How to debug JavaScript on Firefox without Firebug?

How to debug JavaScript on Firefox without Firebug? - javascript

How can I debug JavaScript on Firefox without Firebug? I found that not all JavaScript source files, which were being loaded initially, are shown on the left panel of the debug tool. Any chances for me to dig them out?
More background: We have to do remote debugging on a customer's machine and this machine has everything blocked except our remote connection. Also, the customer is refused to install Firebug.

Firefox has built-in devtools, which can be opened via F12 when Firebug is not installed, Ctrl+Shift+I or via Firefox menu > Developer > Toggle Tools:
Their Debugger panel can be opened via Ctrl+Shift+S. It works similar to the one in Firebug.
Note that to be able to debug dynamically evaluated scripts (e.g. scripts executed via eval()), the scripts need to include a //# sourceURL comment.
Note: If they don't include that comment, they won't be shown within the Sources side panel!
See the following example for such a //# sourceURL comment (taken from the linked MDN website):
var button = document.getElementById("clickme");
button.addEventListener("click", evalFoo, false);
var script = "function foo() {" +
" console.log('called foo');" +
"}" +
"foo();//# sourceURL=my-foo.js";
function evalFoo() {
eval(script);
}
The Firefox DevTools have a feature called 'black boxing' to detect JS libraries and automatically exclude them from debugging, because people normally only want to debug their sources but not third party sources. Those scripts will still be listed within the Sources side panel and blacklisting can be turned off for them manually.

Related

warings:: DevTools failed to load source map: Could not load content for chrome-extension://gighmmpiobklfepjocnamgkkbiglidom/browser-polyfill.js.map [duplicate]

I'm trying to display an image selected from the local machine and I need the location of that image for a JavaScript function. But I'm unable to get the location.
To get the image location, I tried using console.log, but nothing returns.
console.log(document.getElementById("uploadPreview"));
Here's the HTML code:
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<div align="center" style="padding-top: 50px">
<img align="center" id="uploadPreview" style="width: 100px; height: 100px;" />
</div>
<div align="center" style="padding-left: 30px">
<input id="uploadImage" type="file" name="myPhoto" onchange="PreviewImage();" />
</div>
<script type="text/javascript">
function PreviewImage() {
var oFReader = new FileReader();
oFReader.readAsDataURL(document.getElementById("uploadImage").files[0]);
oFReader.onload = function (oFREvent) {
document.getElementById("uploadPreview").src = oFREvent.target.result;
console.log(document.getElementById("uploadPreview").src);
};
}
</script>
</body>
</html>
Console Output:
Here's the warning:
DevTools failed to load SourceMap: Could not load content for
chrome-extension://alplpnakfeabeiebipdmaenpmbgknjce/include.preload.js.map:
HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME

That's because Chrome added support for source maps.
Go to the developer tools (F12 in the browser), then select the three dots in the upper right corner, and go to Settings.
Then, look for Sources, and disable the options:
"Enable JavaScript source maps"
"Enable CSS source maps"
If you do that, that would get rid of the warnings. It has nothing to do with your code. Check the developer tools in other pages and you will see the same warning.

Go to Developer tools → Settings → Console → tick "Selected context only". The warnings will be hidden. You can see them again by unticking the same box.
The "Selected context only" means only the top, iframe, worker and extension contexts. Which is all that you'll need, the vast majority of the time.

Fixing "SourceMap" error messages in the Development Tools Console caused by Chrome extensions:
Examples caused by McAfee extensions:
DevTools failed to load SourceMap: Could not load content for chrome-extension://klekeajafkkpokaofllcadenjdckhinm/sourceMap/content.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load SourceMap: Could not load content for chrome-extension://fheoggkfdfchfphceeifdbepaooicaho/sourceMap/chrome/content.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load SourceMap: Could not load content for chrome-extension://fheoggkfdfchfphceeifdbepaooicaho/sourceMap/chrome/iframe_handler.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
If you are developing, then you need "Enable JavaScript source maps" and "Enable CSS source maps" checked to be able see your source code in Chrome Developer Tools. Unchecking those takes away your ability to debug your source code. It is like turning off the fire alarm instead of putting out the fire. You do not want to do that.
Instead you want to find the extensions that are causing the messages and turn them off. Here is how you do that:
Go to the three dots in the upper right hand corner of Chrome.
Go to "More Tools" and click on "Extensions".
Do this for one extension at a time until no more "SourceMap" errors are in the console:
Turn off the extension by sliding the switch to the left.
Reload the page that you were using the Development Tools on.
Check if any of the "SourceMap" error messages disappeared.
If any did, then that extension was causing those messages.
Otherwise, that extension can be turned back on.
After determining which extensions caused the issue either:
If you need it, then contact the maker to have them fix the issue.
Otherwise, remove the extension.

