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

问题

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

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

But 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??

回答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 giving 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.

回答2:

Old question. But this simple solution worked better for me. Use

function(e) {
  console.dir(e);
}

回答3:

It's a good practice when inspecting objects or arrays that can change later to drop in a

debugger

...after the point in the code you want to debug. This pauses the execution of the code at this point and means you know for sure you're seeing its state at this point, rather than any subsequent changes.

The failure to expand can mean that the object was subsequently deleted or garbage-collected, or (especially if you're using Chrome dev tools on a Node.js script) that the script is complete, so all refs point nowhere.

---

In fact, an arguably better practice is to use debugger instead of console.log, then find what you want to inspect in the local variables.

window.onbeforeunload = function (e) {
  debugger; // Now go find `e` in the local variables section
}

回答4:

I just came across this question with a problem I had where my API PUT request was showing as cancelled in my console tab in chrome dev tools and I was seeing the same behavior where I couldn't expand the object and the little i icon was showing next to the console entry. I decided to post this answer with a link to my question in case it might help anyone else.

Api PUT request showing as "cancelled" with error message "TypeError: failed to fetch"

来源：https://stackoverflow.com/questions/18776467/why-cant-i-expand-this-event-object-in-the-chrome-console