.length  // When the initial value is not passed, use the first value of the array as the initial value    if (typeof pre === "undefined") {    pre = this[0]    i = 1  }  while (i < length) {    if (this.hasOwnProperty(i)) {      pre = callback(pre, this[ i ], i, this)    }    i++  }  return pre}

测试一下

const sum = [1, 2, 3, 4].reduce2((prev, cur) => {  return prev + cur;})console.log(sum) // 10

reduceRight() 方法对累加器和数组的每个值（从右到左）应用函数以将其减少为单个值。

除了reduceRight是从右向左遍历之外，它与reduce非常相似。

const sum = [1, 2, 3, 4].reduce((prev, cur) => {  console.log(prev, cur)  return prev + cur;})// 1 2// 3 3// 6 4console.log(sum) // 10const sum2 = [1, 2, 3, 4].reduceRight((prev, cur) => {  console.log(cur)  return prev + cur;})// 4 3// 7 2// 9 1console.log(sum2) // 10

代码

Array.prototype.reduceRight2 = function (callback, initValue) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  let pre = initValue  const length = this.length  // Start with the last element  let i = length - 1  // If no initial value is passed, the last element is taken as the initial value  if (typeof pre === "undefined") {    pre = this[i]    i--  }  while (i >= 0) {    if (this.hasOwnProperty(i)) {      pre = callback(pre, this[ i ], i, this)    }    i--  }  return pre}

测试一下

const sum = [1, 2, 3, 4].reduceRight2((prev, cur) => {  console.log(cur)  return prev + cur;})// 4 3// 7 2// 9 1console.log(sum) // 10

find() 方法返回提供的数组中满足提供的测试函数的第一个元素。如果没有值满足测试函数，则返回 undefined。

代码

Array.prototype.find2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  while (i < length) {    const value = this[ i ]    // As long as there is an element that matches the logic of the callback function, the element value is returned    if (callback.call(thisCtx, value, i, this)) {      return value    }    i++  }  // otherwise return undefined    return undefined}

测试一下

let arr = [ 0, 1, 2, 3, 4,, 5 ]let ele = arr.find2(function (it, i, array) {  console.log(it, i, array, this)  return it > 3}, { name: "fatfish" })console.log(ele) // 4

findIndex() 方法返回数组中满足提供的测试函数的第一个元素的索引。否则返回-1，表示没有元素通过测试。

let arr = [ 0, 1, 2, 3, 4,, 5 ]let index = arr.findIndex((it, i, array) => {  return it > 2})console.log(index) // 3

代码

Array.prototype.findIndex2 = function (callback, thisCtx) {  if (typeof callback !== "function") {    throw `${callback} is not a function`  }  const length = this.length  let i = 0  while (i < length) {    // Return index i that conforms to callback logic    if (callback.call(thisCtx, this[ i ], i, this)) {      return i    }    i++  }  return -1}

测试一下

let arr = [ 0, 1, 2, 3, 4,, 5 ]let index = arr.findIndex2(function (it, i, array) {  console.log(it, i, array, this)  return it > 2}, { name: "fatfish" })console.log(index) // 3

indexOf() 方法返回可以在数组中找到给定元素的第一个索引，如果不存在则返回 -1。

arr.indexOf(searchElement[, fromIndex])

注意：

如果开始搜索的索引值大于等于数组长度，则表示不在数组中进行搜索，返回-1。

如果fromIndex为负数，则按照-1表示从最后一个元素开始查找，-2表示从倒数第二个元素开始查找，以此类推。

如果 fromIndex 为负数，则仍然从前到后查找数组。

const array = [2, 5, 9]console.log(array.indexOf(2))      // 0console.log(array.indexOf(7))      // -1console.log(array.indexOf(9, 2))   // 2console.log(array.indexOf(2, -1))  // -1console.log(array.indexOf(2, -3))  // 0console.log(array.indexOf(2, -4))  // 0