I stumbled upon this Stack Overflow question after discovering loads of source map errors in the console for the Edge browser. (I think I had disabled the warnings in the Chrome browser long ago.)
For me it meant first realising what a source map is; please refer to Macro Mazzon's answer to understand this. Since it's a good idea, it was just a case of finding out how to turn them on.
It's as simple as adding this line in your webpack.config.js file -
module.exports = {
devtool: "source-map",
}
Now that Edge could detect a source map, the errors disappeared.
Apologies if this answer insults anybody's intelligence, but maybe somebody reading this will be as clueless about source maps as I was.

The include.prepload.js file will have a line like below, probably as the last line:
//# sourceMappingURL=include.prepload.js.map
Delete it and the error will go away.

For me, the problem was caused not by the application in development itself, but by the Chrome extension React Developer Tool. I solved it partially by right-clicking the extension icon in the toolbar, clicking "Manage extension" and then enabling "Allow access to files URLs." But this measure fixed just some of the alerts.
I found issues in the React repository that suggests the cause is a bug in their extension and is planned to be corrected soon - see issues 20091 and 20075.
You can confirm is extension-related by accessing your application in an anonymous tab without any extension enabled.

Chrome has changed the UI in 2022, so this is a new version of the most upvoted reply.
Open the dev tools (hit F12 or Option + Command + J)
Select the gear at the top. There are two gears in that area, so be sure to select the one at the top, top.
Locate the Sources section
Deselect "Enable JavaScript source maps"
Check to see if it worked!

Right: it has nothing to do with your code. I've found two valid solutions to this warning (not just disabling it). To better understand what a source map is, I suggest you check out this answer, where it explains how it's something that helps you debug:
The .map files are for JavaScript and CSS (and now TypeScript too) files that have been minified. They are called SourceMaps. When you minify a file, like the angular.js file, it takes thousands of lines of pretty code and turns it into only a few lines of ugly code. Hopefully, when you are shipping your code to production, you are using the minified code instead of the full, unminified version. When your app is in production, and has an error, the sourcemap will help take your ugly file, and will allow you to see the original version of the code. If you didn't have the sourcemap, then any error would seem cryptic at best.
First solution: apparently, Mr Heelis was the closest one: you should add the .map file and there are some tools that help you with this problem (Grunt, Gulp and Google closure for example, quoting the answer). Otherwise you can download the .map file from official sites like Bootstrap, jQuery, font-awesome, preload and so on... (maybe installing things like popper or swiper by the npm command in a random folder and copying just the .map file in your JavaScript/CSS destination folder)
Second solution (the one I used): add the source files using a CDN (content delivery network). (Here are all the advantages of using a CDN). Using content delivery network (CDN) you can simply add the CDN link, instead of the path to your folder. You can find CNDs on official websites (Bootstrap, jquery, popper, etc.) or you can easily search on some websites like Cloudflare, cdnjs, etc.

Extensions without enough permissions on Chrome can cause these warnings, for example for React developer tools. Check if the following procedure solves your problem:
Right click on the extension icon.
Or
Go to extensions.
Click the three-dot in the row of React developer tool.
Then choose "This can read and write site data".
You should see three options in the list. Pick one that is strict enough based on how much you trust the extension and also satisfies the extension's needs.

I appreciate this is part of your extensions, but I see this message in all sorts of places these days, and I hate it: how I fixed it (this fix seems to massively speed up the browser too) was by adding a dead file
physically create the file it wants it/where it wants it, as a blank file (for example, "popper.min.js.map")
put this in the blank file
{
"version": 1,
"mappings": "",
"sources": [],
"names": [],
"file": "popper.min.js"
}
make sure that "file": "*******" in the content of the blank file matches the name of your file ******.map (minus the word ".map")
(I suspect you could physically add this dead file method to the addon yourself.)

