JavaScript basic knowledge points-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

JavaScript basic knowledge points

黄舟

Feb 07, 2017 pm 02:41 PM

javascript

JavaScript learning

Tag (space separated) variable

1. What is a variable
A variable is a container that stores book values;

2 .Game Rules
【Variable Naming】Variables can be composed of letters, numbers, underscores (_) or dollar signs ($).

1. Must start with a letter, underscore, or dollar sign, and can be followed by letters, underscores, dollar signs, and numbers.

2. Variable names are case-sensitive, such as: myvar and myVarhi are two different variables.

3. JavaScript keywords and reserved words are not allowed to be used as variable names, such as break and Boolean.

3. Variable declaration and assignment

 var myvar=123;

Copy after login

4. Data type

string (string)
Nubmber (number)
Boolean (for example, true and false have only two types)
Array (array)
Object (object)

##undefined and null

var mychar1="双引号包起来的字符串"；//这是字符串
var mychar2=&#39;单引号包起来的字符串&#39;；//这也是字符串
var mychar3=&#39;小蒜："我喜欢我们班的小可。"&#39;；//字符串中有双引号，用单引号包含
var mychar4="Uncle Wang:"\"小蒜啊，&#39;学习好&#39;才能吸引女孩哦~\""; //或者在特定符号（引号）前使用\符号，使其转义输出
var mynum1=6； //这是数字6var mynum2=6.00； //这也是数字6> 
var mynum3=123e；//这是使用科学（指数）计算法来书写的12300000
var mynum4=123e-5；//这是0.00123var mynum5=ture；//这是布尔值
var mynum6=[1,2,3];//这是数组
var myobject={"p":"Hello"};//这是对象

Copy after login

Basic expressions and operators

1. Basic expression

In JavaScript, when + is used to connect strings, other variables will also be converted into strings for connection~

var y="you";
var mysay="I"+"love"+y; //=后面是串表达式，mysay值是字符串
var mynum=12+6*2;//=后面是数值表达式，mynum值是数值
var mynum>12;//=后面是布尔表达式，mysay值是布尔值

Copy after login

2. Operation Symbol

2.1 Arithmetic operator

For example: +-8*/

var num=24;
var myresult1=++num%4+6*2;//myresult是多少呢？
var myresult2=num%4+6*2;//myresult是多少呢？

Copy after login

2.2 Assignment operator

It can be simplified by placing the arithmetic operator before =. For example, num%=4 is equivalent to num=num%4.

2.3 Comparison operators

For example:>,<,>=,<=
==equal to
===all equal to
! = is not equal to

2.4 Logical operators

&& (series connection)
|| (parallel connection)

2.5 Operator precedence (high to low):

-* /etc. arithmetic operators

= && || ! Logical operators such as
= copy symbol.
If operations at the same level are performed from left to right, multi-level brackets are from inside to outside.
As a reminder, when you can't tell the priority, just add parentheses to remember the order of operations.

Exercise: Link numbers and strings

Indicate the following non-string result

Array

What is an array

1. Definition of array

One sentence understanding: Variables that can store multiple data

Array (Arry) is a set of values arranged in order. A single value is called an element, and their positions are numbered (starting from 0 as well) That is to say, the index of the first element is 0, the second element is 1, and so on). The entire array is represented by square brackets.

//表达形式一var arr=[];
var arr[0]=&#39;a&#39;;
var arr=[1]=&#39;b&#39;;
var arr=[2]=&#39;c&#39;;
var arr=[3]=&#39;d&#39;;//表达形式二
var arr=[&#39;a&#39;&#39;b&#39;&#39;c&#39;&#39;d&#39;];

Copy after login

2. What can be installed?

Any type of data can be put into an array.

var arr=[&#39;x&#39;,{a:1},[1,2,3], 
fucation(){return true;}];
arr[0];  //stringarr[1];  //Objectarr[2];  //Arrayarr[3];  //fucation

Copy after login

It can be seen that the elements in the array can also be an array. We call this form a multi-dimensional array.

var arr=[[1,2],[3,4]];
arr[0][1];  //2arr[1][1];  //4

Copy after login

3.length attribute

3.1 The length attribute of the array can return the number of members of the array.

The length attribute of an array is different from the length attribute of an object. As long as it is an array, it must have a length attribute, but the object may not have it.