代码

Array.prototype.indexOf2 = function (targetEle, fromIndex) {  const length = this.length  fromIndex = +fromIndex || 0  // If the array is empty or the search starts from a place greater than or equal to the length of the array, it will directly return -1  if (length === 0 || fromIndex >= length) {    return -1  }  /*    1. Search elements from fromIndex    2. Use it directly when fromindex is greater than 0    3. If it is less than 0, first subtract the absolute value of fromIndex from the length. If it is still less than 0, take 0 directly  */  let i = Math.max(fromIndex >= 0 ? fromIndex : length - Math.abs(fromIndex), 0)  while (i < length) {    // element in the array and equal to targetEle    if (this.hasOwnProperty(i) && targetEle === this[ i ]) {      return i    }    i++  }  return -1}

测试一下

const array = [2, 5, 9]console.log(array.indexOf2(2))      // 0console.log(array.indexOf2(7))      // -1console.log(array.indexOf2(9, 2))   // 2console.log(array.indexOf2(2, -1))  // -1console.log(array.indexOf2(2, -3))  // 0console.log(array.indexOf2(2, -4))  // 0

lastIndexOf() 方法返回可以在数组中找到给定元素的最后一个索引，如果不存在则返回 -1。从 fromIndex 开始向后搜索数组。

除了 lastIndexOf 从右到左遍历之外，它与 indexOf 非常相似。

let array = [2, 5, 9, 2]console.log(array.lastIndexOf(2)) // 3console.log(array.lastIndexOf(7)) // -1console.log(array.lastIndexOf(2, 3)) // 3console.log(array.lastIndexOf(2, 2)) // 0console.log(array.lastIndexOf(2, -2)) // 0console.log(array.lastIndexOf(2, -1)) // 3

代码

Array.prototype.lastIndexOf2 = function (targetEle, fromIndex) {  const length = this.length  fromIndex = typeof fromIndex === "undefined" ? length - 1 : fromIndex  // // Empty array, when fromIndex is negative and the absolute value is greater than the length of the array, the method returns -1, that is, the array will not be searched.  if (length === 0 || fromIndex < 0 && Math.abs(fromIndex) >= length) {    return -1  }  let i  if (fromIndex >= 0) {    // If `fromIndex` is greater than or equal to the length of the array, the entire array is searched.    i = Math.min(fromIndex, length - 1)  } else {    i = length - Math.abs(fromIndex)  }  while (i >= 0) {    // Returns the index when it is equal to targetEle    if (i in this && targetEle === this[ i ]) {      return i    }    i--  }  // Returns -1 when the current value is not found  return -1}

测试一下

let array = [2, 5, 9, 2]console.log(array.lastIndexOf2(2)) // 3console.log(array.lastIndexOf2(7)) // -1console.log(array.lastIndexOf2(2, 3)) // 3console.log(array.lastIndexOf2(2, 2)) // 0console.log(array.lastIndexOf2(2, -2)) // 0console.log(array.lastIndexOf2(2, -1)) // 3

includes() 方法确定数组是否在其条目中包含某个值，并根据需要返回 true 或 false。

arr.includes(valueToFind[, fromIndex])

注意:

includes 方法将从 fromIndex 索引开始搜索 valueToFind。

如果 fromIndex 为负数，则开始搜索 array.length + fromIndex 的索引。

如果数组中存在 NaN，则 [..., NaN] Includes (NaN) 为真。

console.log([1, 2, 3].includes(2))     // trueconsole.log([1, 2, 3].includes(4))     // falseconsole.log([1, 2, 3].includes(3, 3))  // falseconsole.log([1, 2, 3].includes(3, -1)) // trueconsole.log([1, 2, NaN].includes(NaN)) // true

代码

Array.prototype.includes2 = function (targetEle, fromIndex) {  const length = this.length  fromIndex = +fromIndex || 0  if (length === 0 || fromIndex >= length) {    return false  }  // Search for elements from the position of fromIndex  let i = Math.max(fromIndex >= 0 ? fromIndex : length - Math.abs(fromIndex), 0)  while (i < length) {    const value = this[ i ]    // Please note NaN    if (targetEle === value || typeof targetEle === "number" && typeof value === "number" && isNaN(targetEle) && isNaN(value)) {      return true    }    i++  }  return false}