I do not think the warnings you have received are related. I had the same warnings which turned out to be the Chrome extension React Dev Tools. I removed the extension and the errors were gone.

You have just missing files.
Go to the website https://www.cdnpkg.com/.
Download what you need and copy it to the right folder.

For me, the warnings were caused by the Selenium IDE Chrome extension. These warnings appeared in the Console on every page load:
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/atoms.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/polyfills.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/escape.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/playback.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/record.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
Since Selenium IDE was already set to be able to read site data on all sites, I uninstalled it. (I read in another comment here that you might try enabling more permissions for an extension instead of removing it.) In my case, removing Selenium IDE (Chrome extension) got rid of the warnings.

It is also possible to add the file that is missing, aside with other .js libraries in the same folder (no need to reference the .map in the .html file, <script> tag).
I had the same error, when trying to code in Backbone.js.
The problematic file was backbone-min.js, and the line that created the error was sourceMappingURL=backbone-min.map.
After downloading the missing file (the link comes from here), the error disappeared.

I had the same problem. I tried to disable the extensions one by one to check it, and finally realized I had Adblock enabled, which was causing this issue. To remove that error I followed the step below,
Three dots (top right corner).
Click More tools --> extensions.
Disable the Adblock.
Reload the page.
And it should work now.

DevTools failed to load source map: Could not load content for chrome-extension://cfhdojbkjhnklbpkdaibdccddilifddb/browser-polyfill.js.map: System error: net::ERR_FILE_NOT_FOUND
Disable the Chrome extension "Adblock Plus - free ad blocker". https://chrome.google.com/webstore/detail/adblock-plus-free-ad-bloc/cfhdojbkjhnklbpkdaibdccddilifddb
Lately this error is caused by the extension.

Problems with Debugging and Sourcemaps in Web Browsers
Hope this clarifies the technicals behind the problem...knowing how things works helps some :)
This browser error means it has some compiled version of your JavaScript in a sourcemap intermediate file it or some 3rd party created that is now needed when debugging that same script in "devtools" in your web browser.
This can happen if your script fails (or in your case trying to get an image source hidden in the sourcemap code that created the script) but whose script error is tied to some JavaScript that got created from an original sourcemap file that now cannot be found to debug that same error. So it's an error about an error, a missing debugging file creating a new error. (crazy, huh?)
This error is likely coming from an extension in the web browser and is reporting it has generated a script error it has recorded in the console.log window of devtools (press F12 in the browser). The error is likely from the extension (not your code) saying it has some code that contains an address to a sourcemap file it cannot access, has a bad URI/URL address, is blocked, or that is missing.
The browser only needs this sourcemap file if a developer using devtools will need to debug the original script again.
A sourcemap, by the way, is a file that translates or transpiles code from one language to another language. Often this is a file that the browser uses to translate this source code into a child script like JavaScript/ECMAScript, or when it needs to do the opposite and recreate the source file from the child script. In most cases this file is not needed at all as a 3rd party software program has already compiled or transpiled the source code into the child script for the browser. For example, developers who like TypeScript use it to create JavaScript. This source code gets transpiled into JavaScript so the browser script engine can run it. The URI/URL to this sourcemap file is usually at the top of the javaScript or application compiled code file in a format like //#....
When this intermediary transpile file is missing or blocked for security reasons in a web browser, the application will usually not care unless it needs the source file for debugging the child script using this source file. In that case it will complain when it feels it needs this file and cannot find it, as it uses it to recreate the source file for the code running in the browser when debugging the script in order to allow a developer to debug the original source code. When it cannot find it, it means that any developer trying to debug it will not be able to do so, and is stuck with the compiled code only. So it is safe to turn off these errors in the various ways mentioned in this post. It should not affect your own scripts if it is connected to an extension. Even if it is related to your own scripts, it is still unlikely you need it unless you plan to run debugging from devtools.

