Sum up the most complete JavaScript Array method in a week

Preface

In our daily development, we deal with the interface the most. The front-end accesses the back-end interface, and then re-processes the interface data and renders it to the page.

The secondary treatment process is a test Coder to Array whether skilled and which method to use in which scenario best deal.

Editor, I encountered the Array in recent development. When dealing with complex business requirements, I did not expect Array have a similar method, and then combine the methods to solve the current problem.

article took a week to finish writing after work hours. After reading it, likes and forwards are my biggest support.

Array traversal method

Will not change the traversal method of the original array

`forEach()`

forEach() method executes the given function once for each item in the array in ascending order.

Syntax

arr.forEach(callback(currentValue , index , array) ，thisArg)

currentValue : the current item value of the array
index : the index of the current item in the array
arr : the array object itself
thisArg : Optional parameter. When the callback function callback is executed, it is used as the value of this

note

If the arrow function expression used to pass in the function parameters, the thisArg parameter will be ignored because the arrow function is lexically bound to the this value.
forEach will not directly change the object that calls it, but that object may be callback function.
every will not change the original array.

//防盗贴：微信公众号: 前端自学社区

const arr = [2,3,4,1,44]

arr.forEach(val =>{
    console.log(`值为${val*2}`)
    
})
console.log(`原数组为${arr}`);
// 值为4
// 值为6
// 值为8
// 值为2
// 值为88
// 原数组为2,3,4,1,44

`reduce()`

reduce() array element accumulator, returns a combined result value.

Syntax

arr.reduce(callback(accumulator, currentValue, index, array), initialValue)

accumulator : accumulator, the default is the first value of the array element
currentValue : current value
index : current element index optional
array : array optional
initialValue : Initial valueoptional

reduce has two parameters, one is the callback function, and the other is the initial value.

It has two values:

1. When it provided initialValue initial value, accumulator value initialValue , currentValue value array of a first value
1. When not provided initialValue initial value, accumulator first value is an array, currentValue for the second value.

Note

If the array is empty and does not provide initialValue initial value, it throws TypeError .
If the array has one element, and does not provide initialValue or provide initialValue , the array is empty, then the only value to be returned does not perform callback callback function.

Sum

//防盗贴：微信公众号: 前端自学社区/
const arr = [1, 2, 3, 4]

const sum = arr.reduce((accumulator, currentValue) => accumulator + currentValue, 10)

console.log(sum) //20 
// accumulator  累计器
// currentValue  当前值
// initialValue  累计 初始值 为10 

//10 + 1 + 2 + 3 + 4


## 注意
// 回调函数第一次执行时，accumulator 和currentValue的取值有两种情况：
// 如果调用reduce()时提供了initialValue，accumulator取值为initialValue，currentValue取数组中的第一个值；
// 如果没有提供 initialValue，那么accumulator取数组中的第一个值，currentValue取数组中的第二个值。

Calculate the value in the object

To accumulate the values contained in the object array, you must provide an initial value so that each item can pass your function correctly.

/*
 * @Description: 
 * @Author: 微信公众号: 前端自学社区
 * @Date: 2021-08-07 00:53:51
 * @LastEditTime: 2021-08-07 00:53:51
 * @LastEditors: Do not edit
 */
const data = [
    {
        date: '2021-8-1',
        income: 200
    },
    {
        date: '2021-8-2',
        income: 400
    },
    {
        date: '2021-8-3',
        income: 300
    },
]

console.log(`总收入： ${data.reduce( (pre,currentValue) => pre + currentValue.income,0)}`);
//总收入： 900

two-dimensional array to one-bit array

const array = [[1,2],[3,4]]

console.log(array.reduce((a,b) => a.concat(b)));
//[ 1, 2, 3, 4 ]

`find()`

find() returns the element object or element value that meets specific conditions, and returns undefined

Syntax

arr.find((element,index,array), thisArg)

element : current element
index : current element index optional
array : The array itself optional
thisArg : The object this when executing callbacks. Optional

// 从数据中找出第一个满足特定条件的对象

const data = [
    {
        name:'张三',
        article: 3
    },
    {
        name:'老王',
        article: 9
    },
    {
        name:'老李',
        article: 10
    }
]

console.log(data.find(item => item.article > 9 ));

// { name: '老李', article: 10 }

`findIndex()`

findIndex() returns the index of the first element in the array that meets the criteria, if not, it returns -1 .

