<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
</body>
</html>
<script type="text/javascript">
var a = 100;
var b = "100";
alert(b- a);
alert(typeof (b- a));
alert(a + b - a);
alert(typeof (a + b - a));
alert(a + b);
alert(typeof (a + b));
alert(a + b - b);
alert(typeof (a + b - b));
alert(a + b - b - b);
alert(typeof (a + b - b - b));
</script>
运行结果
0
number
100000
number
100100
string
100000
number
99900
number
由此可知道JS在进行这样的运算时 只会在2个值相加时优先判断为string类型 其他情况下都默认为number型运算！！！！