js ajax return false了，仍然会往下执行

 function checkMust(){
  var flag=false;
  $.getJSON("../Ajax/Carton/Ajax_TMSOrder_Create.ashx?r=" + Math.random(), { "action": "GetToWaybill","WAYBILLNO": $("#tbWaybillNo").val(), "FWDCD": $("#ddlForwarder").val() }, function (r) {
                    if (r.length > 0) {
                       alert("<%= Resources.GlobalResource_Carton.ErMsg91%>");
                           flag=true;
                    }  
            });
       if(flag){
10         return false;
11        }
      return true;  
     }