JS这个为什么什么都弹不出来？

Question

<script type="text/javascript"> 
        function A(){        
         this.B=function(){
          return 20;
         }   
        }
        function C(){ 
          var D = new A(); 
          alert(D.B());
       }  
</script>

Answer 1

因为你没执行......

        function A(){        
         this.B=function(){
          return 20;
         }   
        }
        function C(){ 
          var D = new A(); 
          alert(D.B());
       }
       C(); 

或者

        function A(){        
         this.B=function(){
          return 20;
         }   
        }
       +(function C(){ 
          var D = new A(); 
          alert(D.B());
       })();