if判断

<%@ page language="java" import="java.util.*" pageEncoding="GBK"%>
<%
String emailUser = (String)request.getSession().getAttribute("email");
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>My JSP 'index.jsp' starting page</title>
<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript" src="js/popup_layer.js"></script>
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
<script type="text/javascript"> $(document).ready(function() {
var email = "<%=emailUser%>" ;
alert("email+"+email);
if (email!= null){
alert(11);
}else{
alert(222);
}
               var email1 = "${sessionScope.emailUser}" ;
               alert("email1:"+email1);
              if (email1 != ""){
              alert(333);
}else{
        alert(444444);
}
});
</script>
  </head>
  
  <body>
  </body>
</html>
为什么
var email = "<%=emailUser%>" ;
alert("email+"+email);
if (email!= null){
alert(11);
}else{
alert(222);
}
它怎么取都是alert(111);换成 if(email != "" )也是弹出111面下面的就可以  
var email1 = "${sessionScope.emailUser}" ;
               alert("email1:"+email1);
              if (email1 != ""){
              alert(333);
}else{
        alert(444444);
}
它就可以正常判断