Develop Reference

Chrome Developer Tools keeps breaking where no breakpoints are set

When debugging JavaScript code using Chrome Developer Tools the debugger pauses on code where breakpoints are not set. I don't have the Pause on exceptions feature enabled, and there definitely are not breakpoints set (see attached image).
I asked a similar question before which was helpful but didn't quite solve this issue (previously I had the Pause on exceptions enabled). In the example below I swapped out the minified version of kendo.all.min.js for the unminified version, which allows me to see where the script execution is being paused, but I don't know why it is being paused. This happens a lot with jquery.min.js too.

I fixed my breakpoints problem by clicking the "Restore defaults and reload" button located in the “Settings” section. To find the button: In Developer Tools click the cogged wheel next to the top right three vertical dots. (Note: If using older versions of Google Chrome, click on the top right three vertical dots and then select “Settings F1”). The button “Restore defaults and reload” is at the bottom right.
Also note that it really restores defaults - you lose all previously saved breakpoints and any files you have open in Developer Tools will be closed.
I got this solution from: Chrome javascript debugger breakpoints don't do anything?

Accepted answer to Import/export Chrome devtools breakpoints & settings between computers reminded me to check "devtools-on-devtools" and in the DevTools' inner Application Local Storage remove the faulty element(s) of the JSON list value for key breakpoints to fix my same issue without resetting everything.
Later it happened again, so I wrote this console snippet:
((stores, matchKey, removeMatchingRegExp, dryRun)=>{for(let store of stores){
let r = JSON.parse(localStorage[store]), l = r.length;
r = r.filter(b=>!b[matchKey].match(removeMatchingRegExp));
if(!dryRun) localStorage[store] = JSON.stringify(r);
console.log(`${dryRun ? 'Would' : 'Did'} remove ${l - r.length} entries from ${store}:`, r)}; return 'OK'})
(['breakpoints', 'domBreakpoints'], 'url',
new RegExp('^https?://example.com/script.js$'), true)
Note:
Provided without warranty: Use at own risk. Backup your data.
Edit it - at least the RegExp argument - to suite your needs.
The last-most boolean should be false to disable dryRun.
Outer DevTools must be reopened for effect.

I was able to resolve this by updating Chrome. I don't think the version matters, just the process of updating resets all of the breakpoints that have been stored in the cache.

F8 has two functionalitys. 1. Skip ot next Breakpoint, 2.stop wherever the Browser is executing code. This works even if you have no breakpoint set. For example spamming F8 when you forgot to deactivate breakpoints might cause the browser to stop anywhere.
When you go to the debugger shortcuts you can see that F8 has the functionality to Pause / Continue. But it sadly seems like you can't split them on two different keybinds.
Since I know this feature I never ran into this "bug" anymore.

I'm posting this answer in case it will help someone who, like me, missed an important clue as to the cause of the phantom breakpoint behavior. In my case, it was "user error" --mine. The root cause was a forgotten "debugger;" statement in a JavaScript file that was itself generated from TypeScript. I had removed the debugger; statement from TypeScript locally, run and tested without issue from localhost. But I had pushed the version with the statement to remote and it built and released to our dev site with the statement present. The dev site build excludes the TypeScript source files. When the debugger statement was hit, Chrome tried to load the .ts source and displayed "Could not load content..." I just assumed it was at a breakpoint (I'd set many during testing). And when I saw "No breakpoint" I assumed Chrome was experiencing the issue addressed in this thread. If I'd bothered to look in the Call Stack trace, I would have seen the source code line in the .ts file and pretty quickly figured it out. Here's a screenshot:

just disable the cache and reload the page, the breakpoints will show up again

Chrome User Agent not displaying script - CSS fix?

I installed a WP theme recently and am having trouble getting a Facebook conversion box to display when I visit my site through Chrome. This problem doesn't exist on Firefox.
The response I get on Chrome is no visible pop-up box, but upon inspecting the element, I do see the code for the box in my header.php with a line from the Chrome User Agent saying:
script {
display: none;
Is there a CSS solution to override the User Agent and get the script to display? Again, I have no problems with the theme on Firefox. Here's a link to my site.
Thanks in advance!

I think I found the source of your problem. It's a browser extension.
Your conversion box appears on my personal laptop. However, it doesn't on my work laptop. That lead me to believe that it was a conflict with the script & a browser extension. So I opened your site in an incognito window. Voila! it worked. The next step was figuring out which extension was blocking it. For me, it is LastPass (my work requires the use of LastPass).
If you have LastPass enabled as an extension, disable it. Then test again.
It'd also be a good idea to get these console errors figured out:
Uncaught SyntaxError: Unexpected token ; (index):476
carouFredSel: No element found for "#foo1". jquery.carouFredSel-6.2.1.js?ver=3.8.3:4232
Uncaught ReferenceError: paginateVal is not defined cgview.js?ver=3.8.3:28
Solutions
Unfortunately, there isn't much you can do about people's individual extensions. If this is an absolutely critical feature for you, I'd look at finding a different implementation of it. One that does not conflict with common extensions (that's the easy option).
The other option (harder) is to look into the code of the plugin/script that you're using for the conversion box and identify the conflict.
Last option (hardest) is to look at LastPass extension source code and identify the bug that is preventing other scripts from running. I've had issues with LastPass interfering with javascript in the past.