测试一下

console.log([1, 2, 3].includes2(2))     // trueconsole.log([1, 2, 3].includes2(4))     // falseconsole.log([1, 2, 3].includes2(3, 3))  // falseconsole.log([1, 2, 3].includes2(3, -1)) // trueconsole.log([1, 2, NaN].includes2(NaN)) // true

push() 方法将一个或多个元素添加到数组的末尾并返回数组的新长度。

const animals = ["pigs", "goats", "sheep"]animals.push("cows")console.log(animals, animals.length) // ["pigs", "goats", "sheep", "cows"], 4animals.push("chickens", "cats", "dogs")console.log(animals, animals.length) // ["pigs", "goats", "sheep", "cows", "chickens", "cats", "dogs"], 7

代码

Array.prototype.push2 = function (...pushEles) {  const pushEleLength = pushEles.length  const length = this.length  let i = 0  while (i < pushEleLength) {    this[ length + i ] = pushEles[ i ]    i++  }  return this.length}

测试一下

const animals = ["pigs", "goats", "sheep"]animals.push2("cows")console.log(animals, animals.length) // ["pigs", "goats", "sheep", "cows"], 4animals.push2("chickens", "cats", "dogs")console.log(animals, animals.length) // ["pigs", "goats", "sheep", "cows", "chickens", "cats", "dogs"], 7

pop() 方法从数组中删除最后一个元素并返回该元素。此方法更改数组的长度。

let arr = [ 1, 2 ]let arr2 = []console.log(arr.pop(), arr) // 2 [1]console.log(arr2.pop(), arr2) // undefined []

代码

Array.prototype.pop2 = function () {  const length = this.length  // If it is an empty array, return undefined  if (length === 0) {    return undefined  }  const delEle = this[ length - 1 ]  this.length = length - 1  return delEle}

测试一下

let arr = [ 1, 2 ]let arr2 = []console.log(arr.pop2(), arr) // 2 [1]console.log(arr2.pop2(), arr2) // undefined []

unshift() 方法将一个或多个元素添加到数组的开头并返回数组的新长度。

如果传入多个参数调用一次unshift，与传入一个参数多次调用unshift（比如循环调用）得到的结果是不同的。

let arr = [4,5,6]// Insert multiple elements at oncearr.unshift(1,2,3)console.log(arr) // [1, 2, 3, 4, 5, 6]let arr2 = [4,5,6]// Insert multiple timesarr2.unshift(1)arr2.unshift(2)arr2.unshift(3)console.log(arr2); // [3, 2, 1, 4, 5, 6]

代码

Array.prototype.unshift2 = function (...unshiftEles) {  // With "...", Insert the element to be added in front of the array  let newArray = [ ...unshiftEles, ...this ]  let length = newArray.length  let i = 0  if (unshiftEles.length === 0) {    return length  }  // Recopy to array  while (i < length) {    this[ i ] = newArray[ i ]    i++  }  return this.length}

测试一下

let arr = [4,5,6]// Insert multiple elements at oncearr.unshift2(1,2,3)console.log(arr) // [1, 2, 3, 4, 5, 6]let arr2 = [4,5,6]// Insert multiple timesarr2.unshift2(1)arr2.unshift2(2)arr2.unshift2(3)console.log(arr2); // [3, 2, 1, 4, 5, 6]

shift() 方法从数组中删除第一个元素并返回删除的元素。此方法更改数组的长度。

let arr = [ 1, 2 ]console.log(arr.shift(), arr) // 1 [2]console.log(arr.shift(), arr) // 2 []

代码

Array.prototype.shift2 = function () {  const length = this.length  const delValue = this[ 0 ]  let i = 1  while (i < length) {    // Starting from the first element, the following elements move forward one bit    this[ i - 1 ] = this[ i ]    i++  }  // Set the length of the array  this.length = length - 1  // Return deleted value  return delValue}

