Explanation of JavaScript related content-JS Tutorial-php.cn

1. JavaScript Introduction:

JavaScript is the most popular scripting language on the Internet, and all modern HTML uses JavaScript. Since it is a scripting language, it has three characteristics:

(1) Weak type;

(2) Interpreted type Language (no compilation required);

(3) Line by line execution, if one line of code is wrong, subsequent code blocks will not continue to execute;

(4) The tag can be directly embedded into the HTML file. The position is arbitrary. It is usually placed below the modified content or in the head tag, but writing it as a separate js file is conducive to the separation of structure and behavior

2.JavaScript content (with pictures):

##

## Where ECMAScript is the core of JavaScript;

DOM It is the document object model (using js to operate the web page);

BOM is the browser object model (using js to operate the browser)

3. JavaScript information output:

(1) alert() method: in the form of a prompt box on the page Output, for example; alert("hello,javascript") Copy after login (2) console.log() method: output information on the console, for example:

console.log("hello,javascript") Copy after login (3) document.write() : Write the content directly in the HTML page, for example:

document.write("hello,javascript") Copy after login

4. JavaScript variable: ## Unlike Java, ECMAScript The variables in have no specific type. When defining a variable, only use the var operator. It can be initialized to any value. The initialization format of the variable: var variable name = variable value; example:

var a = "hello"; var b = 123; Copy after login If you want to define multiple variables, you can write multiple variables on one line and separate them with commas; example:

var a = "你好", b = 123, c = "hello"; Copy after login Variable rules for variable names:

(1) consists of letters, numbers, underscores, and $ symbols

(2) It cannot start with a number, and it is not recommended to start with an underscore;

(3 ) Strictly case-sensitive;

(4) Cannot be keywords and reserved words

5. JavaScript data type: JavaScript can be divided into two types: primitive data types and reference data types:

(1) Primitive data types: Number, String, Boolean, undefined, null

Number : Numeric type, is a number, including positive numbers, negative numbers, integers, decimals, 0, NaN, Infinity (positive infinity), -Infinity (negative infinity); Note:

NaN : abbreviation of not a number, indicating that the value is not a numerical value (also belongs to Number) String ： 字符串：用双引号""或单引号''包起来的0个或多个字符，如果引号中什么也没有，那么这个字符串被称为空字符串

Boolean ： 布尔型：包含true： 表示真（成立） 和false： 表示假（不成立） 两个值

undefined ： 表示变量未定义，或变量被定义出来，但是没有被赋值

null ： 表示一个变量没有指向任何一片存储空间，即变量存在，但是里面是空的，类似于Undefined

（小提示：在Chrome浏览器控制台输出时，会发现Number类型为深蓝色，String为黑色，Boolean为浅蓝色，undefined和null都为浅灰色）

（2）引用数据类型：

Object（对象），Array（数组），Date（日期），RegExp（正则）。。等等

（3）如何查看一个变量的数据类型（typeof 运算符）：

数值型数据：返回值为number

console.log(typeof 123) //输出number Copy after login 字符串型数据：返回值为string

console.log(typeof "你好") //输出string Copy after login 布尔型数据：返回值为boolean

console.log(typeof true/false) //输出boolean Copy after login Undefined：返回值为undefined

console.log(typeof undefined) //输出undefined Copy after login Null：返回值为Object（历史遗留问题，说明null也是一个对象）

console.log(typeof null) //输出object Copy after login NaN：返回值为number

console.log(typeof NaN) //输出number Copy after login 6.JavaScript 数据类型的转换：

（1） 在使用加法（+）运算符时，任何数据与字符串类型数据相加都为字符串类型数据；

console.log("你好" + 123) //输出"你好123" Copy after login 注（简单理解）： 在JavaScript 中 空字符串""转换为false，非空字符串转换为true（除“0”，“1”外）；

false转换为 0 或“0”，true转换为 1 或“1”；

做逻辑判断的时候，null，undefined，""(空字符串)，0， NaN都默认为false；

==在比较的时候可以转换数据类型，===是严格比较，只要类型不匹配就返回false；

其实 == 的比较确实是转换成字符串来比较但，但是在布尔型转换为字符串之前，要先转换成 Number

console.log("123" == true) //输出false console.log("1" == true) //输出true console.log("" == true) //输出false console.log(1 == true) //输出true

console.log("" == false) //输出true console.log('123' == false) //输出fasle console.log('0' == false) //输出true console.log(0 == false) //输出true

console.log('1' == 1) //输出true console.log('0' == 0) //输出true console.log(-true) //输出-1 Copy after login （2） parseInt：将字符串转换成整数（只识别字符串中的数值）：

注：会 忽略字符串中的前后空格（当数值后的空格后面还有数值时，将不会再识别）；

能够正确识别正负号，即保留正负号；

在转换时，遇到非数值型的字符就会停止转换；

如果字符串的第一个字符是非数值型的，那么转换的结果为NaN；

console.log(parseInt("123")) //输出123 console.log(parseInt(" 1 2")) //只会输出1 console.log(parseInt(-123)) //输出-123 console.log(parseInt("hello")) //输出NaN console.log(parseInt(true)) //输出NaN console.log(parseInt("123hello")) //输出123，后面非数值型不会识别 console.log(parseInt(" 1 ")) //输出1，忽略空格 Copy after login （3） parseFloat：将字符串转换成小数（识别小数点，注意事项同上）

console.log(parseFloat("123.55")) //输出123.55 console.log(parseFloat(".1hello")) //输出0.1 Copy after login （4） Number：将其它类型的数据转换成数值型，注意被转换的数据必须是纯数值构成，否则无法转换，其它注意事项同上

console.log(Number(true)) //1 console.log(Number(false)) //0 console.log(Number(null)) //0 console.log(Number("123hello")) //NaN console.log(Number("12.22")) //12.22 console.log(Number(undefined)) //NaN Copy after login （5）页面中的信息框：

alert（），弹出个提示框，只有确定；

window.alert("今天天气很好") Copy after login

confirm（），弹出个确认框，有确定和取消；

window.confirm("今天心情也很好") Copy after login

prompt（），弹出个输入框，可以输入内容；

window.prompt("password","请输入密码") Copy after login

JavaScript的基础暂时先写到这里，后续都会补上。。。

本文讲解了JavaScript相关的内容讲解，更多相关之请关注php中文网。

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Explanation of JavaScript related content