Firebug says "No Javascript on this page", even though JavaScript does exist on the page

Why does Firebug say there is No Javascript on this page when there are clearly loads of JavaScript on the page. I even reloaded the page several time to make sure but it still show the same message.
It never used to do that before, but all of a sudden it misbehaves.
Is it because of some configuration issues or because of something else?

As of Firefox version 50, it seems that Firebug will no longer work as Mozilla are migrating to Firefox Devtools. The tab Tools/Web Developer has a 'debugger' option. See this page for a discussion on the migration. Read all the posts dated 19 November for further information.
It seems to me that the option gives less functionality but Firebug per se is never going to be fixed for Firefox 50 and beyond

EDIT 2017 April
This answer is now outdated and I'm not even testing it anymore because I'm no longer using Firebug (a shame, I loved it years ago in a time it was The tool of trade for web developing).
EDIT 2014 November
The original answer is now more than a year outdated due to firebug evolution.
Thanks to #AmadeusDrZaius to keep it on tracks
Answer for outdated firebug versions
For anyone still with this problem after updating Firebug.
Just click Clear Activation List option inside the Firebug menu.
For those who can't find it, note that "Clear Activation List" shows up in the dropdown from the Firebug icon in the toolbar, but not the Tools > Web Developer > Firebug menu.
I guess due to the bug previous hitting your working page it stays listed as "no activate FB for this site please".

I encountered No Javascript on this page after upgrading Firefox to ver 50.0, with Firebug ver 2.0.18. I tried refreshing Firefox, even completely uninstall Firefox and installed again. Nothing works for me.
A couple of hours later, I downgraded Firefox to 49.0.2, and voila, it works right away. I downloaded the old version here.

I tried everything on this list and nothing worked for me except resetting Firefox like so:
https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-problems

I realize there are already 10 answers to this question, but in my case none of them helped. I'm running Firefox version 39.0 and Firebug version 2.0.11. The only fix is to completely restart Firefox (i.e. not just the tab or instance with the problem).
Perhaps it's due to Firefox's over-usage of memory, or maybe there's a bug in Firebug. I can't speak to that, but what I do know is it didn't work. I killed Firefox, opened the exact same page again with no changes made and now it works. This has happened to me numerous times and a complete restart always fixes it, but that's the only thing that does.

Since Firefox 49.0 and Firebug 2.0.18 the Script panel is broken. This is due to some internal Firefox API changes. And because Firebug is officially discontinued, this unfortunately won't get fixed anymore.
There was also a bug in Firebug 2.0.11 (and below) in combination with Firefox 39.0 causing this problem. This bug is filed as issue 7918.
It happens when the Script panel is enabled and you close Firebug and reopen it.
You can see two related error messages logged to the Browser Console (can be opened via Ctrl+Shift+J or Firebug menu > Developer > Browser Console):
error occurred while processing 'attach: TypeError: docShell is null
Stack: getChildDocShells#resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/webbrowser.js:53:7
...
and
onPacket threw an exception: Error: Server did not specify an actor, dropping packet: {"error":"unknownError","message":"error occurred while processing 'attach: TypeError: docShell is null\nStack: getChildDocShells#resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/webbrowser.js:53:7
...
Solution:
The only solution until this bug is fixed is to restart Firefox completely.

Similar things have happened to me because I forgot to add the closing tag.
DON'T do this:
<script src="some.url" type="text/javascript">
or
<script src="some.url" type="text/javascript"/>
DO this instead:
<script src="some.url" type="text/javascript"></script>

The same problem just happened to me. I thought that it was a problem in my scripts, but I checked google.com and the problem persisted.
My problem was that I had updated my version of FF and kept the old version of FireBug. When I updated FireBug and everything was solved.

From firefox 49, the fire bug will be built in and it is called fire-bug next. So the fire bug add on won't work properly and no issues will be solved. So you can use firefox inspector. See here for details information

If the problem only shows up on a single page, check that your script folder is referenced from current folder (".") or that's reachable from the server root ("/").
<script src="./scripts/pagescripts.js" type="text/javascript"></script>
may not be the same as
<script src="/scripts/pagescripts.js" type="text/javascript"></script>

As mentioned, sometimes there's a Javascript syntax error that invalidates the whole script. But I have also had this, and simply opening up the page in a new tab or window and refreshing firebug (disable, enable) gets the javascript to show up again.

One time this caught me out was when I published one of my sites and I was checking it out. No Javascript was working, there were no errors in the console and firebug showed no scripts.
It turned out that in that instance, I'd forgotten to allow the site in NoScript - so there really were no scripts being loaded.
Not sure if that's the problem you're facing but it's worth being aware of.

