你知道这20个数组方法是怎么实现的吗？

前言

你们一定对JavaScript中的数组很熟悉，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？如此出色，您一定不想停在这里。我想和你一起挑战实现20+数组方法的功能。

1、forEach

forEach 是我们工作中非常常用的数组方法，实现起来也比较简单。这是我们需要完成的第一个功能。

代码

Array.prototype.forEach2 = function (callback, thisCtx) {if (typeof callback !== 'function') {throw`${callback} is not a function`  }const length = this.lengthlet i = 0while (i < length) {// Deleted, the newly added element index i is not in the array, so it will not be accessedif (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)  } elseif (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 的功能。

2、map

你一般用map做什么？大多数时候是将一个数组转换为另一个数组。

代码

Array.prototype.map2 = function (callback, thisCtx) {if (typeof callback !== 'function') {throw`${callback} is not a function`  }const length = this.lengthlet i = 0// The return value of the map method is a new arraylet newArray = []while (i < length) {// Deleted and uninitialized values will not be accessedif (this.hasOwnProperty(i)) {      newArray.push(callback.call(thisCtx, this[ i ], i, this))    }    i++  }// Return new arrayreturn 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？那是因为你太厉害了。

3、every

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。

每种方法都有你以前可能没有注意到的三点，它们是什么？

• 在空数组上调用 every 方法将返回 true。

• 回调方法只会被已经赋值的索引调用。

• 如果值被删除，回调将不会被调用

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.lengthlet i = 0// If the length of the array is 0, the while loop will not be enteredwhile (i < length) {// False will be returned as long as a value does not conform to the judgment of callbackif (this.hasOwnProperty(i) && !callback.call(thisCtx, this[ i ], i, this)) {returnfalse    }    i++  }returntrue}

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

4、some

some() 方法测试数组中的至少一个元素是否通过了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.lengthlet i = 0while (i < length) {// Returns true if any element meets the callback conditionif (this.hasOwnProperty(i) && callback.call(thisCtx, this[ i ], i, this)) {returntrue    }    i++  }returnfalse}

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

5、filter

filter() 方法创建一个新数组，其中包含所有通过所提供函数实现的测试的元素。

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 arraylet newArray = []let i = 0while (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 ]

6、reduce

这个函数稍微复杂一些。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 = initValuelet i = 0const 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

7、reduceRight

reduceRight() 方法对累加器和数组的每个值（从右到左）应用一个函数，以将其减少为单个值。

它与 reduce 非常相似，只是 reduceRight 从右到左遍历。

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 = initValueconst length = this.length// Start with the last elementlet i = length - 1// If no initial value is passed, the last element is taken as the initial valueif (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

8、find

find() 方法返回提供的数组中满足提供的测试功能的第一个元素。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.lengthlet i = 0while (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 returnedif (callback.call(thisCtx, value, i, this)) {return value    }    i++  }// otherwise return undefined  returnundefined}

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

9、findIndex

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，它返回 -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.lengthlet i = 0while (i < length) {// Return index i that conforms to callback logicif (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

10、indexOf

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。

笔记：

如果开始搜索的索引值大于等于数组的长度，则表示不会在数组中进行搜索，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的规则进行查找，以此类推。

如果 fromIndex 为负数，则仍然从前向后搜索数组。

constarray = [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 -1if (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 targetEleif (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

11、lastIndexOf

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。

它与 indexOf 非常相似，只是 lastIndexOf 从右到左遍历。

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 iif (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 targetEleif (i inthis && targetEle === this[ i ]) {return i    }    i--  }// Returns -1 when the current value is not foundreturn-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

12、includes

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

arr.includes(valueToFind[, fromIndex])

笔记

include 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 || 0if (length === 0 || fromIndex >= length) {returnfalse  }// Search for elements from the position of fromIndexlet i = Math.max(fromIndex >= 0 ? fromIndex : length - Math.abs(fromIndex), 0)while (i < length) {const value = this[ i ]// Please note NaNif (targetEle === value || typeof targetEle === 'number' && typeof value === 'number' && isNaN(targetEle) && isNaN(value)) {returntrue    }    i++  }returnfalse}

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

13、push

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.lengthconst length = this.lengthlet i = 0while (i < pushEleLength) {this[ length + i ] = pushEles[ i ]    i++  }returnthis.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

14、pop

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 undefinedif (length === 0) {returnundefined  }const delEle = this[ length - 1 ]this.length = length - 1return delEle}

测试一下

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

15、unshift

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。

笔记

如果传入多个参数调用 unshift 一次，与传入一个参数调用 unshift 多次（例如循环调用）会得到不同的结果。

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 arraylet newArray = [ ...unshiftEles, ...this ]let length = newArray.lengthlet i = 0if (unshiftEles.length === 0) {return length  }// Recopy to arraywhile (i < length) {this[ i ] = newArray[ i ]    i++  }returnthis.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]

16、 shift

shift() 方法从数组中删除第一个元素并返回该删除的元素。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.lengthconst delValue = this[ 0 ]  let i = 1while (i < length) {// Starting from the first element, the following elements move forward one bitthis[ i - 1 ] = this[ i ]    i++  }// Set the length of the arraythis.length = length - 1// Return deleted valuereturn delValue}

测试一下

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

17、reverse

（来自 MDN） reverse() 方法将数组反转到位。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 - 1while (i < j) {    [ this[ i ], this[ j ] ] = [ this[ j ], this[ i ] ]    i++    j--  }returnthis}

测试一下

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

18、fill

fill() 方法将数组中的所有元素更改为静态值，从开始索引（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.lengthstart = start >> 0  // The defaultvalueofendislengthend = typeof end === 'undefined' ? length : end >> 0  // The minimumvalueofstartis0and the maximum valueislengthstart = start >= 0 ? Math.min(start, length) : Math.max(start + length, 0)  // The minimumvalueofendis0and the maximum valueislengthend = end >= 0 ? Math.min(end, length) : Math.max(end + length, 0)  // The element that fills the specified rangeisvaluewhile (start < end) {    this[ start ] = valuestart++  }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]

19、concat

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 layerlet newArray = [ ...this ]let i = 0while (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]]

20、join

join() 方法通过连接数组（或类似数组的对象）中的所有元素创建并返回一个新字符串，用逗号或指定的分隔符字符串分隔。如果数组只有一个项目，则将返回该项目而不使用分隔符。

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 formatlet lastEle = this[ length - 1 ]letstring = ''if (length === 0) {returnstring  }for (i = 0; i < length - 1; i++) {string += this[ i ] + format  }returnstring + 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

最后

以上就是我今天跟你分享的20个数组方法的实现，希望对你有用，如果你觉得有帮助的话，请记得点赞我，关注我，并将他分享给你身边做开发的朋友。