js Array common methods

1. Array.push()

Add one or more elements... To the end of the array , And returns the new array length . The original array changes .

let arr = ["a","b","c"]
arr.push("d")
console.log(arr)  // ["a","b","c","d"]

2. Array.pop()

Delete and return the last element of the array , If the array is empty , Then return to undefined. The original array changes .

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

3. Array.unshift()

Add one or more elements... To the beginning of the array , And returns the new array length . The original array changes .

let arr = [1,2,3]
let length = arr.unshift(0)
console.log(arr,length) // [0,1,2,3] 4

4. Array.shift()

Delete the first item of the array , And returns the value of the first element . If the array is empty , Then return to undefined. The original array changes .

let arr = [1,2,3,4]
let first = arr.shift()
console.log(arr,first) // [2,3,4] 1

5. Array.concat(arr1,arr2…)

Merge two or more arrays , Generate a new array . The original array remains the same .

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

6. Array.join()

Connect each item of the array with a specified character to form a string . The default connection character is “,” comma .

let arr = [1,2,3]
let str1 = arr.join()
let str2 = arr.join('-')
conosle.log(str1,str2) // 1,2,3 1-2-3

7. Array.reverse()

Reverse array . The original array changes .

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

8. Array.sort()

Sort array elements . According to the string UniCode Sort code , The original array changes .

arr = [2,1,3]
strArr = [{
    name:"zhangsan",age:18},{
    name:"lisi",age:17},{
    name:"wangwu",age:19}]
let comparator1 = function(a,b){
    
    return a - b
}
arr.sort(comparator1) // [1,2,3]
let comparator2 = function(a,b){
    
    return b - a
}
arr.sort(comparator2) // [3,2,1]
function compare(param){
    
    return function sortAge(a,b){
    
        return a[param] - b[param]
    }
}
arr.sort(compare("age"))

9. Array.some()

Judge each item in the array , If none of them meet the requirements, return to false, Otherwise return to true.

let arr = [1,2,3]
console.log(arr.some(e => e > 2)) //true

10. Array.every()

Judge each item in the array , If all match, return true, Otherwise return to false.

let arr = [1,2,3]
console.log(arr.every(e => e > 2)) // false

11. Array.reduce()

Receive a function as accumulator , Every value in the array （ From left to right ） Start to shrink , The final calculation is a value . There is also a corresponding Array.reduceRight() Method , The difference is that this is operated from right to left

let numbers = [65, 44, 12, 4];
function getSum(total, num) {
    
    return total + num;
}
console.log(numbers.reduce(getSum)) // 125

12. Array.filter()

Filter the array , Qualified elements and return a new array .

let arr = [1,2,3]
console.log(arr.filter( e => e < 3)) // [1,2]

13. Array.forEach()

Used to call each element of an array , And pass the element to the callback function . The original array remains the same .（ If you print directly Array.forEach, The result is undefined）

let sum = 0
let arr = [1,2,3]
arr.forEach(e => sum += e)
console.log(sum) // 6

14. Array.map()

Returns a new array , The element in the array is the value after the original array element calls the function . The original array remains the same .

let numbers = [4,9,16]
console.log(numbers.map(Math.sqrt)) // [2,3,4]

15. Array.slice(start,end)

from start Start ,end It's over , Less than end; If not end value , from start From the beginning to the end of the array .start You can give a negative value ,-1 Represents the last position of the array ,-2 It means the next to last , And so on , Look before and ignore behind

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

16. Array.splice(index,howmany,arr1,arr2…)

Delete elements and add elements , from index Location start delete howmany Elements , And will arr1、arr2… Data from index Insert... In sequence .howmany by 0 when , The element is not deleted . The original array changes .

let fruits = ["Banana", "Orange", "Apple", "Mango"];
//  In position  2, add to  2  Elements , Delete  1  Elements ：
fruits.splice(2, 1, "Lemon", "Kiwi");
console.log(fruits) // [Banana,Orange,Lemon,Kiwi,Mango]

17. Array.isArray

Determine whether an object is an array , The return is a Boolean value

18. Array.toString()

Convert array to string

Array weight removal

function unique1(arr) {
    
 	return […new Set(arr)]
}
function unique2(arr) {
    
    var obj = {
    };
    return arr.filter(ele => {
    
        if (!obj[ele]) {
    
            obj[ele] = true;
            return true;
        }
    })
}
function unique3(arr) {
    
    var result = [];
    arr.forEach(ele => {
    
        if (result.indexOf(ele) == -1) {
    
            result.push(ele)
        }
    })
    return result;
}

Enter a value and return its data type

function dataType(para) {
    
	 return Object.prototype.toString.call(para)
}