练习：----计算阶乘按钮

计算阶乘按钮
 1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 2 <html xmlns="http://www.w3.org/1999/xhtml">
 3 <head>
 4 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 5 <title>计算阶乘按钮</title>
 6 </head>
 7 
 8 <body>
 9 
10 <body>
11 <div style="margin:0px auto">
12     这是计算阶乘的按钮:
13     <input type="button" value="点击输入整数" onclick="jiecheng()" />
14     
15 </div>
16 </body>
17 </html>
18 <script>
19 function jiecheng()
20 {
21 var a = prompt("输入数字吧","只能是整数哦！！！");
22 if(isNaN(a)){
23 alert("输入的不是整数");    
24         }else{
25             var b =1;
26             for(var i=1;i<=a;i++){
27                 b= b*i;
28             }
29             alert(b);
30         }
31     }
32 </script>
33 
34 
35 
36 </body>
37 </html>
显示的效果：