Moreover, the length attribute of the array is a dynamic value, which is equal to the maximum value in the key name plus 1.

var arr=[&#39;a&#39;,&#39;b&#39;];
arr.length; //2arr[2]=;&#39;c&#39;;
arr.length;  //3arr[9]=&#39;d&#39;;
arr.length;  //10arr[1000]=&#39;e&#39;;
arr.lengh;  //10001

Copy after login

It can be found that the numeric key values of the array do not need to be consecutive, and the value of the length attribute is always equal to the largest key value greater than 1.

3.2 The length attribute is writable. If you manually set a value for the current number of members in Xiaoyu, the members of the array will automatically be reduced to the length set by length.

var arr=[&#39;a&#39;,&#39;b&#39;,&#39;c&#39;];
arr.length;  //3arr.length=2;
arr;    //[&#39;a&#39;,&#39;b&#39;]

Copy after login

When the length attribute of the array is set to 2, that is, the largest integer can only be 1, so the element ('c') corresponding to the key value 2 is automatically deleted. Therefore, an effective way to clear an array is to set the length attribute of the array to 0.

3.3 The length of the array

It should be noted that because the index of the array always starts from 0, the upper and lower limits of an array are: 0 and length-1 respectively. If the length of the array is 5, the upper and lower limits of the array are doubled to 0 and 4.

4. Create an array

var myarr=new Array(6);
console.log(myarray);

Copy after login

5. Assign an array

var myarr=new Array(3);
myarr[0]="小五";
myarr[1]="小明";
myarr[2]="月影";
console.log("班里学号为0的是："+myarr[0]);
console.log("班里学号为1的是："+myarr[1]);
console.log("班里学号为2的是："+myarr[2]);
var arr=["1","abc","myarr"];
console.log(arr[1]);

Copy after login

6. Add a new element

myarr[0]="小五";
myarr[1]="小明";
myarr[2]="月影";
console.log("班里学号为0的是："+myarr[0]);
console.log("班里学号为1的是："+myarr[1]);
console.log("班里学号为2的是："+myarr[2]);
myarr[3]="小新";
console.log(myarr[3]);
myarr[0]="小五";
myarr[1]="小明";
myarr[2]="月影";
console.log("班里学号为0的是："+myarr[0]);
console.log("班里学号为1的是："+myarr[1]);
console.log("班里学号为2的是："+myarr[2]);
myarr[3]="小新";
console.log(myarr[3]);

Copy after login

7. Use an array literal

To get the value of an array element, just use the array variable and provide an index.

var myarr=["小雷","小可","小新","小明","月影"];
var mynum=4;
console.log("学号为4的是"+myarr[mynum]);

Copy after login

8. Multidimensional array nesting

var myarr=[[0,2,3],[1,2,3]]
myarr[0][1]=5;//将5的值传入数组中，覆盖原有值。
console.log(myarr[0][1]);

Copy after login

Knowledge point expansion

Simple for loop:

var arr=[&#39;a&#39;,&#39;b&#39;,&#39;c&#39;];
for(var i=0; i<arr.length; i++){
console.log(arr[i]);
        }

Copy after login

The above is JavaScript Basic knowledge points, please pay attention to the PHP Chinese website (www.php.cn) for more related content!

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

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

1 months ago By DDD

How to fix KB5055523 fails to install in Windows 11?

3 weeks ago By DDD

How to fix KB5055518 fails to install in Windows 10?

3 weeks ago By DDD

Strength Levels for Every Enemy & Monster in R.E.P.O.

