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...

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