Common methods of array

Array methods

1.concat() Merge array , Do not change the original array

let arr = [1, 2, 3];
let arr1 = ["a", "b", "c"];
let arr2 = arr.concat(arr1)
 console.log(arr2);// [1, 2, 3, "a", "b", "c"]

2.push() Add the contents to the end of the array , And return the modified length

let arr = [1, 2, 3];
console.log(arr.push(6)); //  Length of new array  4

3.pop() Remove the last entry in the array , Return the removed value , Reduce the length

let arr = [1, 2, 3];
console.log(arr.pop());//  Length of new array after removal  4

5.unshift（） Add parameters to the beginning of the original array , And returns the length of the array

let arr = [1, 2, 3];
console.log(arr.push(6)); //  Length of new array  4

5.shift() Delete the first item of the original array , And returns the value of the deleted element ; Returns if the array is empty undefined

let arr = [1, 2, 3];
console.log(arr.shift(2)); //1  Subscript of deleted value 

6.sort() Sort the items in the array from small to large

let arr = [3,2,7,4,6];
console.log(arr.sort());// [2, 4, 6, 7]

7.reverse() Reverse the order of array items

let arr = [3,2,7,4,6];
console.log(arr.reverse());// [6, 4, 7, 2, 3]

8.slice() Remove elements Two parameters of the method , The first parameter deletes the starting subscript , The second is the ending subscript, but does not include the ending subscript , If the second parameter is none , Then delete the subscript corresponding to the first parameter to the end of the array , in addition , The other parameters can be negative

let arr = [3,2,7,4,6];
console.log(arr.slice(1,3));//[2, 7]
console.log(arr.slice(-3,-1));//[ 7,4]

9.splice() Have deleted , Replace , The function of insertion ; It can be transmitted 3 Parameters , The first starting subscript , the second Delete / Replace Number of elements of , Third Replace / Insert The data of When the second parameter is 0 Time is the function of inserting

let arr = [3, 2, 7, 4, 6];
let arr1 = arr.splice(1, 2, 112, 33, 444)
console.log(arr1); //[2, 7]
console.log(arr); //[3, 112, 33, 444, 4, 6]

10.indexOf() Take two parameters , The first is the item to be found , The second is the subscript of the starting position , If you find the first one , Stop looking , That is, it will not find subsequent elements , Return value when the index of the returned element is found , Return when you can't find it -1

let arr = [3, 2, 7, 4, 6];
console.log(arr.indexOf(2,3));//-1
console.log(arr.indexOf(7,1));//2

11.lastIndexOf（） Receive two parameters ： The items to find and （ Optional ） Index indicating the location of the search start point , Start at the end of the array and look forward

let arr = [3, 2, 7, 4, 6];
console.log(arr.lastIndexOf(2,1));//1

12.join（“ Put quotes ”） Is to convert an array into a string , Connect with the symbols in brackets , If you don't write, the default is comma ( ,)

let arr = [3, 2, 7, 4, 6];
console.log(arr.join());//3,2,7,4,6
console.log(arr.join("-"));//3-2-7-4-6

Array higher order functions
13.forEach( Traverse )

 let arr = [3, 2, 7, 4, 6];
 arr.forEach((item, index) => {
    
            console.log( ` Elements ：${
      item}  Subscript ${
      index}`);
        });

The operation results are as follows

14.filter( Filter , Return a new array )

   let arr = [3, 2, 7, 4, 6];
        console.log(arr.filter((item) => {
    
            return item > 3
        })); //[7, 4, 6]

15.map( mapping , Return a new array ) Do not change the original array

  let arr = [3, 2, 7, 4, 6];
        console.log(arr.map((item) => {
    
            return item * 10
        })); //[30, 20, 70, 40, 60]

16.some( Determine whether there are qualified elements , Returns a Boolean value ) Return as long as one of the conditions is met ture Return only when you are not satisfied with one false

  let arr = [3, 2, 7, 4, 6];
      console.log(arr.some((item) => {
    
            return item > 3
        }));//true

17.every( Determine whether all elements meet the conditions , Returns a Boolean value ) As long as one of the conditions is not met, it returns false

  let arr = [3, 2, 7, 4, 6];
       console.log(arr.every((item) => {
    
            return item > 3
        }));//false

18.reduce( Add up )

  let arr = [3, 2, 7, 4, 6];
      console.log(arr.reduce((a, b) => {
    
            return a + b
        })); //22