20 個程序員需要知道的 JavaScript 數組方法的實現
作者：佚名
我想你們一定對JavaScript中的數組非常熟悉，我們每天都會用到它的各種方法，比如push、pop、forEach、map……等等。
我想你們一定對JavaScript中的數組非常熟悉，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？太棒了，你一定不想在這里停下來。想和你一起挑戰，實現20+個數組方法的功能。
1. forEach
forEach是我們工作中使用頻率非常高的一個數組方法，實現起來也比較簡單。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 `${callback} is not a function`
}
const length = this.length
let i = 0
while (i < length) {
// Deleted, the newly added element index i is not in the array, so it will not be accessed
if (this.hasOwnProperty(i)) {
callback.call(thisCtx, this[ i ], i, this)
}
i++
}
}測試一下
let demoArr = [ 1, 2, 3, 4, , 5 ]
demoArr.forEach2((it, i) => {
if (i === 1) {
// 5 will not be printed out
demoArr.push(5)
} else if (i === 2) {
// 4 will not be printed out, but "4-4" will be printed out
demoArr.splice(3, 1, '4-4')
}
console.log(it)
})
/*
1
2
3
4-4
5
*/哇，恭喜！我們已經實現了 forEach 的特性。
2.map
您通常使用map做什么？大多數時候是將一個數組轉換成另一個數組。
代碼
Array.prototype.map2 = function (callback, thisCtx) {
if (typeof callback !== 'function') {
throw `${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？那是因為你太優秀了。
3. every
every() 方法測試數組中的所有元素是否通過提供的函數實現的測試。它返回一個布爾值。
every方法有3個你以前可能沒有注意到的點，它們是什么？
- 在空數組上調用 every 方法將返回 true。
- 回調方法只會被已經賦值的索引調用。
- 如果值被刪除，回調將不會被調用
let emptyArr = []
// Calling every method on an empty array returns true
console.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 `${callback} is not a function`
}
const length = this.length
let i = 0
// If the length of the array is 0, the while loop will not be entered
while (i < length) {
// False will be returned as long as a value does not conform to the judgment of callback
if (this.hasOwnProperty(i) && !callback.call(thisCtx, this[ i ], i, this)) {
return false
}
i++
}
return true
}測試一下
let emptyArr = []
// Calling every method on an empty array returns true
console.log(emptyArr.every2((it) => it > 0)) // true
// The `callback` method will only be called by an index that has already been assigned a value.
let arr = [ 0, 1, 2, 3, 4,, 5, -1 ]
// The `callback` method will not be called when an array value is deleted or an index that has never been assigned a value.
delete arr[7]
console.log(arr.every2((it) => it >= 0)) // true4.some
some() 方法測試數組中的至少一個元素是否通過了由提供的函數實現的測試。
代碼
Array.prototype.some2 = function (callback, thisCtx) {
if (typeof callback !== 'function') {
throw `${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 false
console.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)) // false
console.log(arr.some2((it) => it > 0)) // true5. filter
filter() 方法創建一個新數組，其中包含通過提供的函數實現的測試的所有元素。
Array.prototype.filter2 = function (callback, thisCtx) {
if (typeof callback !== 'function') {
throw `${callback} is not a function`
}
const length = this.length
// The return value will be a new array
let newArray = []
let i = 0
while (i < length) {
if (this.hasOwnProperty(i) && callback.call(thisCtx, this[ i ], i, this)) {
newArray.push(this[ i ])
}
i++
}
return newArray
}測試一下
// The position with index 5 will not be traversed because it has no initialization value
let arr = [ 0, 1, 2, -3, 4,, 5 ]
// we try to remove the last element
delete arr[6]
// filter out values greater than 0
let filterArr = arr.filter2((it) => it > 0)
console.log(filterArr) // [ 1, 2, 4 ]6. reduce
這個函數有點復雜。讓我們用一個例子來看看它是如何使用的。
const sum = [1, 2, 3, 4].reduce((prev, cur) => {
return prev + cur;
})
console.log(sum) // 10
// initialization
prev = initialValue = 1, cur = 2
// step 1
prev = (1 + 2) = 3, cur = 3
// step 2
prev = (3 + 3) = 6, cur = 4
// step 3
prev = (6 + 4) = 10, cur = undefined (quit)代碼
Array.prototype.reduce2 = function (callback, initValue) {
if (typeof callback !== 'function') {
throw `${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) // 107. reduceRight
reduceRight() 方法對累加器和數組的每個值（從右到左）應用函數以將其減少為單個值。
除了reduceRight是從右向左遍歷之外，它與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 4
console.log(sum) // 10
const sum2 = [1, 2, 3, 4].reduceRight((prev, cur) => {
console.log(cur)
return prev + cur;
})
// 4 3
// 7 2
// 9 1
console.log(sum2) // 10代碼
Array.prototype.reduceRight2 = function (callback, initValue) {
if (typeof callback !== 'function') {
throw `${callback} is not a function`
}
let pre = initValue
const length = this.length
// Start with the last element
let i = length - 1
// If no initial value is passed, the last element is taken as the initial value
if (typeof pre === 'undefined') {
pre = this[i]
i--
}
while (i >= 0) {
if (this.hasOwnProperty(i)) {
pre = callback(pre, this[ i ], i, this)
