Cacher is the code snippet organizer for pro developers

 window.onload = function() {
  let w;
  document.getElementById('open').onclick = function() {
    // ウィンドウを開く
    w = open('newpage.html', 'new page', 'width=400, height=300');
    document.getElementById('close').onclick = function() {
      // ウィンドウを閉じる
      if(w) w.close();
    }
  }
}

input#open(type='button' value='open')
input#close(type='button' value='close')