测试一下

let arr = [ 1, 2 ]console.log(arr.shift2(), arr) // 1 [2]console.log(arr.shift2(), arr) // 2 []

reverse() 方法原地反转数组。第一个数组元素成为最后一个，最后一个数组元素成为第一个。

const arr = [1, 2, 3]console.log(arr) // [1, 2, 3]arr.reverse()console.log(arr) // [3, 2, 1]

代码

Array.prototype.reverse2 = function () {  let i = 0  let j = this.length - 1  while (i < j) {    [ this[ i ], this[ j ] ] = [ this[ j ], this[ i ] ]    i++    j--  }  return this}

测试一下

const arr = [1, 2, 3]console.log(arr) // [1, 2, 3]arr.reverse2()console.log(arr) // [3, 2, 1]

fill() 方法将数组中的所有元素更改为静态值，从起始索引（默认 0）到结束索引（默认 array.length）。 它返回修改后的数组。

const array1 = [1, 2, 3, 4];console.log(array1.fill(0, 2, 4)) // [1, 2, 0, 0]console.log(array1.fill(5, 1)) // [1, 5, 5, 5]console.log(array1.fill(6)) // [6, 6, 6, 6]

代码

Array.prototype.fill2 = function (value, start, end) {  const length = this.length  start = start >> 0  // The default value of end is length  end = typeof end === "undefined" ? length : end >> 0  // The minimum value of start is 0 and the maximum value is length  start = start >= 0 ? Math.min(start, length) : Math.max(start + length, 0)  // The minimum value of end is 0 and the maximum value is length  end = end >= 0 ? Math.min(end, length) : Math.max(end + length, 0)  // The element that fills the specified range is value  while (start < end) {    this[ start ] = value    start++  }  return this}

测试一下

const array1 = [1, 2, 3, 4];console.log(array1.fill2(0, 2, 4)) // [1, 2, 0, 0]console.log(array1.fill2(5, 1)) // [1, 5, 5, 5]console.log(array1.fill2(6)) // [6, 6, 6, 6]

concat() 方法用于合并两个或多个数组。 此方法不会更改现有数组，而是返回一个新数组。

let num1 = [[1]]let num2 = [2, [3]]let num3=[5,[6]]let nums = num1.concat(num2) // [[1], 2, [3]]let nums2 = num1.concat(4, num3) // [[1], 4, 5,[6]]

代码

Array.prototype.concat2 = function (...concatEles) {  const length = concatEles.length  // The array itself needs to be expanded one layer  let newArray = [ ...this ]  let i = 0  while (i < length) {    const value = concatEles[ i ]    Array.isArray(value) ? newArray.push(...value) : newArray.push(value)    i++  }  return newArray}

测试一下

let num1 = [[1]]let num2 = [2, [3]]let num3=[5,[6]]let nums = num1.concat2(num2) // [[1], 2, [3]]let nums2 = num1.concat2(4, num3) // [[1], 4, 5,[6]]

join() 方法通过连接数组（或类似数组的对象）中的所有元素来创建并返回一个新字符串，以逗号或指定的分隔符字符串分隔。 如果数组只有一项，则将在不使用分隔符的情况下返回该项。

const elements = ["Fire", "Air", "Water"]const elements2 = ["Fire"]console.log(elements.join()) // Fire,Air,Waterconsole.log(elements.join("")) // FireAirWaterconsole.log(elements.join("-")) //  Fire-Air-Waterconsole.log(elements2.join("-")) // Fire

代码

Array.prototype.join2 = function (format = ",") {  const length = this.length  // Save the last element because it does not participate in the connection of format  let lastEle = this[ length - 1 ]  let string = ""  if (length === 0) {    return string  }  for (i = 0; i < length - 1; i++) {    string += this[ i ] + format  }  return string + lastEle}

测试一下

const elements = ["Fire", "Air", "Water"]const elements2 = ["Fire"]console.log(elements.join2()) // Fire,Air,Waterconsole.log(elements.join2("")) // FireAirWaterconsole.log(elements.join2("-")) //  Fire-Air-Waterconsole.log(elements2.join2("-")) // Fire

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