Why can't I expand this event object in the chrome console?

Question

Simplified, what I'm doing is running this in the console:

window.onbeforeunload = function (e) {
    console.log(e);
}

In the console, when the event fires (by trying to "leave page" in the middle of writing an SO question) what I see is this:

Event {clipboardData: undefined, cancelBubble: false, returnValue: true, srcElement: document, defaultPrevented: false…}

With a little "i" graphic next to it. When I click the arrow next to it to expand the object in the console, nothing happens. The arrow turns to indicate that it has expanded, but it doesn't expand.

What am I missing here??

Answer 1

This is happening because although you're letting the console persist over page changes, the Object no longer exists - it was destroyed when you left the page. This means it's simply not available to be inspected anymore, so clicking the down triangle is not helpful.

Try this instead, to prevent the page actually changing:

window.onbeforeunload = function (e) {
    console.log(e);
    return true;
}

Now the page will prompt to ask you what to do. Click 'cancel' in the prompt that comes up in order to remain on the page. Now you can inspect the Event in the console as desired.

The difference is that the onbeforeunload function is now returning a value that isn't null/undefined. The return value could be anything, even '' or false, etc...anything except null and undefined, and it will still cause the page to prompt before navigating away and thus give you an opportunity to inspect the event. Remember that with no return statement, JavaScript functions return undefined by default.

Whenever you can't inspect something in the Chrome Dev Tools, 90% of the time it's because some action has caused that thing to become unavailable...the page has moved on from when that object existed.