JavaScript学习－if语句

一、IF语句

例：

       <% -- JavaScript学习 -- %>
     < script  type ="text/javascript"  language ="JavaScript" >
         // if语句
         function  FunShowIf()
        {
             var  a = 1 ;
             var  b = a ++ ;
             if (a > b)
            {
                window.alert(a + ' > ' + b);   
            }
             else
            {
                window.alert(a + ' < ' + b);   
            }                      
        } 
      </ script >

             <% -- 测试使用的按钮 -- %>
             < input  id ="Button1"  type ="button"  runat ="server"  value ="测试"  onclick ="FunShowIf();"   />

  

   二：变量为空

  如果定义了变量但没有赋值，此时X类型为Undefined。如果要检查X是否定义，则用X==null()或typeof(x)=='undefined'(类型为未定义)。例：   

     < script  type ="text/javascript"  language ="JavaScript" >
         function  FunIfNull()
        {
             var  x;
             if (x == null )
            {
                alert( " X为空! " );
            }
        }
         function  FunIfUndefine()
        {        
             var  x;
             if ( typeof (x) == ' undefined ' )
            {
                alert( " X为空! " );
            }
        }
         function  FunIfSign()
        {        
             var  x;
        
         x = ()
             if ( typeof (x) == ' undefined ' )
            {
                alert( " X为空! " );
            }
        }
      </ script >

             <% -- 测试使用的按钮 -- %>
             < input  id ="Button1"  type ="button"  runat ="server"  value ="测试"  onclick ="FunIfNull();"   />
             < input  id ="Button2"  type ="button"  runat ="server"  value ="测试"  onclick ="FunIfUndefine();"   />

三：? : 语句

例：

         < script  type ="text/javascript"  language ="JavaScript" >
         function  FunIfSign()
        {        
             var  x;
             var  a = 0 ;b = 3 ;            
            x = (a > b) ? " a>b " : " a<b " ;
            alert(x);
        }
      </ script >
             <% -- 测试使用的按钮 -- %>
             < input  id ="Button1"  type ="button"  runat ="server"  value ="测试"  onclick ="FunIfSign();"   />

    结果：a<b