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< form  name ="form1"  method ="post"  action ="" >
   < table  width ="500"  border ="0"  align ="center" >
     < tr >
       < td  width ="200" >< input  type ="text"  name ="test" ></ td >
       < td >< input  type ="submit"  name ="Submit"  onClick ="return check();"  value ="提交" ></ td >
     </ tr >
   </ table >
</ form >

< script language = " javascript " >
   function  check() {
   if (document.form1.test.value == "" ) {
alert ( " 不能为空！ " );
return   false ;
}
}
</ script >