    <!DOCTYPE html>
    <html>
    <head>
      <meta charset="UTF-8">
      <title>윈도우 닫기</title>
      <link rel="stylesheet" href="style_js.css">
      <style>
        a { text-decoration-line: none; }
       </style>
    </head>
    <body>
      <a href="#" onclick="openWin()">누르면 새 창이 열립니다.</a><hr>
      <a href="#" onclick="closeWin()">열린 창 닫기</a>
      <script>
        let nw;
        function openWin() {
          nw = open("", "_blank", "width=700, height=400, left=10, top=10");
          nw.document.write("<h2>여기는 새 창</h2>");
        }
        function closeWin() {
          if (nw)                  // 창이 존재하면 nw == true
            nw.close();
        }
      </script>
    </body>
    </html>