英文 | https://javascript.plainenglish.io/20-javascript-array-methods-every-developer-should-know-6c04cc7a557d

翻譯 | 楊小愛

前言

你們一定對JavaScript中的數(shù)組很熟悉，我們每天都會用到它的各種方法，比如push、pop、forEach、map……等等。

但是僅僅使用它就足夠了嗎？如此出色，您一定不想停在這里。我想和你一起挑戰(zhàn)實現(xiàn)20+數(shù)組方法的功能。

1、forEach

forEach 是我們工作中非常常用的數(shù)組方法，實現(xiàn)起來也比較簡單。這是我們需要完成的第一個功能。

代碼

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++  }}

測試一下

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！我們已經(jīng)實現(xiàn)了 forEach 的功能。

2、map

你一般用map做什么？大多數(shù)時候是將一個數(shù)組轉(zhuǎn)換為另一個數(shù)組。

代碼

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}

測試一下

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]

朋友們，你們覺得不難嗎？那是因為你太厲害了。

3、every

every() 方法測試數(shù)組中的所有元素是否通過提供的函數(shù)實現(xiàn)的測試。它返回一個布爾值。

每種方法都有你以前可能沒有注意到的三點，它們是什么？

在空數(shù)組上調(diào)用 every 方法將返回 true。
回調(diào)方法只會被已經(jīng)賦值的索引調(diào)用。
如果值被刪除，回調(diào)將不會被調(diào)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

代碼

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}

測試一下

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() 方法測試數(shù)組中的至少一個元素是否通過了提供的函數(shù)實現(xiàn)的測試。

代碼

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}

測試一下

let emptyArr = []// An empty array will return falseconsole.log(emptyArr.some2((it) => it > 0)) // false
let 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() 方法創(chuàng)建一個新數(shù)組，其中包含所有通過所提供函數(shù)實現(xiàn)的測試的元素。

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 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}

測試一下

// 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

這個函數(shù)稍微復(fù)雜一些。讓我們用一個例子來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)

代碼

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}

測試一下

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

7、reduceRight

reduceRight() 方法對累加器和數(shù)組的每個值（從右到左）應(yīng)用一個函數(shù)，以將其減少為單個值。

它與 reduce 非常相似，只是 reduceRight 從右到左遍歷。

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

代碼

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}

測試一下

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() 方法返回提供的數(shù)組中滿足提供的測試功能的第一個元素。如果沒有值滿足測試函數(shù)，則返回 undefined。

代碼

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}

測試一下

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() 方法返回數(shù)組中滿足提供的測試函數(shù)的第一個元素的索引。否則，它返回 -1，表示沒有元素通過測試。

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

代碼

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}

測試一下

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() 方法返回可以在數(shù)組中找到給定元素的第一個索引，如果不存在，則返回 -1。

筆記：

如果開始搜索的索引值大于等于數(shù)組的長度，則表示不會在數(shù)組中進(jìn)行搜索，返回-1。

如果fromIndex為負(fù)數(shù)，則按照-1表示從最后一個元素開始查找，-2表示從倒數(shù)第二個元素開始查找的規(guī)則進(jìn)行查找，以此類推。

如果 fromIndex 為負(fù)數(shù)，則仍然從前向后搜索數(shù)組。

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

代碼

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}

測試一下

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() 方法返回可以在數(shù)組中找到給定元素的最后一個索引，如果不存在，則返回 -1。從 fromIndex 開始向后搜索數(shù)組。

它與 indexOf 非常相似，只是 lastIndexOf 從右到左遍歷。

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

代碼

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}

測試一下

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() 方法確定數(shù)組是否在其條目中包含某個值，根據(jù)需要返回 true 或 false。

arr.includes(valueToFind[, fromIndex])

筆記

include 方法將從 fromIndex 索引開始搜索 valueToFind。

如果 fromIndex 為負(fù)數(shù)，則開始搜索 array.length + fromIndex 的索引。

如果數(shù)組中存在 NaN，則 [..., NaN] Includes (NaN) 為真。

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

代碼

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}

測試一下

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() 方法將一個或多個元素添加到數(shù)組的末尾，并返回數(shù)組的新長度。

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

代碼

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}

測試一下

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() 方法從數(shù)組中刪除最后一個元素并返回該元素，此方法更改數(shù)組的長度。

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

代碼

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}

測試一下

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

15、unshift

unshift() 方法將一個或多個元素添加到數(shù)組的開頭并返回數(shù)組的新長度。

筆記

如果傳入多個參數(shù)調(diào)用 unshift 一次，與傳入一個參數(shù)調(diào)用 unshift 多次（例如循環(huán)調(diào)用）會得到不同的結(jié)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]

代碼

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}

測試一下

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() 方法從數(shù)組中刪除第一個元素并返回該刪除的元素。此方法更改數(shù)組的長度。

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

代碼

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}

測試一下

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

17、reverse

（來自 MDN） reverse() 方法將數(shù)組反轉(zhuǎn)到位。第一個數(shù)組元素成為最后一個，最后一個數(shù)組元素成為第一個。

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

代碼

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}

測試一下

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

18、fill

fill() 方法將數(shù)組中的所有元素更改為靜態(tài)值，從開始索引（默認(rèn) 0）到結(jié)束索引（默認(rèn) array.length），它返回修改后的數(shù)組。

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]

代碼

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}

測試一下

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() 方法用于合并兩個或多個數(shù)組。此方法不會更改現(xiàn)有數(shù)組，而是返回一個新數(shù)組。

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]]

代碼

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}

測試一下

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() 方法通過連接數(shù)組（或類似數(shù)組的對象）中的所有元素創(chuàng)建并返回一個新字符串，用逗號或指定的分隔符字符串分隔。如果數(shù)組只有一個項目，則將返回該項目而不使用分隔符。

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

代碼

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}

測試一下

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

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你知道這20個數(shù)組方法是怎么實現(xiàn)的嗎？

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