3 weeks ago By 尊渡假赌尊渡假赌尊渡假赌

Blue Prince: How To Get To The Basement

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

Java Tutorial

1653

CakePHP Tutorial

1413

Laravel Tutorial

1305

PHP Tutorial

1251

C# Tutorial

1224

Related knowledge

How to implement an online speech recognition system using WebSocket and JavaScript Dec 17, 2023 pm 02:54 PM

How to use WebSocket and JavaScript to implement an online speech recognition system Introduction: With the continuous development of technology, speech recognition technology has become an important part of the field of artificial intelligence. The online speech recognition system based on WebSocket and JavaScript has the characteristics of low latency, real-time and cross-platform, and has become a widely used solution. This article will introduce how to use WebSocket and JavaScript to implement an online speech recognition system.

WebSocket and JavaScript: key technologies for implementing real-time monitoring systems Dec 17, 2023 pm 05:30 PM

WebSocket and JavaScript: Key technologies for realizing real-time monitoring systems Introduction: With the rapid development of Internet technology, real-time monitoring systems have been widely used in various fields. One of the key technologies to achieve real-time monitoring is the combination of WebSocket and JavaScript. This article will introduce the application of WebSocket and JavaScript in real-time monitoring systems, give code examples, and explain their implementation principles in detail. 1. WebSocket technology

How to use JavaScript and WebSocket to implement a real-time online ordering system Dec 17, 2023 pm 12:09 PM

Introduction to how to use JavaScript and WebSocket to implement a real-time online ordering system: With the popularity of the Internet and the advancement of technology, more and more restaurants have begun to provide online ordering services. In order to implement a real-time online ordering system, we can use JavaScript and WebSocket technology. WebSocket is a full-duplex communication protocol based on the TCP protocol, which can realize real-time two-way communication between the client and the server. In the real-time online ordering system, when the user selects dishes and places an order

How to implement an online reservation system using WebSocket and JavaScript Dec 17, 2023 am 09:39 AM

How to use WebSocket and JavaScript to implement an online reservation system. In today's digital era, more and more businesses and services need to provide online reservation functions. It is crucial to implement an efficient and real-time online reservation system. This article will introduce how to use WebSocket and JavaScript to implement an online reservation system, and provide specific code examples. 1. What is WebSocket? WebSocket is a full-duplex method on a single TCP connection.

JavaScript and WebSocket: Building an efficient real-time weather forecasting system Dec 17, 2023 pm 05:13 PM

JavaScript and WebSocket: Building an efficient real-time weather forecast system Introduction: Today, the accuracy of weather forecasts is of great significance to daily life and decision-making. As technology develops, we can provide more accurate and reliable weather forecasts by obtaining weather data in real time. In this article, we will learn how to use JavaScript and WebSocket technology to build an efficient real-time weather forecast system. This article will demonstrate the implementation process through specific code examples. We

Simple JavaScript Tutorial: How to Get HTTP Status Code Jan 05, 2024 pm 06:08 PM

JavaScript tutorial: How to get HTTP status code, specific code examples are required. Preface: In web development, data interaction with the server is often involved. When communicating with the server, we often need to obtain the returned HTTP status code to determine whether the operation is successful, and perform corresponding processing based on different status codes. This article will teach you how to use JavaScript to obtain HTTP status codes and provide some practical code examples. Using XMLHttpRequest

How to use insertBefore in javascript Nov 24, 2023 am 11:56 AM

Usage: In JavaScript, the insertBefore() method is used to insert a new node in the DOM tree. This method requires two parameters: the new node to be inserted and the reference node (that is, the node where the new node will be inserted).

How to get HTTP status code in JavaScript the easy way Jan 05, 2024 pm 01:37 PM

Introduction to the method of obtaining HTTP status code in JavaScript: In front-end development, we often need to deal with the interaction with the back-end interface, and HTTP status code is a very important part of it. Understanding and obtaining HTTP status codes helps us better handle the data returned by the interface. This article will introduce how to use JavaScript to obtain HTTP status codes and provide specific code examples. 1. What is HTTP status code? HTTP status code means that when the browser initiates a request to the server, the service

See all articles