Syntax

arr.findIndex((element,index,array), thisArg)

element : current element
index : current element index optional
array : The array itself optional
thisArg : The object this when executing callbacks. Optional

const arr = [22,33,44,55]
console.log(arr.findIndex(val => val > 33));    //2
console.log(arr.findIndex(val => val > 99));    //-1

`key()`

key() returns a new Array Iterator object that contains the key for each index in the array.

Syntax

keys()

Note

If there are empty original elements in the array, they will also be added to the traversed queue when obtaining the key.

const inputModal = [
    {
        name:''
    },
    {
        age:''
    },
    {
        hobby:''
    }
]
for(const key of inputModal.keys()){
    console.log(key)
}
// 0
// 1
// 2

const arr = [1,2,,3]
for(const key of arr.keys()){
    console.log(key);
}
// 0
// 1
// 2
// 3


//Object.keys() 方法会返回一个由一个给定对象的自身可枚举属性组成的数组
// 所以 Object.keys(arr) = [ '0', '1', '3' ]
for(const key of Object.keys(arr)){
    console.log(key);
}
// 0
// 1
// 3

`values()`

values() method returns a new Array Iterator object, which contains the value of each index in the array.

Syntax

arr.values()

const Color = ['red','yelloe','orange']

for(val of Color.values()){
    console.log(val);
}
// red
// yelloe
// orange

<hr/>

`Return boolean`

`every()`

every used to determine whether all elements in the array meet a certain condition, and return Boolean value

Syntax

arr.every(callback(currentValue , index , array) ，thisArg)

currentValue : The current item value of the array Required
index : The index of the current item in the array optional
arr : The array object itself optional
thisArg : Optional parameter. When the callback function callback is executed, it is used as the value of this Optional

Note

true will be returned when all elements meet the conditions
every will not change the original array.
If an empty array is passed in, true will be returned anyway.

//防盗贴：微信公众号: 前端自学社区

const arr = [2,3,4,1,44]

console.log(arr.every(val =>  val > 0 ));   //true

console.log(arr.every(val => { val > 2 })) //false

`some()`

some() used to determine whether an array element meets a certain condition, as long as one element meets, then true is returned.

Syntax

arr.some(callback(currentValue , index , array) ，thisArg)

currentValue : The current item value of the array required
index : The index of the current item in the array optional
arr : The array object itself optional
thisArg : Optional parameter. When the callback function callback is executed, it is used as the value of this Optional

Note

some() will not change the array when it is called.
If you test with an empty array, it will return false in any case.
some() During the traversal, the element range has been determined, and the elements added during the traversal process will not be added to the traversed sequence.

const arr = [2,3,4,1,44]

console.log(arr.some(val => val > 2))  //true
console.log([].some(val => val > 2 )); //false


const newList = [11,22,33,44]
console.log(newList.some(val => {
    newList.push(55)
    newList.push(66)
    val > 55
}));   //false

`Do not change the original array, form a new array`

`filter()`

filter() used to traverse the original array, filter the array elements that meet the conditions, and form new array elements.

Syntax

arr.some(callback(currentValue , index , array) ，thisArg)

currentValue : The current item value of the array Required
index : The index of the current item in the array optional
arr : The array object itself optional
thisArg : Optional parameters. When the callback function callback is executed, it is used as the value of this Optional

Note

filter does not change the original array, it returns the filtered new array.
filter() When traversing, the element range has been determined, and the elements added during the traversal process will not be added to the traversed sequence.

const arr = [11,22,33,44,55,66]
 

console.log(arr.filter(val => val > 44 ))
console.log(`原数组为${arr}`);

// [ 55, 66 ]
// 原数组为11,22,33,44,55,66

`map()`

map() creates a new array, and the result is the result returned by calling a provided function for each element in the array.