In my case, it was JSON Viewer extension that was blocking the source map files from being loaded

In my case i made silly mistake by adding bootstrap.min.js instead of bootstrap.bundel.js :)

You need to open Chrome in developer mode: select More tools, then Extensions and select Developer mode

Chrome browser console warning [https://s3.amazonaws.com/onelogin-sourcemaps/extensions/chrome/production~] [duplicate]

I'm trying to display an image selected from the local machine and I need the location of that image for a JavaScript function. But I'm unable to get the location.
To get the image location, I tried using console.log, but nothing returns.
console.log(document.getElementById("uploadPreview"));
Here's the HTML code:
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<div align="center" style="padding-top: 50px">
<img align="center" id="uploadPreview" style="width: 100px; height: 100px;" />
</div>
<div align="center" style="padding-left: 30px">
<input id="uploadImage" type="file" name="myPhoto" onchange="PreviewImage();" />
</div>
<script type="text/javascript">
function PreviewImage() {
var oFReader = new FileReader();
oFReader.readAsDataURL(document.getElementById("uploadImage").files[0]);
oFReader.onload = function (oFREvent) {
document.getElementById("uploadPreview").src = oFREvent.target.result;
console.log(document.getElementById("uploadPreview").src);
};
}
</script>
</body>
</html>
Console Output:
Here's the warning:
DevTools failed to load SourceMap: Could not load content for
chrome-extension://alplpnakfeabeiebipdmaenpmbgknjce/include.preload.js.map:
HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME

That's because Chrome added support for source maps.
Go to the developer tools (F12 in the browser), then select the three dots in the upper right corner, and go to Settings.
Then, look for Sources, and disable the options:
"Enable JavaScript source maps"
"Enable CSS source maps"
If you do that, that would get rid of the warnings. It has nothing to do with your code. Check the developer tools in other pages and you will see the same warning.

Go to Developer tools → Settings → Console → tick "Selected context only". The warnings will be hidden. You can see them again by unticking the same box.
The "Selected context only" means only the top, iframe, worker and extension contexts. Which is all that you'll need, the vast majority of the time.

I stumbled upon this Stack Overflow question after discovering loads of source map errors in the console for the Edge browser. (I think I had disabled the warnings in the Chrome browser long ago.)
For me it meant first realising what a source map is; please refer to Macro Mazzon's answer to understand this. Since it's a good idea, it was just a case of finding out how to turn them on.
It's as simple as adding this line in your webpack.config.js file -
module.exports = {
devtool: "source-map",
}
Now that Edge could detect a source map, the errors disappeared.
Apologies if this answer insults anybody's intelligence, but maybe somebody reading this will be as clueless about source maps as I was.

The include.prepload.js file will have a line like below, probably as the last line:
//# sourceMappingURL=include.prepload.js.map
Delete it and the error will go away.

For me, the problem was caused not by the application in development itself, but by the Chrome extension React Developer Tool. I solved it partially by right-clicking the extension icon in the toolbar, clicking "Manage extension" and then enabling "Allow access to files URLs." But this measure fixed just some of the alerts.
I found issues in the React repository that suggests the cause is a bug in their extension and is planned to be corrected soon - see issues 20091 and 20075.
You can confirm is extension-related by accessing your application in an anonymous tab without any extension enabled.

Chrome has changed the UI in 2022, so this is a new version of the most upvoted reply.
Open the dev tools (hit F12 or Option + Command + J)
Select the gear at the top. There are two gears in that area, so be sure to select the one at the top, top.
Locate the Sources section
Deselect "Enable JavaScript source maps"
Check to see if it worked!

Extensions without enough permissions on Chrome can cause these warnings, for example for React developer tools. Check if the following procedure solves your problem:
Right click on the extension icon.
Or
Go to extensions.
Click the three-dot in the row of React developer tool.
Then choose "This can read and write site data".
You should see three options in the list. Pick one that is strict enough based on how much you trust the extension and also satisfies the extension's needs.

I appreciate this is part of your extensions, but I see this message in all sorts of places these days, and I hate it: how I fixed it (this fix seems to massively speed up the browser too) was by adding a dead file
physically create the file it wants it/where it wants it, as a blank file (for example, "popper.min.js.map")
put this in the blank file
{
"version": 1,
"mappings": "",
"sources": [],
"names": [],
"file": "popper.min.js"
}
make sure that "file": "*******" in the content of the blank file matches the name of your file ******.map (minus the word ".map")
(I suspect you could physically add this dead file method to the addon yourself.)

