1. What is JavaScript
1. Overview
JavaScript is a scripting language that belongs to the network, has been widely used for Web application development, used to add functionality to a wide range of dynamic pages, to provide users with a more smooth appearance browsers.
JavaScript is one of the world's most popular scripting language.
2. Historical
originally designed by Brendan Eich Netscape's. JavaScript is a registered trademark of Oracle Corporation. JavaScript-based Ecma International has developed ECMAScript standard. JavaScript can be used in other applications, such as server-side programming.
Full JavaScript implementation consists of three parts:
ECMAScript, DOM, browser object model
ECMAScript It can be understood as a standard JavaScript, the latest version has to es6 version, but most browsers support es5 just stop at the code!
2.JavaScript Quick Start
1. The introduction of JavaScript
- 1. Internal tags: in HTML, JavaScript code must be inserted between the tag
<script> //......</script>
- 2. External introduced: scripts can be placed with an external file
abs.js
JavaScript 文件的文件扩展名是 .js
//。。。
test.html
如需使用外部脚本，需在 <script> 标签的 src (source) 属性中设置脚本的名称
<script src="abc.js"></script>
External JavaScript advantages :
- HTML codes and separated
- The HTML and JavaScript easier to read and maintain
- Cached JavaScript file can speed up page load
Test code :
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<!--script标签内，写Javascript代码-->
<!--<script>-->
<!--alert('hello,world');-->
<!--</script>-->
<!--外部引入-->
<!--注意：script标签必须成对出现-->
<script src="js/yy.js"></script>
<!--不用显示定义type，也默认就是 javascript-->
<script type="text/javascript">
</script>
</head>
<body>
<!--这里也可以存放-->
</body>
</html>
2. Basic Grammar
JavaScript严格区分大小写
1. 定义变量 变量类型 变量名 = 变量值;
2. 条件控制 alert(num);
<!--JavaScript严格区分大小写！-->
<script>
// 1. 定义变量 变量类型 变量名 = 变量值;
var score = 71;
// alert(num);
// 2. 条件控制
if (score>60 && score<70){
alert("60~70")
}else if(score>70 && score<80){
alert("70~80")
}else{
alert("other")
}
//console.log(score) 在浏览器的控制台打印变量！ System.out.println();
/*
* asdasdasd
* */
</script>
3. Data Types
1. Variables
var 王者荣耀 = "荣耀王者";
2.number
123 //整数123
123.1 // 浮点数123.1
1.123e3 //科学计数法
-99 //复数
NaN // not a number
Infinity //表示无限大
3. String
’abc‘ “abc”
4. Boolean value
true ， false
The logical operation
&& 两个都为真，结果为真
|| 一个为真，结果为真
! 真即假，假即真
6. Comparison operators
=
== 等于（类型不一样，值一样，也会判断为true）
=== 绝对等于(类型一样，值一样，结果true)
This is a JS defect, insist not to use == comparison
- NaN === NaN, this is not all of equal value, including their own
- It can only be judged by isNaN (NaN) if the number is NaN
Floating-point problem ::
console.log((1/3) === (1-2/3))
Try to avoid the use of floating point operations carried out, there are accuracy problems!
Math.abs(1/3-(1-2/3))<0.00000001
7.null 和 undefined
- null null
- undefined undefined
8. Array: Java value must be the same type of object ~, JS unnecessary so!
//保证代码的可读性，尽量使用 []
var arr = [1,2,3,4,5,'hello',null,true];
new Array(1,12,3,4,4,5,'hello');
Take array subscript: If you cross the line willundefined
9. Object: The object is braces, arrays are in parentheses
- Using commas to separate each attribute, a need to add the last
//Person person = new Person(1,2,3,4,5);
var person = {
name: "yy",
age: 18,
tags: ['js','java','web','...']
}
- Value takes the object
person.name
> "yy"
person.age
> 18
4. Strict inspection format
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<!--
前提：IEDA 需要设置支持ES6语法
'use strict'; 严格检查模式，预防JavaScript的随意性导致产生的一些问题
必须写在JavaScript的第一行！
局部变量建议都使用 let 去定义~
-->
<script>
'use strict';
// 全局变量
let i = 1;
// ES6 let
</script>
</head>
<body>
</body>
</html>
