Related explanations about JavaScript basics-JS Tutorial-php.cn

var variable name = value; In js, var can be used to define variables of any data type and can be omitted, but it is not recommended; Because JavaScript is a weakly typed language, and Java is a strongly typed language;

3> If a variable has no value (undefined), the undefined variable cannot be used directly; They are all number type (lowercase);
3>

<script type="text/javascript">
		
	//定义变量
	var a = 10 ;
	
	//重复定义
	//var a  ;
	
	var b = 12.34 ;
	var c = &#39;a&#39; ;
	var d = "hello" ;
	var e = true ; 
	var f = new Object() ;
	
	//输出变量
	document.write("a的值是:"+a+",a的数据类型是:"+typeof(a)+"<br/>") ;
	document.write("b的值是:"+b+",b的数据类型是:"+typeof(b)+"<br/>") ;
	document.write("c的值是:"+c+",c的数据类型是:"+typeof(c)+"<br/>") ;
	document.write("d的值是:"+d+",d的数据类型是:"+typeof(d)+"<br/>") ;
	document.write("e的值是:"+e+",e的数据类型是:"+typeof(e)+"<br/>") ;
	document.write("f的值是:"+f+",f的数据类型是:"+typeof(f)+"<br/>") ;
</script>

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Data conversion function:
String----Integer of type number
parseInt (variable);

STIRNG ---- Number Type of Number Parsefloat (variable);
Example:

Operators in JavaScript:
Arithmetic operators: (addition), - (subtraction), * (multiplication), / (division); Comparison operators: > ;=, <=, <, > . . . ; Logical operators: && (logical double AND), || (logical double OR), ! (logical NOT);

Ternary operation Symbol: expression? The result of true: the result of false;

Note:

In JavaScript, true represents 1 and false represents 0;

Example:

<script type="text/javascript">
	//定义一个变量
	var a = "10" ; //string :里面的值10
	document.write("转换前a的数据类型是:"+typeof(a)+"<br/>") ;
	
	//转换
	a = parseInt(a) ;
	document.write("转换后a的数据类型是:"+typeof(a)+",值是:"+a) ;
	
	document.write("<hr/>") ;
	
	var b = "3.14" ;
	document.write("转换前b的数据类型是:"+typeof(b)+"<br/>");
		
	//转换
	b = parseFloat(b) ;
	document.write("转换后b的数据类型是:"+typeof(b)+",值是:"+b+"<br/") ;
	
</script>

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Process statement:

if statement:

Format:

if(表达式){
语句1；
} else {
语句2；
}
表达式的类型：
1> 如果是numbler类型,(整数类型,小数),0表示条件不成立；
2> 如果是string类型,要么字符要么字符串,必须有数据,如果是空串,条件不成立；
3> 如果是boolean类型，true成立，false不成立；
4> 如果是object类型，必须有对象，才成立，为null不成立；
switch语句：
格式：
和Java相同；
和Java的区别：
1> js中的case后面也可以跟常量,还可以跟变量,还可以跟表达式；
2> java中case后面只能跟常量；
举例：
<script type="text/javascript"> //case后面跟表达式 var age = 25 ; switch(true){ case age >=18 : document.write("你是成年人...") ; break; case age<18 : document.write("你是未成年人呢...") ; break ; } document.write("<hr/>"); var i = "b" ; //和Java语言一样,case后面可以常量, var b = "b" ; switch(i){ case "a" : document.write("a") ; break ; case b : document.write("b") ; break ; case "c" : document.write("c") ; break ; case "d": document.write("d") ; break ; } </script>
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JavaScript中的循环语句：
如：do-while，for，while 循环语句与java中的循环语法结构相同；
练习：
<script type="text/javascript"> //显示"*"形,第一行有一个... for(var i = 0 ; i<5; i++){//外层循环控制行数 for(var j= 0 ; j<=i; j ++){//内层循环控制列数 document.write("* ") ; // 空格转义字符 } document.write("<br/>") ; } document.write("<hr/>") ; //table标签: 使用js里面嵌套表格标签 //document.write("<table align='center'>") ; for(var i = 1 ; i <=9 ; i++){ // for(var j = 1 ; j<=i ;j++){ document.write(i+"*"+j+"="+(i*j)+" ") ; } document.write("<br/>") ; } </script>
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wIth语句：
格式：
with(document){
语句；
}
特点：将document对象当做witch语句参数,然后直接使用它里面的write()；
举例：
<script type="text/javascript"> //with语句:格式 with(document){....} //将document对象当做witch语句参数,然后直接使用它里面的write() ; //with语句 with(document){ for(var i = 0 ; i < 5 ; i ++){ for(var j = 0 ; j <=i ; j++){ write("* "); } write("<br/>") ; } write("<hr/>") ; for(var i = 1 ; i <=9 ; i++){ for(var j = 1 ; j <= i ;j++){ write(i+"*"+j+"="+(i*j)+" ") ; } write("<br/>") ; } } </script>
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for-in语句：
作用：针对数组或者对象进行遍历；
格式：
for(var 遍历的变量名 in 数组名称/对象名称){
输出变量名；
}
举例：
<script type="text/javascript"> var arr = [10,20,30,40] ; //类似于Java中的静态初始化 for(var i =0 ; i < arr.length; i++){ document.write(arr[i] +" ") ; } document.write("<hr/>"); //使用for-in语句 for(var i in arr){ document.write(arr[i] +" ") ; }
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</script>
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函数：
格式：
function 函数名称(形式参数列表){
语句；
}
函数调用：
函数名称(实际参数列表)；
函数中注意的问题：
1> 函数的形式参数不能有var关键字定义，否则报错；
2> 在js中函数是可以有return语句的，直接return，但是没有返回值；
3> 在js中，函数是不存在的重载概念，后面定义的函数回覆盖掉前面定义的函数；
4> 当实际参数的数量小于形式参数的数量，就会出现一个值肯定是NaN；
如果实际参数大于形式参数，那么最终会计算前面的值，然后将后面的数据丢弃掉；
5> 在js的每一个函数中，里面存在默认的数组：arguments，它的作用就是将实际参数从左到右依次赋值给形式参数(从左到右)；
举例：

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