Several methods of defining javascript arrays (graphic tutorial)-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

Several methods of defining javascript arrays (graphic tutorial)

亚连

May 19, 2018 am 09:23 AM

javascript js method

This article mainly introduces several methods of defining JavaScript arrays. Generally speaking, they are divided into implicit creation, direct instantiation, creating an array and specifying the length. Later, for convenience, you can directly use the json format to define the array. Friends who need it can For reference

1. What is an array

An array is a collection of data
Its expression is a continuous memory address in the memory
The array name is actually the first address of the continuous memory address

2. About the characteristics of arrays in js

There is no need to specify the data type when defining an array
You can when defining an array There is no need to specify the array length
Arrays can store data of any data type (for example, one element stores integer type, and one element stores string type. This is possible in JS)

Create array Syntax:

var arr=[value 1, value 2, value 3]; //Implicitly created

var arr=new Array(value 1, value 2, value 3); //Direct instantiation

var array=new Array(size); //Create an array and specify the length

//方法一
var arr1 = [1,3,5,7,9];
document.write(arr1[2] + &#39;<br>&#39;);
 
//方法二
var arr2 = new Array(2,4,6,8,10);
document.write(arr2[3] + &#39;<br>&#39;);
 
//方法三
var arr3 = new Array(3);//固定数组长度为3
arr3[0] = 1;
arr3[1] = 2;
arr3[2] = 3;
document.write(arr3[2] + &#39;<br>&#39;);

Copy after login

The meaning of symbols in JS:

() means function execution
[] means syntax simulation, means simulates an instance of the Array class (=new Array())
{} means syntax simulation, means simulates an instance of the Object class (=new Object ())
//Indicates syntax simulation (regular object), which means simulating instances of the RegExp class (=new RegExp())

3. About the array length

Array object.length

In js, each array object can call the length attribute, which indicates how many array elements there are under the array object

Example:

var row = [&#39;zhangsan&#39;,&#39;lisi&#39;,&#39;wangwu&#39;];
doucument.write(&#39;共有&#39;+row.length+&#39;个人<br>&#39;);

var length = row.length;//对数组进行遍历
for (var i=0;i<length;i++){
doucument.write(row[i]+&#39;<br>&#39;);
}

Copy after login

4. for...in statement

In js, the array is not a data type. The data type of the array is actually the object

For... in Js The ..in statement can traverse all attributes of an object

You can also use the for...in statement to traverse all elements of an array

Syntax:

for( var i in array ){

}

Principle: There are several elements in the array, and the for..in statement will be executed in a loop as many times as possible

Each time When executed, the subscript of the current array element is stored in variable i

var row = [&#39;zhangsan&#39;,&#39;lisi&#39;,&#39;wangwu&#39;,&#39;xiaoqiang&#39;];
for (var i in row){
 document.write(i + &#39;:&#39; + row[i] + &#39;<br>&#39;);
}

Copy after login

Result:

　0:zhangsan
　1:lisi
　2:wangwu
　3: xiaoqiang

5. Text subscript

format:

arr['key'] = value;

in js , the array elements with text subscripts are not included in the array length

Added to the array in the form of text subscripts, but actually added to the array object in the form of attributes

var arr = [1,2,3];
arr[&#39;first&#39;] = &#39;zhangsan&#39;;
arr[&#39;second&#39;] = &#39;lisi&#39;;

document.write(arr.length + &#39;<br>&#39;);
document.write(arr.first + &#39;<br>&#39;);
document.write(arr.second + &#39;<br>&#39;);

Copy after login

Result:

　3
　zhangsan
　lisi

Traverse the array with text subscripts:

var arr = [1,2,3];
arr[&#39;first&#39;] = &#39;zhangsan&#39;;
arr[&#39;second&#39;] = &#39;lisi&#39;;

for(var i in arr){
 document.write(i + &#39;:&#39; + arr[i] + &#39;<br>&#39;);
}

Copy after login

Result:

　0:1
　1:2
　2:3
　First:zhangsan
　Second:lisi

6. Multi-dimensional array

var arr = [
  [10,&#39;zhangsan&#39;,&#39;male&#39;],
  [11,&#39;lisi&#39;,&#39;female&#39;],
  [12,&#39;wangwu&#39;,&#39;male&#39;]
 ];
for (var i in arr){
 for(var j in arr[i]){
  document.write(arr[i][j]);
 }
 document.write(&#39;<br>&#39;);
}

