JS common array methods

1.sort() Sorting method :

arr.sort()// Press default ascii Sort code 
// From small to large 
candidates.sort((a, b) => a - b)
// From big to small 
candidates.sort((a, b) => b - a )

2.push() This method is to add a new element after the array , This method changes the length of the array ：

const arr=[];
const ans=[];
ans.push([...arr])
console.log(ans);//[[1,2,3],[4,5,6]]

3.pop() This method deletes the last element after the array , And returns an array , This method changes the length of the array ：

let arr = [1, 2, 3, 4, 5]
    arr.pop()
    console.log(arr)//[1, 2, 3, 4]
    console.log(arr.length)//4

4.shift() Delete before shift() Used to delete and return the first element , Delete the first element , Returns the deleted element , Change the original array

var a = [1,2,3]
var b = a.shift()
 
console.log(a) // [2,3]
console.log(b) // 1

5.unshift() It's increasing unshift() You can add one or more elements to the front of the array , And return the new length header to add , Return length , Change the original array

var a = [2,3,4]
var b = a.unshift(0,1)
 
console.log(a)  // [0,1,2,3,4]
console.log(b)  // 5

6.Array.toString() This method converts the array to a string ：

let arr = [1, 2, 3, 4, 5];
   let str = arr.toString()
   console.log(str)// 1,2,3,4,5

7.Array.splice( Starting position , Number of deletions , Elements ) Omnipotent method , You can add, delete and modify ： Change the array itself

let arr = [1, 2, 3, 4, 5];
     let arr1 = arr.splice(2, 0'haha')
     let arr2 = arr.splice(2, 3)
     let arr1 = arr.splice(2, 1'haha')
     console.log(arr1)//[1, 2, 'haha', 3, 4, 5] Add an element 
     console.log(arr2)//[1, 2]  Delete three elements 
     console.log(arr3)//[1, 2, 'haha', 4, 5]  Replace an element 

8.slice() shear slice(startIndex,endIndex) Return from startIndex Start ( Include ), To endIndex( barring ) The new array is returned from the array composed of the original attributes between , Do not change the original array

var a = [1,2,3]
var b = a.slice(0,1)
//  If no parameter is filled in, the entire array will be cut  
var c = a.slice() 
console.log(a) // [1,2,3] 
console.log(b) // [1]
console.log(c) // [1,2,3] 
console.log(a===c)  // false //  Be careful  a !== c 
 
//  A negative number means to count from the back to the front 
var d = a.slice(-1,-2)   
console.log(d) // []  Intercept from left to right , So it's []
 
var e = a.slice(-1)  
console.log(e)  // [3]

9.concat() Splicing concat() Method is used to merge two or more arrays , Return a new array , It doesn't change the original array

var a = [1,2,3]
var b = [4,5]
var c = a.concat(b)
 
console.log(a) // [1,2,3]
console.log(b) // [4,5]
console.log(c) // [1,2,3,4,5] 

10.join();join() Method is used to convert an array to a string without changing the original array , Return the converted String

var a = [1,2,3,4,5]
 
console.log(a.join(','))  // 1,2,3,4,5
console.log(a)  // [1,2,3,4,5]

11.reverse() Reverse the order ;reverse() Method to reverse the order of the elements in an array . It returns the inverted array , It's going to change the array .

var a  = [1,3,2,7,6]
 
console.log(a.reverse())  // [6,7,2,3,1]
console.log(a)  // [6,7,2,3,1]

12.indexOf() and lastIndexOf();indexOf( A particular element ,startIndex) from startIndex Start , Find the position of an element in the array , If exist , Returns the subscript of the first position , Otherwise return to -1

var a = [1,2,4,3,4,5]
 
console.log(a.indexOf(4))  // 2
console.log(a.indexOf(4,3)) // 4

13.filter() Filter filter() Method returns a new array of qualified elements in the array , The original array remains the same

filter() The argument to is a method

var a = [1,2,3,4,11]
//  The first parameter is a method , There are three parameters ,current: Current value  index: Current value subscript  array: This array object 
var b = a.filter(function(current,index,array){
    
    return current < 10
})
 
console.log(b) // [1,2,3,4]
console.log(a) // [1,2,3,4,11]

14.map() Format array

map() Method to format the original array as required , Returns the formatted array . The original array remains the same

var a = [1,2,3,4,5]
//  The parameters are the same as filter Method 
var b = a.map(function(current,index,array){
    
   return current + 1
})

console.log(b) // [2,3,4,5,6]
console.log(a) // [1,2,3,4,5]

15.every() Run the given function for each item of the array , If each item returns ture, Then return to true

var a = [1,2,3,4,5]
 
var b = a.every(function(current,index,array){
    
       return current < 6
})
 
var c = a.every(function(current,index,array){
    
       return current < 3
})
console.log(b)  // true 
console.log(c)  // false 

16.some() Run the given function for each item of the array , If one or more items exist, return ture, Then return to true

var a = [1,2,3,4,5]
 
var b = a.some(function(current,index,array){
    
       return current > 4
})
 
var c = a.some(function(current,index,array){
    
       return current > 5
})
console.log(b)  // true 
console.log(c)  // false 

17.forEach() Array traversal Traverse the entire array , You can't interrupt

var arr = ['a','b','c']
var copy = []
arr.forEach(function(item){
    
     copy.push(item)   
})
console.log(copy)

ES6 The new method

1. find() Find the element in the array that meets the condition for the first time , And back to , If not, return undefined. Do not change the original array .

and filter() The difference is ：filter The return value is an array of all the elements that meet the conditions ,
Generally, when you need to use the found elements , use find() Method

var a = [1,2,3,4]
// b Below you need to use , Generally use find
var b = a.find(function(ele,index,array){
    
    return ele == 1
})
 
var c = 3
var d = b + c
console.log(a) // [1,2,3,4]
console.log(b) // 1
console.log(d) // 4
 
//  If you just need to judge whether the element exists 
//  If it's a simple array ( Non object array ), It is generally used Array.includes(value) Method 
//  If it is an object array , You can use Array.some() Method 
 
var a = [1,2,3]
console.log(a.includes(1))  // true
 
var a = [{
    "name": "xiaoming" },{
    "name": "xiaohong"}]
 
console.log(a.some(function(ele){
    
    return ele.name == 'xiaoming'
}))                            // true
 

2.findIndex() Method

findIndex() The effect is the same as indexOf(), Returns the first subscript that satisfies the condition , And stop looking .
The difference is that findIndex() The parameter of is a callback function , And is generally used for object arrays

var a = [1,2,3,4]
 
var b = a.findIndex(function(ele,index,array){
    
    return ele === 2
})
 
var c = a.indexOf(2)  
 
console.log(a)  // [1,2,3,4]
console.log(b)  // 1
console.log(c)  // 1

3.includes()

includes() Method , Returns a Boolean value . The parameter is one value, Generally used for simple arrays .
For complex arrays , You can use some() Method substitution includes() Method

var a = [1,2,3]
console.log(a.includes(1))  // true

4.Array.isArray() Method Used to determine whether an element is an array

Array.isArray([])  // true
Array.isArray({
    })  // false