jQuery how to build a link with json?, error

i have this:

    $.get('xxx.php', { username: userName },
    function(data){
        var get_back = data;
        alert(get_back);
});

this will return get_back=12345

and i'm trying to build this:

url: "http://www.test.com/users/" + get_back,

the result to be http://www.test.com/users/12345

for some reason it doesn't want to work. If i hard-code the 12345 in the link it will work. i've also tryed url: "http://www.test.com/users/" + get_back + "", and url: 'h开发者_StackOverflow社区ttp://www.test.com/users/' + get_back,

any ideas?

edit:

$.ajax({
type: "POST",
data: JSON.stringify(formData),
dataType: "json",
url: "http://www.test.com/users/" + get_back + "",
success: function(t){    alert(t);  }
});

that because get_back might be in the scope of everything else. either you can call get_back before the $.get call and it will be global or you can put it all into on object like this:

var get = {
   get_back: null,
   init: function(username){
      var self = this;
      $.get('xxx.php', { username: userName },
          function(data){
              self.get_back = data;
              self.runAjax(); //run the ajax when get_back is instantiated
          }
      });

   },
   runAjax: function(){
      var self = this;
      $.ajax({
         type: "POST",
         data: JSON.stringify(formData),
         dataType: "json",
         url: "http://www.test.com/users/" + self.get_back + "",
         success: function(t){    alert(t);  }
         });
   }
}

//to use it:
get.init(username);
//to use get_back
alert(get.get_back);

UPDATED WITH AJAX

you might have to add formdata to the object somehow, i dont know where that was created