Syntax

arr.map(callback(currentValue , index , array) ，thisArg)

currentValue : The current item value of the array required
index : The index of the current item in the array optional
arr : The array object itself optional
thisArg : Optional parameter. When the callback function callback is executed, it is used as the value of this Optional

Note

map does not modify the original array itself that calls it
map() During the traversal, the element range has been determined, and the elements added during the traversal process will not be added to the traversed sequence.

const arr = [1,2,3,4]

console.log(arr.map(val => val*3 ))  // [ 3, 6, 9, 12 ]
console.log(arr)  // [ 1, 2, 3, 4 ]

<hr/>

`Array CRUD`

`Change the original array method`

`reverse()`

reverse() method reverses the position of the elements in the array and returns the array. The first element of the array becomes the last, and the last element of the array becomes the first. This method will change the original array.

const arr = [1,2,3]

console.log(arr.reverse(11,22,33))  //[ 3, 2, 1 ]

`sort()`

sort() method uses the place algorithm to sort and return the array. The default sort order is to the elements at 16111cfa043e13 to the string , and then compare their UTF-16 code unit value sequence

Place Algorithm is an algorithm that uses auxiliary data structures to transform the input. However, it allows a small amount of additional storage space to store auxiliary variables. When the algorithm is running, the input is usually overwritten by the output. The in-place algorithm only updates the input sequence by replacing or exchanging elements.

const arr = [23,11,33,44,1]

console.log(arr.sort())  //[ 1, 11, 23, 33, 44 ]


const arr = [23,11,33,44,1000000000]

console.log(arr.sort())  
// [ 1000000000, 11, 23, 33, 44 ]

`Delete element`

`shift()`

shift() method removes from the array first element, and returns the value of the element. This method changes the length of the array.

Syntax

arr.shift()

Note

The element removed from the array; if the array is empty, it returns undefined

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
    {
        id:3,
        name:'移动端'
    },
    {
        id:4,
        name:'嵌入式开发'
    },
]

const deleObj = data.shift()



console.log('==============删除后的元素======================');
console.log(data);
console.log('=================删除后的元素===================');


console.log('===============被删除的元素=====================');
console.log(deleObj);
console.log('================被删除的元素====================');

//  ==============删除后的元素======================
// [
//     { id: 2, name: '后端' },
//     { id: 3, name: '移动端' },
//     { id: 4, name: '嵌入式开发' }
//   ]
//   =================删除后的元素===================

  
//   ===============被删除的元素=====================
//   { id: 1, name: '前端' }
//   ================被删除的元素====================

`pop()`

pop() method deletes the last element from the array and returns the value of that element. This method changes the length of the array.

The usage is similar to shift

Syntax

arr.pop()

Attention

The element removed from the array; if the array is empty, it returns undefined

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
    {
        id:3,
        name:'移动端'
    },
    {
        id:4,
        name:'嵌入式开发'
    },
]

const deleObj = data.pop()




console.log(data);
// [
//     { id: 1, name: '前端' },
//     { id: 2, name: '后端' },
//     { id: 3, name: '移动端' }
// ]
console.log(deleObj);
// { id: 4, name: '嵌入式开发' }

`splice()`

splice() method deletes or replaces the or adds new elements in place to modify the array, and returns the modified content in the form of an array. This method will change the original array.

Syntax

array.splice(start,deleteCount, [item1,item2....])

start : starting index
deleteCount : The number of deleted optional
[item1，item2 .....] ; elements added and replaced from the starting index, optional

Note

An array of deleted elements. If only one element is deleted, an array containing only one element is returned. If no elements are deleted, an empty array is returned.

If only the starting index position is passed, all element objects after the index will be deleted

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
    {
        id:3,
        name:'移动端'
    },
    {
        id:4,
        name:'嵌入式开发'
    },
]
data.splice(1)
console.log(data)
// [ { id: 1, name: '前端' } ]

Starting from index 2, delete 1 array element object and add two array element objects

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
    {
        id:3,
        name:'移动端'
    },
    {
        id:4,
        name:'嵌入式开发'
    },
]


data.splice(2,1,...[{id:5,name:'人工智能'},{id:6,name:'大数据开发'}])

console.log(data);
// [
//     { id: 1, name: '前端' },
//     { id: 2, name: '后端' },
//     { id: 5, name: '人工智能' },
//     { id: 6, name: '大数据开发' },
//     { id: 4, name: '嵌入式开发' }
// ]

`Add elements`

`splice()`

Already introduced above

`push()`

push() method adds one or more elements to the end of , and returns the new length of the array.