}
i--
}
return pre
}測試一下
const sum = [1, 2, 3, 4].reduceRight2((prev, cur) => {
console.log(cur)
return prev + cur;
})
// 4 3
// 7 2
// 9 1
console.log(sum) // 108. find
find() 方法返回提供的數組中滿足提供的測試函數的第一個元素。如果沒有值滿足測試函數，則返回 undefined。
代碼
Array.prototype.find2 = function (callback, thisCtx) {
if (typeof callback !== 'function') {
throw `${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) // 49. findIndex
findIndex() 方法返回數組中滿足提供的測試函數的第一個元素的索引。否則返回-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) // 310. indexOf
indexOf() 方法返回可以在數組中找到給定元素的第一個索引，如果不存在則返回 -1。
arr.indexOf(searchElement[, fromIndex])注意：
如果開始搜索的索引值大于等于數組長度，則表示不在數組中進行搜索，返回-1。
如果fromIndex為負數，則按照-1表示從最后一個元素開始查找，-2表示從倒數第二個元素開始查找，以此類推。
如果 fromIndex 為負數，則仍然從前到后查找數組。
const array = [2, 5, 9]
console.log(array.indexOf(2)) // 0
console.log(array.indexOf(7)) // -1
console.log(array.indexOf(9, 2)) // 2
console.log(array.indexOf(2, -1)) // -1
console.log(array.indexOf(2, -3)) // 0
console.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)) // 0
console.log(array.indexOf2(7)) // -1
console.log(array.indexOf2(9, 2)) // 2
console.log(array.indexOf2(2, -1)) // -1
console.log(array.indexOf2(2, -3)) // 0
console.log(array.indexOf2(2, -4)) // 011. lastIndexOf
lastIndexOf() 方法返回可以在數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 開始向后搜索數組。
除了 lastIndexOf 從右到左遍歷之外，它與 indexOf 非常相似。
let array = [2, 5, 9, 2]
console.log(array.lastIndexOf(2)) // 3
console.log(array.lastIndexOf(7)) // -1
console.log(array.lastIndexOf(2, 3)) // 3
console.log(array.lastIndexOf(2, 2)) // 0
console.log(array.lastIndexOf(2, -2)) // 0
console.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)) // 3
console.log(array.lastIndexOf2(7)) // -1
console.log(array.lastIndexOf2(2, 3)) // 3
console.log(array.lastIndexOf2(2, 2)) // 0
console.log(array.lastIndexOf2(2, -2)) // 0
console.log(array.lastIndexOf2(2, -1)) // 312. includes
includes() 方法確定數組是否在其條目中包含某個值，并根據需要返回 true 或 false。
arr.includes(valueToFind[, fromIndex])注意:
includes 方法將從 fromIndex 索引開始搜索 valueToFind。
如果 fromIndex 為負數，則開始搜索 array.length + fromIndex 的索引。
如果數組中存在 NaN，則 [..., NaN] Includes (NaN) 為真。
console.log([1, 2, 3].includes(2)) // true
console.log([1, 2, 3].includes(4)) // false
console.log([1, 2, 3].includes(3, 3)) // false
console.log([1, 2, 3].includes(3, -1)) // true
console.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)) // true
console.log([1, 2, 3].includes2(4)) // false
console.log([1, 2, 3].includes2(3, 3)) // false
console.log([1, 2, 3].includes2(3, -1)) // true
console.log([1, 2, NaN].includes2(NaN)) // true13. push
push() 方法將一個或多個元素添加到數組的末尾并返回數組的新長度。
const animals = ['pigs', 'goats', 'sheep']
animals.push('cows')
console.log(animals, animals.length)
// ["pigs", "goats", "sheep", "cows"], 4
animals.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"], 4
animals.push2('chickens', 'cats', 'dogs')
console.log(animals, animals.length)
// ["pigs", "goats", "sheep", "cows", "chickens", "cats", "dogs"], 714. pop
pop() 方法從數組中刪除最后一個元素并返回該元素。此方法更改數組的長度。
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組的開頭并返回數組的新長度。
如果傳入多個參數調用一次unshift，與傳入一個參數多次調用unshift（比如循環調用）得到的結果是不同的。
let arr = [4,5,6]
// Insert multiple elements at once
arr.unshift(1,2,3)
console.log(arr) // [1, 2, 3, 4, 5, 6]
let arr2 = [4,5,6]
// Insert multiple times
arr2.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 once
arr.unshift2(1,2,3)
console.log(arr) // [1, 2, 3, 4, 5, 6]
let arr2 = [4,5,6]
// Insert multiple times
arr2.unshift2(1)
arr2.unshift2(2)
arr2.unshift2(3)
console.log(arr2); // [3, 2, 1, 4, 5, 6]16. shift
shift() 方法從數組中刪除第一個元素并返回刪除的元素。此方法更改數組的長度。
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
reverse() 方法原地反轉數組。第一個數組元素成為最后一個，最后一個數組元素成為第一個。
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() 方法將數組中的所有元素更改為靜態值，從起始索引（默認 0）到結束索引（默認 array.length）。 它返回修改后的數組。
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組。 此方法不會更改現有數組，而是返回一個新數組。
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() 方法通過連接數組（或類似數組的對象）中的所有元素來創建并返回一個新字符串，以逗號或指定的分隔符字符串分隔。 如果數組只有一項，則將在不使用分隔符的情況下返回該項。
const elements = ['Fire', 'Air', 'Water']
const elements2 = ['Fire']
console.log(elements.join()) // Fire,Air,Water
console.log(elements.join('')) // FireAirWater
console.log(elements.join('-')) // Fire-Air-Water
console.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,Water
console.log(elements.join2('')) // FireAirWater
console.log(elements.join2('-')) // Fire-Air-Water
console.log(elements2.join2('-')) // Fire最后
感謝您的閱讀，同時，也期待您的關注，閱讀更多優質文章。
責任編輯：華軒
來源：
web前端開發