Copy after login

7. Define first and then assign value

var myArray1 = new Array(); //不定义元素个数
  myArray1[0] = "P";
  myArray1[1] = "H";
  myArray1[2] = "P";
var myArray2 = new Array(4); //定义元素的个数
  myArray1[0] = "P";
  myArray1[1] = "H";
  myArray1[2] = "P";

Copy after login

8. Assign values when defining an array

var　myArray3 = new Array("P","H","P");

//You need to specify a numerical or logical value in the array, then the variable type should be a numerical variable or a Boolean variable, not a character variable

9. Definition of json format

var myArray4 = [
   {"num":"1001","name":"jack","age":16},
   {"num":"1002","name":"Mary","age":17},
   {"num":"1003","name":"Tom","age":19}
   ];
//注意中括号和大括号的位置

Copy after login

Students who don’t understand can check this article: http://www.jb51.net/article/20688.htm

The difference between Json definition usage and array

Json: It is a method used to store data in javascript.
Json format: var Json={a:1, b:2, c:3};
Array format: var arr=[1,2,3];

Json The difference between two arrays:

1. The subscript of Json is a string. For example, when calling the first data, use: Json.a. The subscript of the array is a number such as arr[0]
2. Json does not have length, and the array has lengtht

Use Json to store data. If you need to loop, you need to use for (element variable in variable name) {}; Because Json does not have a length, a for in loop must be used. The for in loop can be used for Json and arrays

for(i in Json){
Statement
};

When to use for loop and when to use for in loop?
Array: Use for loop for(i=0,iJson: Use for in loop for(i in Json){};

10. Note

· Arrays only have one attribute: array length
· If no value is assigned to the array, the result is undefined, and the print value is ""
· If the array length is not long enough, it can be automatically expanded and does not exist Cross-border issues

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

Related articles: Detailed explanation of the use of key-value pairs stored in

NodeJS parent process Detailed explanation of resource sharing steps with child processes

Concept and usage instructions of JS anonymous self-executing function

The above is the detailed content of Several methods of defining javascript arrays (graphic tutorial). For more information, please follow other related articles on the PHP Chinese website!

Statement of this Website

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress images for free

Clothoff.io

AI clothes remover

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Assassin's Creed Shadows: Seashell Riddle Solution

1 months ago By DDD

What's New in Windows 11 KB5054979 & How to Fix Update Issues

3 weeks ago By DDD

Where to find the Crane Control Keycard in Atomfall

1 months ago By DDD

How to fix KB5055523 fails to install in Windows 11?

2 weeks ago By DDD

InZoi: How To Apply To School And University

3 weeks ago By DDD

Hot Tools

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

God-level code editing software (SublimeText3)

Hot Topics

Where is the login entrance for gmail email?

7774

Java Tutorial

1644

CakePHP Tutorial

1399

Laravel Tutorial

1296

PHP Tutorial

1234

Related knowledge

How to write a novel in the Tomato Free Novel app. Share the tutorial on how to write a novel in Tomato Novel. Mar 28, 2024 pm 12:50 PM

Tomato Novel is a very popular novel reading software. We often have new novels and comics to read in Tomato Novel. Every novel and comic is very interesting. Many friends also want to write novels. Earn pocket money and edit the content of the novel you want to write into text. So how do we write the novel in it? My friends don’t know, so let’s go to this site together. Let’s take some time to look at an introduction to how to write a novel. Share the Tomato novel tutorial on how to write a novel. 1. First open the Tomato free novel app on your mobile phone and click on Personal Center - Writer Center. 2. Jump to the Tomato Writer Assistant page - click on Create a new book at the end of the novel.

How to recover deleted contacts on WeChat (simple tutorial tells you how to recover deleted contacts) May 01, 2024 pm 12:01 PM

Unfortunately, people often delete certain contacts accidentally for some reasons. WeChat is a widely used social software. To help users solve this problem, this article will introduce how to retrieve deleted contacts in a simple way. 1. Understand the WeChat contact deletion mechanism. This provides us with the possibility to retrieve deleted contacts. The contact deletion mechanism in WeChat removes them from the address book, but does not delete them completely. 2. Use WeChat’s built-in “Contact Book Recovery” function. WeChat provides “Contact Book Recovery” to save time and energy. Users can quickly retrieve previously deleted contacts through this function. 3. Enter the WeChat settings page and click the lower right corner, open the WeChat application "Me" and click the settings icon in the upper right corner to enter the settings page.

