Trying to set lookup value via Javascript

Here's my code, some of which I got online, so I don't fully understand it all, but I know what it's doing, pretty much.

    // Check if it's a create form
    var CREATE_FORM = 1;
    var userName1;
    var  formType= Xrm.Page.ui.getFormType();
    if (formType == 1)
   {
       // Set 'Completed by' field to record owner
       //Get GUID of logged user
        var user = Xrm.Page.context.getUserId();
        var userId = user.substring(1,37);

        // Read the CRM Context to determine the CRM URL
        var serverUrl = Xrm.Page.context.getServerUrl()
        var ODATA_ENDPOINT = "/XRMServices/2011/OrganizationData.svc";
        var ODATA_EntityCollection = "/SystemUserSet";
        // Specify the ODATA Query
        var ODATA_Query = "(guid\'" + userId + "')";
        // Combine into the final URL
       var ODATA_Final_url = serverUrl + ODATA_ENDPOINT + ODATA_EntityCollection + ODATA_Query;


        //Calls the REST endpoint to retrieve data
        $.ajax({
            type: "GET",
            contentType: "application/json; charset=utf-8",
            datatype: "json",
            url: ODATA_Final_url,
            beforeSend: function (XMLHttpRequest) {
                XMLHttpRequest.setRequestHeader("Accept", "application/json");
            },
            success: function (data, textStatus, XmlHttpRequest) {
            userName = data.d.FullName;

            userName1 = userName ;
//alert("userName1 set to: " + userName1);
            },
            error: function (XmlHttpRequest, textStatus, errorThrown) {
            alert('Error: '+ ODATA_Final_url);

            }
          });

         var lookup = new Array();
         lookup[0] = new Object();
         lookup[0].id = userId ;
//alert("userName1 value now: " + userName1);
         lookup[0].name = userName1 ;
         lookup[0].entityType = "systemuser";
         Xrm.Page.getAttribute("vs_completedby").setValue(lookup);

   } 
}

When I leave the commented out alert boxes in, it runs fine - username1 has a value, and the lookup displays this username correctly. However, when I comment out the alert lines, username1 is undefined. Do I need some sort of wait interval to allow the Rest services to complete the data retrieval?? And the alert box is causing a slight delay, so that's why username1 gets populated when they aren't commented out??? I can't have the alert boxes in my code though!

Thanks.