Develop Reference

How to fix IE9 issue with f12?

I need to be able to run my angularjs app in IE9 but this currently only works with devtools open(F12). From what I am aware is that console.log can cause this but this is stripped out in the app , I am using gulp.stripDebug. What can be another cause or is this a IE9 bug?
Is there a way of debugging/tracking js code without having to open the devtools at the same time?

If console is not defined, calling .log() will break javascript execution. An easy way to overcome this is to check if it exists and create a dummy object if it does not. If you load the page without developer console and open the console afterwards, logging won't work though.
if (!window.console) {
window.console = { log : function () {} };
}
You could make the function alert the debug message, but that would probably annoy more than it would benefit you ;)

Log JavaScript console into a log file with Firefox

We have a web application which runs in a kiosk mode Firefox, using the RKiosk extension to achieve this. We suspect that we have a very rare error in the system which yields in a JavaScript error. However because we can't access the JavaScript console we can't examine the log.
I'm searching for an option to make Firefox log all JavaScript console messages into a file regardless of the tab and page opened. I can't seem to find any extension for this. I'm already using log4javascript which sends errors back to the server, but it seems that our application crashes in a way that it skips the logging altogether.

Writing to a file sounds like a tedious task to me. It requires privileges that browser code doesn't normally have and you'd have to negotiate with an add-on you'd have to write in order to access file I/O.
From what I understand your issue is
I'd like to make Firefox log all errors
There are several approaches we can do to tackle this
First approach - log everything to localStorage too:
Now, rather than writing to an actual file, you can write to localStorage or IndexedDB instead.
localStorage["myApplog"] = localStorage["myApplog"] || "";
var oldLog = console.log;
console.log = function(){
oldLog.apply(console,arguments); // use the old console log
var message = "\n "+(new Date).toISOString() + " :: "+
Array.prototype.join.call(arguments," , "); // the arguments
localStorage["myApplog"] += message;
}
This is rather dirty and rather slow, but it should get the job done and you can access the log later in local storage. LocalStorage has a ~5MB limit if I recall correctly which I think is enough if you don't go crazy with logging. You can also run it selectively.
Second approach - log only errors
This is similar to what Pumbaa80 suggested. You can simply override window.onerror and only log errors.
// put an empty string in loggedWinErrors first
var oldError = window.onerror || function(){};
window.onerror = function(err,url,lineNumber){
oldError.call(this,err,url,lineNumber);
var err ="\n Error: (file: " + url+", error: "+err+", lineNumber: "+lineNumber+")");
localStorage["loggedWinErrors"] += err;
}
Third and drastic approach - use a VM.
This is the most powerful version, but it provides the most problematic user experience. You run the kiosk in a virtual machine, you detect an uncaught exception - when you do you freeze the machine and save its state, and run a backup VM instead. I've only had to do this when tackling the most fearsome errors and it's not pretty. Unless you really want the whole captured state - don't do this.
Really, do the extension before this - this is tedious but it gets very solid results.
In conclusion, I think the first approach or even just the second one are more than enough for what you need. localStorage is an abstracted storage that web pages get for saving state without security issues. If that's not big enough we can talk about an IndexedDB solution.
It all really depends on the use case you have.

You can use XULRunner...a Mozilla runtime environment for XUL applications. It uses Gecko like Firefox and:
You can access the file system or using the SQLite database to store logs.
You can render your kiosk in fullscreen mode without using extensions.

Have you tried jserrorcollector? We are using it and it works fine (only in Firefox). It's only for Java.
// Initialize
FirefoxProfile ffProfile = null;
ffProfile = new FirefoxProfile();
JavaScriptError.addExtension(ffProfile);
// Get the errors
List<JavaScriptError> jsErrors = JavaScriptError.readErrors(webDriver);
More information: https://github.com/mguillem/JSErrorCollector

Have you considered remote logging?
I commonly assign window.onerror to do send a request to a webserver storing the details of the error remotely. You could do the same with console.log if you preferred.