I do not think the warnings you have received are related. I had the same warnings which turned out to be the Chrome extension React Dev Tools. I removed the extension and the errors were gone.

You have just missing files.
Go to the website https://www.cdnpkg.com/.
Download what you need and copy it to the right folder.

For me, the warnings were caused by the Selenium IDE Chrome extension. These warnings appeared in the Console on every page load:
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/atoms.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/polyfills.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/escape.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/playback.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
DevTools failed to load source map: Could not load content for chrome-extension://mooikfkahbdckldjjndioackbalphokd/assets/record.js.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME
Since Selenium IDE was already set to be able to read site data on all sites, I uninstalled it. (I read in another comment here that you might try enabling more permissions for an extension instead of removing it.) In my case, removing Selenium IDE (Chrome extension) got rid of the warnings.

It is also possible to add the file that is missing, aside with other .js libraries in the same folder (no need to reference the .map in the .html file, <script> tag).
I had the same error, when trying to code in Backbone.js.
The problematic file was backbone-min.js, and the line that created the error was sourceMappingURL=backbone-min.map.
After downloading the missing file (the link comes from here), the error disappeared.

I had the same problem. I tried to disable the extensions one by one to check it, and finally realized I had Adblock enabled, which was causing this issue. To remove that error I followed the step below,
Three dots (top right corner).
Click More tools --> extensions.
Disable the Adblock.
Reload the page.
And it should work now.

DevTools failed to load source map: Could not load content for chrome-extension://cfhdojbkjhnklbpkdaibdccddilifddb/browser-polyfill.js.map: System error: net::ERR_FILE_NOT_FOUND
Disable the Chrome extension "Adblock Plus - free ad blocker". https://chrome.google.com/webstore/detail/adblock-plus-free-ad-bloc/cfhdojbkjhnklbpkdaibdccddilifddb
Lately this error is caused by the extension.

In my case, it was JSON Viewer extension that was blocking the source map files from being loaded

In my case i made silly mistake by adding bootstrap.min.js instead of bootstrap.bundel.js :)

You need to open Chrome in developer mode: select More tools, then Extensions and select Developer mode

Eel application: download files in Browser

i´m currently using JavaScript and HTML with eel and Python. But unfortunately when I am trying to create a file in the Chrome-Browser window (as an download) over JS i can only download it once.
The second "download" is not working.
-> Chrome just shows this (as it does when it downloads something) but then nothing happens:
When I am using Edge browser or only JS without eel it works perfectly fine!
My JS function that creates the download: (string is a json string that is generated earlier).
var jsonLink = document.getElementById("jsonLink");
jsonLink.download = "exportedToJson.json";
jsonLink.href = "data:application/json;charset=utf-8," + encodeURIComponent(string);

Ok I found a solution:
My chrome browser was blocking more than one download from "localhost:8000". So I had to go to settings and allow more than one download.
Maybe this helps someone :)
Why did I not find this earlier:
When I started my Python script, it calls:
eel.start('index.html', mode='chrome', port=8000) #starting chrome
Which does open a new Chrome Tab without the Tabbar (so i did neither see the tabs nor my favorite sites). Therefore I did not get a notification when chrome stated that download is blocked.
But after starting the eel-local webserver and open localhost:8000 in my normal chrome window, I did get a notification and I was able to allow the downloads.
-> afterwards it also worked in this eel-chrome window.

How to make a 'protocol' of my own and a Desktop application to use it for a Browser? [duplicate]

How do i set up a custom protocol handler in chrome? Something like:
myprotocol://testfile
I would need this to send a request to http://example.com?query=testfile, then send the httpresponse to my extension.

