Javascript 当textarea不为空时，如何在关闭页面前警告用户？

我试图复制与stackoverflow文本区域相同的内容

编辑：在阅读了这里的建议后，我选择了这样的方式：

window.onbeforeunload = function()
{

    var myTextArea = document.getElementById('post_content');

    if (myTextArea.value.length > 0)
    {
        return "You haven\'t submitted your post; are you sure you want to discard it?"; 
    }

}

这似乎适用于Firefox和其他浏览器，而不会引起双重确认。

使用类似于：

window.onunload = function(){
   var myTextArea = [a ref to your textarea];
   if (myTextArea.value.length > 0) {
   //=>textarea contains text, ask the user:
      return confirm('You didn\'t submit your text yet! Are you sure'+
                     ' you want to navigate away from this page?');
   }
   //=>continue unloading
   return true;
}