离开页面-消息出现两次

Question

I've added this Javascript to my website which detects when the user navigates away from the page, but I only want a warning to appear if the navigator is offline AND one of my elements contains the word "unsaved":

window.thisPage = window.thisPage || {};
window.thisPage.closeEditorWarning = function (event) {
 if (navigator.onLine===false) {
  // See if any fields need saving...
  for (i = 1; i <= 500; i++) { 
   try {
    ToSave = document.getElementById("Q" + i).innerHTML;
    if (ToSave.indexOf("unsaved")!=-1) {
     return "You are currently offline and some of your responses are not yet saved.\r\n\r\nIf you want to save the changes you've made, choose to 'Stay on this Page' and then reconnect to the internet and any unsaved responses will save automatically.";
    }
   }
   catch(err) {  }
   // If got this far, nothing needs saving anyway...
  }
 }
 return undefined;
};

window.onbeforeunload = window.thisPage.closeEditorWarning;

The code works fine except; if the message pops up the first time and I choose "Stay on this page", then try to navigate away again and the second time click "Leave this page" - rather than navigating away it displays the message again but I can't work out why.

Answer 1

Try returning true from your catch, so the unload will execute. You can also remove the return undefined; from the end.

This works for me, is this what you're after?

This works for me in Firefox.

Fiddle: http://jsfiddle.net/518myq0j/3/ (clicking 'Run' triggers the onbeforeunload )

Updated JavaScript code:

window.thisPage = window.thisPage || {};

window.thisPage.closeEditorWarning = function (event) {
    if (navigator.onLine === false) {
        // See if any fields need saving...
        for (i = 1; i <= 5; i++) {
            try {
                var ToSave = document.getElementById("Q" + i).innerHTML;
                if (ToSave.indexOf("unsaved") != -1) {
                    return "You are currently offline and some of your responses are not yet saved.\r\n\r\nIf you want to save the changes you've made, choose to 'Stay on this Page' and then reconnect to the internet and any unsaved responses will save automatically.";
                }
            } catch (err) {
                return true;
            }
        }
    }
};

window.onbeforeunload = window.thisPage.closeEditorWarning;