javascript 基础1第11节

<html>
	<head>
	<title>javascript基础</title>
	</head>

	<body>
		1.NaN  isNaN()</br>
		<script type="text/javascript">
			  //数据类型转换失败 返回NaN类型
			  document.write("parseInt('a') : " ,parseInt('a'),"<br/>");
			  document.write("parseInt('9') : " ,parseInt('9'),"<br/>");
			  document.write("NaN == NaN : " , NaN == NaN,"<br/>");
		</script>     	

		2.关系运算符  >  >=  < <=  !=  ==  ===</br>
		<script type="text/javascript">
			  document.write("'7' < 5  : " ,'7' < 5,"<br/>");//发生类型转换
			  document.write("'7' > 5  : " ,'7' > 5,"<br/>");
			  document.write("'7' > '12'  : " ,'7' > '12',"<br/>");//不发生类型转换
			  document.write("'a' > 12  : " ,'a' > 12,"<br/>");//发生类型转换
			  document.write("'a' < 12  : " ,'a' < 12,"<br/>");//发生类型转换
			  document.write("isNaN('a')  : " ,isNaN('a') ,"<br/>");//是否是非数字
			  document.write("isNaN('8')  : " ,isNaN('8') ,"<br/>");

			  document.write("100 == '100'   : " , 100 == '100' ,"<br/>");
			  document.write("100 === '100'   : " , 100 === '100' ,"<br/>");//值相等  类型相同
			  document.write("'100' === '100'   : " , '100' === '100' ,"<br/>");
		</script>     	

		3.赋值运算符  =  += -= *= /=   %=</br>

		4.逻辑运算符  ！&& ||</br>
		<script type="text/javascript">
			 //任何类型使用！结果是布尔型  true  --- null '' 0  undefined
			 document.write("!true   : " , !true ,"<br/>");
			 document.write("!5   : " , !5 ,"<br/>");
			 document.write("!0   : " , !0 ,"<br/>");
			 document.write("!-2   : " , !-2 ,"<br/>");
			 document.write("!null   : " , !null ,"<br/>");
			 document.write("!new Date()   : " , !new Date() ,"<br/>");
		</script>     	

		5.类型转换</br>
		<script type="text/javascript">
				document.write("parseInt('123') : " ,parseInt('123'),"<br/>");
				document.write("parseInt('123a') : " ,parseInt('123a'),"<br/>");
				document.write("parseInt('a123') : " ,parseInt('a123'),"<br/>");
				document.write("parseInt('123.456') : " ,parseInt('123.456'),"<br/>");

				document.write("parseFloat('123.456') : " ,parseFloat('123.456'),"<br/>");
				document.write("parseFloat('123.456.789') : " ,parseFloat('123.456.789'),"<br/>");
				document.write("parseFloat('123.a.789') : " ,parseFloat('123.a.789'),"<br/>");
				document.write("parseFloat('a123.789') : " ,parseFloat('a123.789'),"<br/>");
				var a = 20;
				var b = 50;
				document.write("a + b = " ,a + b,"<br/>");
				document.write("a.toString() + b = " ,a.toString() + b,"<br/>");

				document.write("'10' + b = " ,'10' + b,"<br/>");//字符串 连接
				document.write("'10' - b = " ,'10' - b,"<br/>");//类型转换 进行计算
				document.write("'a' - b = " ,'a' - b,"<br/>");
		</script>
	</body>

</html>

结果：

2.