팝업창이 존재하는지 여부 확인

var popup;

$("#btnPopup").click(function () {
           if (popup == undefined) {
                    popup = window.open("Popup.aspx", "Test", "top=10, left=20, width=600, height=600");
           }
           else {
                    if (popup.closed) {
                        popup = window.open("Popup.aspx", "Test", "top=10, left=20, width=600, height=600");
                    }
                    else {
                        alert("이미 팝업창이 떠있습니다.");
                    }
          }
});