javaScript 载入自执行

1.注册可以直接调用f()中的b(),c(),d() .原因？自己想。

<!DOCTYPE html>
<html>
     <head>
    <meta charset=gb2312 />
    <title>js</title>
    <script>
         function f(){
            var a = 1;
            b = function(){
                alert(a);
            }
            c = function(){
                a++;
            }
            d = function(x){
                a = 100;
            }
         }
    </script>                
     </head>

     <body>
        <input type="button" value="f" onclick="f()"/>    //需要先点击这个，才能点击其他，否则出错   
        <input type="button" value="alert a" onclick="b()"/>       
        
        <input type="button" value="a++" onclick="c()"/>  
        <input type="button" value="a=100" onclick="d()"/>  
        
     </body>
</html>

2.f()改为匿名函数----载入自执行

 1 <!DOCTYPE html>
 2 <html>
 3      <head>
 4     <meta charset=gb2312 />
 5     <title>js</title>
 6     <script>
 7          (function(){
 8             var a = 1;
 9             b = function(){
10                 alert(a);
11             }
12             c = function(){
13                 a++;
14             }
15             d = function(x){
16                 a = 100;
17             }
18          })();
19     </script>                
20      </head>
21 
22      <body>
23         <div>f()改为匿名函数，载入后自执行</div>
24         <input type="button" value="alert a" onclick="b()"/>       
25         
26         <input type="button" value="a++" onclick="c()"/>  
27         <input type="button" value="a=100" onclick="d()"/>  
28         
29      </body>
30 </html>