如何检测浏览器/选项卡是否关闭

我想在浏览器关闭或浏览器选项卡关闭时发出警告框。我有一些代码，但在这1额外的消息即将到来。我想知道它从哪里来，在警报框里。如果你有其他代码，请发给我或告诉我该怎么做。实际上，我想避免第二个用户登录(为此，我创建1会话id并保存在DB之后，每次当用户登录时，它检查会话id是否在DB中，如果用户可以登录，当用户注销会话id也被删除，但浏览器或选项卡关闭，那么会话id未被删除。为此，我想检测一个之后，我想将控件发送到ajax控制器)，这就是为什么我想知道从哪里来1个额外的消息。请解决我的问题…

<script type="text/javascript" >
var validNavigation = false;
function wireUpEvents() {
  var dont_confirm_leave = 0; //set dont_confirm_leave to 1 when you want the user to be able to leave withou confirmation
  var leave_message = 'You sure you want to leave?'
  function goodbye(e) {
    if (!validNavigation) {
      if (dont_confirm_leave!==1) {
        if(!e) e = window.event;
        //e.cancelBubble is supported by IE - this will kill the bubbling process.
        e.cancelBubble = true;
        e.returnValue = leave_message;
        //e.stopPropagation works in Firefox.
        if (e.stopPropagation) {
          e.stopPropagation();
          e.preventDefault();
        }
        //return works for Chrome and Safari
        return leave_message;
      }
    }
  }
  window.onbeforeunload=goodbye;
  // Attach the event keypress to exclude the F5 refresh
  $('document').bind('keypress', function(e) {
    if (e.keyCode == 116){
      validNavigation = true;
    }
  });
  // Attach the event click for all links in the page
  $("a").bind("click", function() {
    validNavigation = true;
  });
  // Attach the event submit for all forms in the page
  $("form").bind("submit", function() {
    validNavigation = true;
  });
  // Attach the event click for all inputs in the page
  $("input[type=submit]").bind("click", function() {
    validNavigation = true;
  });
}
// Wire up the events as soon as the DOM tree is ready
$(document).ready(function() {
  wireUpEvents();
}); 
</script>