Syntax

arr.push(element1, ..., elementN)

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
]

console.log(data.push({id:3,name:'移动端'}))  //3

merged array

const data = [
    {
        id:1,
        name:'前端'
    },
    {
        id:2,
        name:'后端'
    },
]


var obj = [
    {
        id:4,
        name:'嵌入式开发'
    },
]

// 相当于 data.push({id:4,name:'嵌入式开发'});
Array.prototype.push.apply(data, obj);

console.log(data);

[
  { id: 1, name: '前端' },
  { id: 2, name: '后端' },
  { id: 4, name: '嵌入式开发' }
]

`unshift()`

unshift() method adds one or more elements to the at the beginning of the array, and returns the new length of .

const arr = [1,2,3]



console.log(arr.unshift(11,22,33))  //6 
console.log(arr)  //[ 11, 22, 33, 1, 2, 3 ]

`Do not change the original array element method`

`indexOf()`

indexOf() method returns the first index where the given element can be found in the array, or -1 if it does not exist.

Syntax

indexOf(searchElement)
indexOf(searchElement, fromIndex)

searchElement : the element to find
fromIndex : Search for the first index of the specified element that appears according to the specified index. Optional
- If the index is greater than or equal to the length of the array, -1 is returned
- If the provided index value is negative, it will be treated as an offset from the end of the array
- If the provided index is negative, the array is still searched from front to back
- If the index provided is 0, the entire array will be searched.
- Default value: 0 (search the entire array).

const arr = [1,1,2,3,4,5,4,4,6]

console.log(arr.indexOf(3));  //3
console.log(arr.indexOf(9));  //-1


console.log(arr.indexOf(3,4)); //-1
//从索引为 4 的元素进行查找 3, 显然后面没有3 ， 返回 -1

Array deduplication

Create a new empty array and use indexOf to determine whether an element exists in the empty array for the first time.

If it does not exist, return [<0] and push into an empty array.

const newArr = []
arr.forEach(val => {
    if(newArr.indexOf(val) < 0){
       newArr.push(val)
    }
})
console.log(newArr);
// [ 1, 2, 3, 4, 5, 6 ]

`lastIndexOf()`

lastIndexOf() finds the index of the last occurrence of an element in the array, and returns -1 if it is not found.

If it does not exist, it returns -1. Search forward from the back of the array, starting from fromIndex .

Grammar

arr.lastIndexOf(searchElement, fromIndex)

searchElement : the element to find
fromIndex : Find the first index of the specified element that appears according to the specified index. Optional
- Reverse search from the specified index position
- The default is the length of the array minus 1 ( arr.length - 1 ), that is, the entire array is searched.
- If the value is greater than or equal to the length of the array, the entire array will be searched.
- If it is negative, the array will still be searched from back to front.
- If the value is negative and its absolute value is greater than the length of the array, the method returns -1, that is, the array will not be searched.

Note

lastIndexOf using strictly equal === compare searchElement element and the array.

const arr = [1,1,2,3,4,5,4,4,6]

console.log(arr.lastIndexOf(4)); //7

console.log(arr.lastIndexOf(4,11));  
//7    指定的查找的索引 大于 数组的长度， 会进行整个数组查找

console.log(arr.lastIndexOf(4,-33));
// -1   指定的索引为负数，且绝对值大于数组长度， 则返回 -1

console.log(arr.lastIndexOf(4,-5));
//4    指定的索引为负数，且绝对值小于数组长度， 则会 从向前进行查找

`inCludes()`

includes() method is used to determine whether an array contains a specified value, according to the situation, if it contains, it returns true, otherwise it returns false.

Syntax

arr.includes(searchElement, fromIndex)

searchElement : the element to find
When searching, case sensitive
fromIndex : Find the first index of the specified element that appears according to the specified index. Optional
- Search from the specified index
- If it is a negative value, start searching from the index of array.length + fromIndex
- If fromIndex greater than or equal to the length of the array, false will be returned and the array will not be searched.
- Default is 0

const arr = [1,1,2,3,4,5,4,4,6]

console.log(arr.includes(4)); //true

console.log(arr.includes(4,66)); //false

console.log(arr.includes(1,-1)); //false

`concat()`

concat() method is used to merge two or more arrays.

