JavaScript basics (variables, operators, functions, anonymous function declarations, global variables, local variables, function overloading issues)

Javas is a product language developed by the Internet police company, and has nothing to do with java.

Three ways to introduce JS in html page:

• Introduce js syntax format 1: write js code inside the script tag

 <script type="text/javascript">
			alert(1);
		</script>`

• Introduce js syntax format 2: use external link src: import link file address

<script type="text/javascript" src="js/index.js"></script>

• Introduce js syntax format 3: js code can be nested in html code

<a href="#" onclick="alert(3)">点我啊</a>

Notes on using js:

• When using script tags, you must use double tags, not single tags

Not recommended:

 `<a href="#" onclick="alert(3)">点我啊</a>` 
  不推荐的原因：
				1.它会修改你当前url地址
				2.它会跳转到当前页面的最顶部

	推荐使用的语法格式：

<a href="javascript:void(0);" onclick="alert(3)">点我啊</a>

或者使用简写

<a href="javascript:;" onclick="alert(3)">点我啊</a>  这种方式是上面方式的简写
	-->

Wording

<!--href="javascript:void(0);":我当前a标签不再超链接到一个新的界面，而是作为JS触发源而存在-->

variable

In js, variables are declared with var
. As long as it is a numeric type
in js, the number printed is number. In js, there is also Object type, and the Object class is the base class of all classes.

var obj = new Object();
			console.debug(obj,typeof obj);//{}    object

Note: Objects are displayed as {} in js

Operator

Basic operator: +-* / (When the string is operated in js-* /% will be converted to the corresponding number for operation)

console.debug(2+"3");//结果为5

而+，都是在做拼接

Strings and any data type + are stitching.
In JS, there are 6 false values: 0 null "" undefined NaN (not a number) false
Other values ​​are all true
logical operator
&&: false is short circuit, When using js operation, either the first value is false or the last value
||: true is short circuit, when using js operation, the first value is true or the last value is
compared Operator
==: relatively equal, only compare values, not compare types
===: absolutely equal, not only to compare values, but also to compare types

Logical NOT operator
! The result value obtained is either true or false

//判断是否是一个数字的函数 isNaN  不是一个数字返回true，是一个数字返回false
		console.debug(isNaN(e-f));

js declaration function

/*
			 * 在js中函数就相当于我们java中的方法
			 * 函数申明的语法格式：
			 * 		function  函数名([形参列表]){
			 * 			函数体
			 * 			[return 返回结果值];
			 * 		}
			 */
			function show(){
				console.debug("show show show show...");
			}
			//调用函数
			show();
			//申明函数，并且有参数
			function showParam(param1,param2){
				console.debug("show show show 参数1："+param1+" 参数2:"+param2);
			}
			showParam("王天霸","张根锁");			
			//申明函数，并且返回结果值
			function returnValue(){
				return "李很弱";
			}
			//返回结果值，申明变量接收函数返回过来的 结果值
			var value = returnValue();
			console.debug("返回过来的结果值："+value);

Anonymous function

/*什么叫做匿名函数:简单理解,没有名字的函数就叫做匿名函数*/
			var fun = function(){
				console.debug("show show show");
			}
			//注意：匿名函数需要一个变量名来接收
			
			/*
			  var fun = function(){
				console.debug("show show show");
			  } 
			  就相当于
			  function fun(){
			  }
			 */
			
			fun();

Global variables and local variables Global variables and local variables
in js:
* Global variables: Variables declared outside the function or declared within the function without adding var are global variables
* Local variables: Variables declared inside the function and It is the
problem of overloading local variables that are declared with var
. Functions are not overloaded in js. When there are multiple functions with the same function name in js,
* it will only execute the last function, the previous functions are all
Note that * function calls will not be executed : The calling function has nothing to do with the function parameters. When you call the
* function, if no parameters are passed, the parameters of the receiver are displayed as undefined