About array methods and loops in JavaScript

This article brings you relevant knowledge about javascript, which mainly introduces array methods and loops in JavaScript. It has a good reference value and I hope it will be helpful to everyone.

[Related recommendations: javascript video tutorial, web front-end】

1. Basic concepts

JavaScript arrays are used to store multiple values ​​in a single variable. It is a collection of one or more values ​​with the same data type

2. Three ways to create an array

(1) Use the JavaScript keyword new to create an Array object and assign values ​​separately

//1、创建数组  new 一个Array() 对象
    let arr = new Array();
    arr[0] = "html";
    arr[1] = "css";
    arr[2] = "javascript";
    arr[3] = "java";

(2) Assign a value at declaration time

//2、创建数组  在Array()对象里面直接赋值
    let arr1 = new Array("html","css","java","javaweb","javascript");

(3) Use array literals to create directly

 //3、通过[]直接创建
    let arr2 = ["html","css","java","javascript","javaweb"];

For the sake of simplicity, readability and execution speed , please use the third method (array text method).

3. Access the array

(1) Reference an array element by referencing the index number (subscript number) , [0] is the element in the array The first element. [1] is the second one. Array index starts from 0;

document.write(arr1[0]);

(2) The complete array can be accessed by referencing array name

console.log(arr1);

(3) Modify the array elements.

arr[1] = "css";

4. Common properties of arrays

The length property returns the length of the array (the number of array elements).

console.log(arr,arr.length);//控制台输出数组和数组长度

5. Common methods for arrays

(1) join(): Put all the elements of the array into a string and separate them by a separator;

 //1、join()方法 以分隔符将数组分隔转化为string
    let arr = new Array("html","css","javascript","java","web","mysql");
    console.log(arr,typeof(arr));
    let newarr = arr.join("+");
    console.log(newarr,typeof(newarr));

(2) split() method converts the string into array array type through delimiter

// 2、split()方法  将字符串通过分隔符转化为array数组类型
    // split() 函数验证邮箱格式
    let email = prompt("请输入你的邮箱：");
    console.log(email);
    let arr1 = email.split("@");
    console.log(arr1,typeof(arr1));
    document.write("你的账号为："+arr1[0]+"<br>"+"你的网站时："+arr1[1]);

Use the above two methods to eliminate all spaces between strings

//功能  剔除字符串里的所有空格
    function trimAll(str){
        let nowstr = str.trim();//先剔除两端的空格
        let arr = nowstr.split(" ");//split()  转换为数组 用空格分隔
        for(let i = 0;i<arr.length;i++){//循环遍历
            if(arr[i] == ""){
                arr.splice(i,1);//遇到空格删除
                i--;
            }
        }
        return arr.join("");//join() 转化为字符串
    }
 
    let nowstr = trimAll("     1     2    4    5    ");
    console.log(nowstr);

(3) sort(): Sort the array

let arr = [31,23,26,76,45,1,90,6,24,56];
    //sort() 函数  对数组进行排序  默认按数字首位进行排序
    //添加参数  参数为匿名函数
    arr.sort(function(a,b){
        // return a-b;         //正序排序
 
        return b-a;           //倒序排序
    });
 
    console.log(arr);

Note: The following method operates on the array itself

(4) push(): Add an element to the end of the array or more elements, and returns the new length;

(5) pop(): Delete the element at the end of the array;

(6) unshfit(): Add elements to the head of the array;

(7) shfit(): Delete the head element of the array;

(8) splice(): Universal array method: 1. Delete elements in the array; 2. Add elements; 3. Replace elements

 let arr = ["html","java","csss","javascript"];
    console.log("旧数组："+arr);
    //对数组自身进行操作
    arr.push("weeb");//在数组末尾添加元素  可以有多个参数 之间用逗号隔开
    arr.pop();//删除末尾元素  没有参数
    arr.unshift("react","mysql");//在数组头部添加元素  可以有多个参数  之间用逗号隔开
    arr.shift();//删除数组头部的元素  没有参数
    arr.shift();//删除需要多次删除  或者利用循环
    arr.splice(0,2);//数组万能方法  删除任意位置元素  参数为： 起始下标，删除数目
    arr.splice(3,2,"java","html");//添加元素  参数为：数组没有的下标，添加数目，添加的数据
    arr.splice(1,1,"javaweb")//替换元素  参数为：起始下标，替换个数，替换数据  如果替换数据小于替换个数  则执行删除功能
    console.log("新数组："+arr);

6. Commonly used methods of looping through arrays

Loop: Loop is to repeatedly execute a certain function when the conditions are met. One operation

1. Use for loop to traverse the array. Known conditions and known length. Judge first and then loop.

let arr = new Array("html","css","javascript","java","web","mysql");
    //1、利用for循环遍历数组  已知条件  已知长度  先判断后循环
    for (let i = 0;i < arr.length;i++){
        document.write(arr[i]+"<br>");
    }

2. Use while loop to traverse the array. Unknown conditions and unknown length. Judge first and then loop.

//2、利用while循环遍历数组  未知条件 未知长度  先判断后循环
    let i = 0;
    while(i < arr.length){
        document.write(arr[i]+"<br>");
        i++;
    }

3. do while loops through the array and executes it at least once

//3、至少执行一次 do while 循环遍历数组
    let j = 0;
    do{
        document.write(arr[j]+"<br>");
        j++;
    }
    while(j < arr.length);

4. for of loops through the array value is directly the element value

//4、for of  循环遍历数组  value直接元素值  
    for(let value of arr){
        document.write(value+"<br>");
    }

5. for in loops through the object i is key Key is specially used to loop through objects, and can also loop through arrays

//5.for in 循环遍历对象  i 为  key键  专门用来循环遍历对象
    for(let i in arr){
        document.write(arr[i]+"<br>");
    }

6. forEach() array method Anonymous callback function [Loop through arrays]

//6.forEach()  数组方法  匿名回调函数  【循环遍历数组】
    arr.forEach(function(value,index,arr){
        document.write(index+"---"+value+"----"+arr+"<br>");
    })

7. Use map() array method Traversing the array has a return value

//7、利用map() 数组方法遍历数组 有返回值
    // 返回一个新的数组  和老数组长度一定一致，有可能是二维数组
    let newarr = arr.map(function(value,index,oldarr){
        document.write(index+"---"+value+"----"+oldarr+"<br>");
 
        if(index == 0){
            value = "12345";
        }
        return [value,index];
 
    });
    console.log(newarr);

8. Using the flatmap() array method to traverse the array has a return value and also returns a new array

//8、利用flatmap() 数组方法遍历数组 有返回值 同样返回一个新的数组 
    //长度有可能和原来数组不一致，但一定是一维数组  flat() 为降维函数
    let newarr1 = arr.flatMap(function(value,index,oldarr){
        document.write(index+"---"+value+"----"+oldarr+"<br>");
 
        if(index == 0){
            value="321";
        }
        return [value,index];
    });
    console.log(newarr1);

[Related recommendations ：javascript video tutorial、web front-end】

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