How to enter bios on Colorful motherboard? Teach you two methods Mar 13, 2024 pm 06:01 PM

Colorful motherboards enjoy high popularity and market share in the Chinese domestic market, but some users of Colorful motherboards still don’t know how to enter the bios for settings? In response to this situation, the editor has specially brought you two methods to enter the colorful motherboard bios. Come and try it! Method 1: Use the U disk startup shortcut key to directly enter the U disk installation system. The shortcut key for the Colorful motherboard to start the U disk with one click is ESC or F11. First, use Black Shark Installation Master to create a Black Shark U disk boot disk, and then turn on the computer. When you see the startup screen, continuously press the ESC or F11 key on the keyboard to enter a window for sequential selection of startup items. Move the cursor to the place where "USB" is displayed, and then

The secret of hatching mobile dragon eggs is revealed (step by step to teach you how to successfully hatch mobile dragon eggs) May 04, 2024 pm 06:01 PM

Mobile games have become an integral part of people's lives with the development of technology. It has attracted the attention of many players with its cute dragon egg image and interesting hatching process, and one of the games that has attracted much attention is the mobile version of Dragon Egg. To help players better cultivate and grow their own dragons in the game, this article will introduce to you how to hatch dragon eggs in the mobile version. 1. Choose the appropriate type of dragon egg. Players need to carefully choose the type of dragon egg that they like and suit themselves, based on the different types of dragon egg attributes and abilities provided in the game. 2. Upgrade the level of the incubation machine. Players need to improve the level of the incubation machine by completing tasks and collecting props. The level of the incubation machine determines the hatching speed and hatching success rate. 3. Collect the resources required for hatching. Players need to be in the game

How to set font size on mobile phone (easily adjust font size on mobile phone) May 07, 2024 pm 03:34 PM

Setting font size has become an important personalization requirement as mobile phones become an important tool in people's daily lives. In order to meet the needs of different users, this article will introduce how to improve the mobile phone use experience and adjust the font size of the mobile phone through simple operations. Why do you need to adjust the font size of your mobile phone - Adjusting the font size can make the text clearer and easier to read - Suitable for the reading needs of users of different ages - Convenient for users with poor vision to use the font size setting function of the mobile phone system - How to enter the system settings interface - In Find and enter the "Display" option in the settings interface - find the "Font Size" option and adjust it. Adjust the font size with a third-party application - download and install an application that supports font size adjustment - open the application and enter the relevant settings interface - according to the individual

Summary of methods to obtain administrator rights in Win11 Mar 09, 2024 am 08:45 AM

A summary of how to obtain Win11 administrator rights. In the Windows 11 operating system, administrator rights are one of the very important permissions that allow users to perform various operations on the system. Sometimes, we may need to obtain administrator rights to complete some operations, such as installing software, modifying system settings, etc. The following summarizes some methods for obtaining Win11 administrator rights, I hope it can help you. 1. Use shortcut keys. In Windows 11 system, you can quickly open the command prompt through shortcut keys.

Detailed explanation of Oracle version query method Mar 07, 2024 pm 09:21 PM

Detailed explanation of Oracle version query method Oracle is one of the most popular relational database management systems in the world. It provides rich functions and powerful performance and is widely used in enterprises. In the process of database management and development, it is very important to understand the version of the Oracle database. This article will introduce in detail how to query the version information of the Oracle database and give specific code examples. Query the database version of the SQL statement in the Oracle database by executing a simple SQL statement

Recommended: Excellent JS open source face detection and recognition project Apr 03, 2024 am 11:55 AM

Face detection and recognition technology is already a relatively mature and widely used technology. Currently, the most widely used Internet application language is JS. Implementing face detection and recognition on the Web front-end has advantages and disadvantages compared to back-end face recognition. Advantages include reducing network interaction and real-time recognition, which greatly shortens user waiting time and improves user experience; disadvantages include: being limited by model size, the accuracy is also limited. How to use js to implement face detection on the web? In order to implement face recognition on the Web, you need to be familiar with related programming languages and technologies, such as JavaScript, HTML, CSS, WebRTC, etc. At the same time, you also need to master relevant computer vision and artificial intelligence technologies. It is worth noting that due to the design of the Web side

See all articles