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;
      }