Try the following console export. It is a plugin for Firebug of Firefox. It's quite handy.
http://www.softwareishard.com/blog/consoleexport/

If you are able/willing to switch from Firefox to Chrome or Opera you would be able to use the Sandboxed Filesystem API to write a local file. See:
http://www.html5rocks.com/en/tutorials/file/filesystem/
http://caniuse.com/filesystem
Start in kiosk mode using chrome.exe --kiosk <url>
You would then want to disable Alt-F4 and Ctrl-Alt-Del which on Windows can be done with several third-party tools like Auto Hotkey (Disable Ctrl-Alt-Del Script).

You could use a remote logging script like Qbaka. It catches every JS error and sends it to the Qbaka server. There you can login and see all JS errors. Qbaka stores the exact error message, the script, line number, stack trace and the used browser for each error message.

Firefox Web Console Disabled?

How come I get this message from Firefox Web Console
The Web Console logging API (console.log, console.info, console.warn, console.error) has been disabled by a script on this page
The same webpage can print messages in Chrome Console but not Firefox. I opened the same webpage in another computers' Firefox (don't know what version) Web Console can print messages. My Firefox version is the latest, 8.0.

This happens when the page itself defines a global variable called console, for example. If the page is browser-sniffing to decide whether to define it, the behavior could differ in different browsers.
In the case of Firefox it also happens when Firebug is installed and its console is enabled, since that overrides the default window.console.

I had the same exact error message, and once I removed firebug, it went away.
I'm not saying you should remove firebug, I love firebug, but that is most probably the source of the error for you as well. One more note, the error was still there even if firebug was turned off (disabled) for that particular page.

Here is a JavaScript workaround I used to restore console API after it was set to empty function by a script on the page (works in Firefox 46, tested in Firebug and in greasemonkey script):
function restoreConsole() {
var i = document.createElement('iframe');
i.style.display = 'none';
document.body.appendChild(i);
window.console = i.contentWindow.console;
i.parentNode.removeChild(i);
}
More info and credentials: Restoring console.log()

Right click over firebug console tab and uncheck "enabled" option (the first one).

console.log doesn't print the message to the firebug console?

I am facing this weird problem. The WebApp I'm debugging right now, is invoking the javascript console.log/console.log/error/debug/etc., the Firebug console however, doesn't print them at all.
This application uses Dojo/Dijit toolkit. Not sure if there is anything special about it
It doesn't appear to be a problem with the Browser, I tried another simple web-page with a console.debug call, and the message appears on the console as expected.
Please advise about what should I look for. I have also tried Chrome/IE.
Thanks in Advance/

console is not write protected, it can be replaced with anything. You could try
alert(console.log.toString());
to find out what console.log really is
Edit:
A better method would be
var originalConsole = console;
// now include your library
// ...
originalConsole.log(console.log);
In Firebug, clicking on the function takes you directly to its definition.

did you try window.console.log()? Maybe you are not in window scope

In case like this do not forget to check if "Logging" is enabled or active at your console of browser.

Just to update this question -:
Ensure that firebug is Enabled - - > On for all Web Pages.
Reload the application.
Then in the firebug panel - - > Console - - > All.
All the console.log messages will appear.

Any way to debug javascript on Android devices?

Are there any means of debugging JS code, or at least get errors, on Android devices?

This helps:
http://www.nanaze.com/2009/01/debugging-javascript-on-android.html
There are Firebug like methods (e.g. console.debug, console.info) that will log to logcat, so you can snoop there.
In my testing I'm getting logs to W/browser, not D/WebCore, but the info is being logged there.

Error messages should show up in the log with the WebCore tag.
Answered previously.

As it says http://www.technomancy.org/android/javascript-debugging/ you use can use console.log(...) to log messages, and then install a log viewer to view the logs if you filter based on browser (at least on Android 2.2)