Syntax

var new_array = old_array.concat([arr1][arr2])

Note

concat method does not change this or any array provided as a parameter, but returns a shallow copy , which contains a copy of the same elements combined with the original array
- Object reference (not actual object): concat Copy the object reference into the new array. Both the original array and the new array refer to the same object. That is, if the referenced object is modified, the change is visible to both the new array and the original array. This includes elements that are also array parameters of arrays.
- Data types such as string, number and Boolean (not String , Number and Boolean ) objects): concat the string and number values to the new array.

let arr1 = [1,2,3]
let arr2 = [4,5,6]
let arr3 = [[1,2],[3,4]]
console.log(arr1.concat(arr2));
//[ 1, 2, 3, 4, 5, 6 ]

// 嵌套合并
console.log(arr1.concat(arr2).concat(arr3));
// [ 1, 2, 3, 4, 5, 6, [ 1, 2 ], [ 3, 4 ] ]


let obj1 = [{a:1},{b:2}]
let obj2 = [{c:3},{d:4}]
let obj3 = obj1.concat(obj2)  
console.log(obj3); 
//[ { a: 1 }, { b: 2 }, { c: 3 }, { d: 4 } ]


obj1[0].a = 4  //改变obj[0]对象值，会直接影响合并后的数组，因为是浅拷贝
console.log(obj3); 
//[ { a: 4 }, { b: 2 }, { c: 3 }, { d: 4 } ]

`toString()`

toString() returns a string representing the specified array and its elements.

When an array is used as a text value or string concatenation operation, it will automatically call its toString method.

For array objects, the toString method concatenates the arrays and returns a string containing each array element separated by a comma.

Syntax

arr.toString()

const arr = [1,2,3]

console.log(arr.toString());  //1,2,3

`join()`

join() method returns a string by concatenating array elements separated by a comma or a specified delimiter string.

If the array has only one item, the item will be returned without using a separator.

Syntax

join()
join(separator)

separator : Specified split character optional

const arr = ['2021','08','08']

console.log(arr.join());     //2021,08,08
console.log(arr.join('-'));  //2021-08-08
console.log(arr.join('/'));  //2021/08/08

`slice()`

slice() method returns a new array object, that the object is a begin and end original array decision shallow copy (including begin , not including end ). The original array will not be changed.

Syntax

arr.slice(begin, end)

begin : Specify the intercepted start index optional
- Start from 0 by default
- If begin is a negative number, the second element from the end of slice(-2) will be truncated from the absolute value from the end of the array
- If begin exceeds the index range of the original array, an empty array will be returned.
end : Specify the intercepted end index optional
- If end is omitted, slice will be extracted to the end of the original array.
- If end greater than the length of the array, slice will also be extracted to the end of the original array.
- If end is a negative number, it means that the last element in the original array has been extracted.

const arr = [11,22,33,44,55,66,77,88]

console.log(arr.slice(1,4));
// 应该返回 索引 1 - 3 的数组元素
// [ 22, 33, 44 ]


console.log(arr.slice(-4,2))  //[]

console.log(arr.slice(-4));   //[ 55, 66, 77, 88 ]


console.log(arr.slice(0,-1));
// [
//     11, 22, 33, 44,
//     55, 66, 77
//   ]

`references`

Array - JavaScript | MDN

`Previous wonderful articles`

`at last`

The article has spent a week after get off work, and writing is not easy. The content in the article refers to the MDN document. If you like it, you can like it 👍👍👍Follow and support, I hope everyone can gain something after reading this article!

Sum up the most complete JavaScript Array method in a week

Preface

article took a week to finish writing after work hours. After reading it, likes and forwards are my biggest support.

Array traversal method

Will not change the traversal method of the original array

forEach()

reduce()

find()

findIndex()

key()

values()

Return boolean

every()

some()

Do not change the original array, form a new array

filter()

map()

Array CRUD

Change the original array method

reverse()

sort()

Delete element

shift()

pop()

splice()

Add elements

splice()

push()

unshift()

Do not change the original array element method

indexOf()

lastIndexOf()

inCludes()

concat()

toString()

join()

slice()

references

Previous wonderful articles

at last

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`forEach()`

`reduce()`

`find()`

`findIndex()`

`key()`

`values()`

`Return boolean`

`every()`

`some()`

`Do not change the original array, form a new array`

`filter()`

`map()`

`Array CRUD`

`Change the original array method`

`reverse()`

`sort()`

`Delete element`

`shift()`

`pop()`

`splice()`

`Add elements`

`splice()`

`push()`

`unshift()`

`Do not change the original array element method`

`indexOf()`

`lastIndexOf()`

`inCludes()`

`concat()`

`toString()`

`join()`

`slice()`

`references`

`Previous wonderful articles`

`at last`

`引用和评论`