哪错了．．．．
var   f=document.form1;
  var   usrName=f.userName.value;

  用户名: <input   type= "text "   id= "userName "   name= "userName "   value= " ">
提示userName为空或不是对像...

------解决方案--------------------
那你输入个值再试试看
或var usrName=document.GetElemengById( "userName ").value;
------解决方案--------------------
var usrName=document.getElementById( "userName ");
alert(usrName.value);
------解决方案--------------------
通过document.GetElemengById( "userName ").Value来获取值
------解决方案--------------------
var f=document.form1( "userName ");
alert(f.value );