My problem is resolved !!!!
I should write my code like this:
function doLayout(role){
switch(role){
case "admin":
$('#menu').empty();
$('#menu').append('<a href="#" id="linkAdmin">admin</a>');
$('#linkAdmin').linkbutton({
plain:true,
iconCls:'icon-ok'
});
$('#menu').append('<a href="#" id="linkUser">user</a>');
$('#linkUser').linkbutton({
plain:true,
iconCls:'icon-ok'
});
$('#menu').show();
break;
case "user":
$('#menu').empty();
$('#menu').append('<a href="#" id="linkUser">user</a>');
$('#linkUser').linkbutton({
plain:true,
iconCls:'icon-ok'
});
$('#menu').show();
break;
}
}