JS Getting started - writing position, notes and literal variables

First, write the location of JS

1, the label in the HTML body tag

<!--
	可以将js代码编写到标签的onclick属性中；
	当我们点击按钮时，js代码才会执行；		
	虽然可以写在标签的属性中，但是他们属于结构与行为耦合，不方便维护，不推荐使用。
-->
	<button onclick="alert('讨厌，你点我干嘛~~');">点我一下</button>
		
<!--
	可以将js代码写在超链接的href属性中，这样当点击超链接时，会执行js代码
-->
	<a href="javascript:alert('让你点你就点！！');">你也点我一下</a>
	<a href="javascript:;">你也点我一下</a>

2, within the script tag, script tags written in two places: in the head tag , if written in the head tag, recommendations written in the onload event of the window; in the body , if written in the body is generally recommended to write the end of the body label money.

<!--引入外部JS文件-->
<script type="text/javascript" src="js/script.js"></script>

<!--直接写在内部的-->
<script type="text/javascript">
	alert("我是内部的JS代码");
</script>

Second, the comment about the JS

/*
    这是一段多行注释
    JS注释：被注释中的内容不会被执行，但是可以在源代码中查看
    要养成良好的编写注释的习惯来说明自己的代码逻辑
*/

//这是一个单行注释
alert("hello");
document.write("hello");
console.log("hello"); //该语句用来在控制台输出一个日志

Third, the literals and variables

/*
 * 字面量：都是一些不可改变的值
 * 		比如 ：1 2 3 4 5 
 * 		字面量都是可以直接使用，但是我们一般都不会直接使用字面量
 * 
 * 变量：变量可以用来保存字面量（数据），而且变量的值是可以任意改变的
 * 		变量更加方便我们使用，所以在开发中都是通过变量去保存一个字面量，而很少直接使用字面量，可以通过变量对字面量进行描述。
 */

//声明变量:在js中使用var关键字来声明一个变量
var a; // 只是声明了一个变量，并没有进行赋值

//为变量赋值
a = 123;
a = 456;
a = 123124223423424;

//声明和赋值同时进行
var b = 789;
var c = 0;
var age = 80;
console.log(age);

//变量名支持中文，但是千万不要这么用
var 锄禾日当午 = 789;
console.log(锄禾日当午);

• Identifier (also can be said that the variable name)
  • In JS and all that are by our self-named can be called is the identifier
  • For example: variable names, function names, attribute names are all identifiers
• You need to comply with the following rules when naming an identifier:
  • 1. The identifier may contain letters, numbers, _, $
  • 2. Identifiers can not begin with a number
  • 3. ES identifier can not be reserved words or keywords, categories such as: var, const, function ......
  • 4. identifier generally used hump (hump large or a small hump) nomenclature
    • Small hump: the first letter lowercase , beginning letter of each word capitalized, the rest lowercase letters
    • Big Hump: the first letter capitalized , beginning letter of each word capitalized, the rest lowercase letters
• JS bottom during storage identifier Unicode encoding actually employed, so that in theory, all the contents contained in utf-8 can be used as an identifier.
• The first letter capitalized , beginning letter of each word capitalized, the rest lowercase letters
• JS bottom during storage identifier Unicode encoding actually employed, so that in theory, all the contents contained in utf-8 can be used as an identifier.