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javascript 基础1第11节

2015-08-18 14:23 676 查看 
<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>


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