I also got this error when attempting to use Ajax to a different server than the one serving up the page. JavaScript only allows Ajax access to the server that served up the page.

This problem is STILL happening (Aug 2015) with Firefox 39.0.3 and Firebug 2.0.11
For people saying the code is wrong or missing end tags etc. all I did to get all the javascript to reappear in tab was to shut down Firefox and restart it. I didn't change anything. Also that "Clear Activation List" option didn't work (no idea what that means)

I upvoted #Tom_Thomson's answer above, but I am wondering if simply removing and then reinstalling the Firebug add-on might not have accomplished the same thing without losing all the other stuff like bookmarks.

FWIW this problem is still happening in August 2016. I'm using Firebug 2.0.17 with Firefox Dev Edition 50.0a2.
As a workaround, I'm able to use the Developer Tools, though they are not as robust as Firebug (when it's working).

Well Firefox native Inspector is good... I know that you will miss some things, but in general we can survive with it. The good news is... less one addon, FF will be faster :)

JS assets in a subdomain wont load on ff & ie but chrome is all good

We are building a Saas product and have purchased a bootstrap dashboard, all the JS/CSS assets are loaded though a sobdomain via our CDN.
Works perfectly on chrome but on ie and ff the components do not load properly, on ff I get the following errors:
TypeError: can't access dead object
ReferenceError: event is not defined
A link to a non working example is below (We don't want to give access to our working dashboard for commercial reasons) You can see the error when click on the "dropdown" menu item. As far as I can tell, all the assets are loading correctly.
http://hunchbuzz.com/acme/index.html
Any help would be appreciated.

Well, take your bugs one by one. Did you turn on the JavaScript Debugger when you tested your page in IE? In the F12 developer tools, select Script and then Start Debugging.
First there were a couple of errors in jquery.sparkline.min.js which I ignored for the moment. Then I tried clicking your 'dropdown' link and got this error:
SCRIPT438: Object doesn't support property or method 'preventDefault'
custom.js, line 3 character 1193
The highlighted code is (reformatted for readability):
$('.dropmenu').click( function(){
event.preventDefault();
// ...
});
Do you see the problem? What is event? The code should be:
$('.dropmenu').click( function( event ){
event.preventDefault();
// ...
});
The fact that it worked in any browser at all is probably due to the global event variable that some browsers create for compatibility with very old code.
Now back to the sparkline problems. I see it hitting two errors in jquery.sparkline.min.js, but with the minimized code the problem isn't jumping out at me the way the other one did.
When you're debugging, it would help a lot to load the non-minified versions of jquery.sparkline.js, custom.js, jQuery, etc. Then you'll have readable code to look at in the debugger, which should make it easier to spot these problems.

Firebug doesn't load JavaScript files or stop execution on breakpoints

I'm new to Firebug and having a lot of trouble.
JavaScript files usually show up empty, or load partially (some of the time)
Lines are not available to set breakpoints on frequently (line numbers are greyed out)
When I do set breakpoints, script execution often does not stop on them
I'm using Firebug 1.3.3 and Firefox 3.0.11. I have disabled all other Add-ons. I'm loading Javascript from localhost. Sometimes closing the window and re-opening the page I was on clears things up, but that never lasts for more than a couple page loads.
I'm working on learning jQuery, which obviously has a huge library, but I imagine many other people use Firebug for the same, so that shouldn't be a problem. Also, most of the time (but not always), Firefox loads and executes the JavaScript no problem; just Firebug can't see it.
Due diligence:
These discussions seem to cover the same problem, but have no answers:
"Firebug not showing Javscript errors" - http ://groups.google.com/group/firebug/browse_thread/thread/443848cd11be48e1?pli=1
"firebug does not always load javascript" - http ://code.google.com/p/fbug/issues/detail?id=1644&q=empty%20javascript&colspec=ID%20Type%20Status%20Owner%20Test%20Summary
(Sorry I'm new, and not allowed to hyperlink those)

A couple suggestions. Make sure that you have the console, net, and script panels of Firebug all turned on.
You should see in the net panel what js files have downloaded. In the console panel, you should be able to type console.log(jQuery) and get back function().
This should confirm that jQuery is actually loaded and running.
Then go to your script panel, and you should see four options across the top. Inspect, Edit, Static, and then a drop down list of your scripts. That's the one you want. Select the script that you want to debug.
Based on your question, you probably know some of this already, but confirm that all of that is working first.
When you don't see jQuery in the scripts list, can you do console.log(jQuery)?
PS. It's not a matter of size. I routinely load js files that are 10x the size of jQuery.
Edit: A few more suggestions:
1) Reduce to simplest case and add back. Remove all your scripts other than jQuery and then add your other scripts incrementally. Is there one that consistently breaks it.
2) Put try / catch statements around suspicious code blocks. I've often found that FB stops reporting errors after an uncaught exception has been thrown.
try {
// your code here
} catch (e) {
console.log(e)
}
3) Setup another FF profile to test if you get the same problem.