The following method registers an application to a URI Scheme. So, you can use mycustproto: in your HTML code to trigger a local application. It works on a Google Chrome Version 51.0.2704.79 m (64-bit).
I mainly used this method for printing document silently without the print dialog popping up. The result is pretty good and is a seamless solution to integrate the external application with the browser.
HTML code (simple):
Click Me
HTML code (alternative):
<input id="DealerName" />
<button id="PrintBtn"></button>
$('#PrintBtn').on('click', function(event){
event.preventDefault();
window.location.href = 'mycustproto:dealer ' + $('#DealerName').val();
});
URI Scheme will look like this:
You can create the URI Scheme manually in registry, or run the "mycustproto.reg" file (see below).
HKEY_CURRENT_USER\Software\Classes
mycustproto
(Default) = "URL:MyCustProto Protocol"
URL Protocol = ""
DefaultIcon
(Default) = "myprogram.exe,1"
shell
open
command
(Default) = "C:\Program Files\MyProgram\myprogram.exe" "%1"
mycustproto.reg example:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Classes\mycustproto]
"URL Protocol"="\"\""
#="\"URL:MyCustProto Protocol\""
[HKEY_CURRENT_USER\Software\Classes\mycustproto\DefaultIcon]
#="\"mycustproto.exe,1\""
[HKEY_CURRENT_USER\Software\Classes\mycustproto\shell]
[HKEY_CURRENT_USER\Software\Classes\mycustproto\shell\open]
[HKEY_CURRENT_USER\Software\Classes\mycustproto\shell\open\command]
#="\"C:\\Program Files\\MyProgram\\myprogram.exe\" \"%1\""
C# console application - myprogram.exe:
using System;
using System.Collections.Generic;
using System.Text;
namespace myprogram
{
class Program
{
static string ProcessInput(string s)
{
// TODO Verify and validate the input
// string as appropriate for your application.
return s;
}
static void Main(string[] args)
{
Console.WriteLine("Raw command-line: \n\t" + Environment.CommandLine);
Console.WriteLine("\n\nArguments:\n");
foreach (string s in args)
{
Console.WriteLine("\t" + ProcessInput(s));
}
Console.WriteLine("\nPress any key to continue...");
Console.ReadKey();
}
}
}
Try to run the program first to make sure the program has been placed in the correct path:
cmd> "C:\Program Files\MyProgram\myprogram.exe" "mycustproto:Hello World"
Click the link on your HTML page:
You will see a warning window popup for the first time.
To reset the external protocol handler setting in Chrome:
If you have ever accepted the custom protocol in Chrome and would like to reset the setting, do this (currently, there is no UI in Chrome to change the setting):
Edit "Local State" this file under this path:
C:\Users\Username\AppData\Local\Google\Chrome\User Data\
or Simply go to:
%USERPROFILE%\AppData\Local\Google\Chrome\User Data\
Then, search for this string: protocol_handler
You will see the custom protocol from there.
Note: Please close your Google Chrome before editing the file. Otherwise, the change you have made will be overwritten by Chrome.
Reference:
https://msdn.microsoft.com/en-us/library/aa767914(v=vs.85).aspx

Chrome 13 now supports the navigator.registerProtocolHandler API. For example,
navigator.registerProtocolHandler(
'web+custom', 'http://example.com/rph?q=%s', 'My App');
Note that your protocol name has to start with web+, with a few exceptions for common ones (like mailto, etc). For more details, see: http://updates.html5rocks.com/2011/06/Registering-a-custom-protocol-handler

This question is old now, but there's been a recent update to Chrome (at least where packaged apps are concerned)...
http://developer.chrome.com/apps/manifest/url_handlers
and
https://github.com/GoogleChrome/chrome-extensions-samples/blob/e716678b67fd30a5876a552b9665e9f847d6d84b/apps/samples/url-handler/README.md
It allows you to register a handler for a URL (as long as you own it). Sadly no myprotocol:// but at least you can do http://myprotocol.mysite.com and can create a webpage there that points people to the app in the app store.

This is how I did it. Your app would need to install a few reg keys on installation, then in any browser you can just link to foo:\anythingHere.txt and it will open your app and pass it that value.
This is not my code, just something I found on the web when searching the same question. Just change all "foo" in the text below to the protocol name you want and change the path to your exe as well.
(put this in to a text file as save as foo.reg on your desktop, then double click it to install the keys)
-----Below this line goes into the .reg file (NOT including this line)------
REGEDIT4
[HKEY_CLASSES_ROOT\foo]
#="URL:foo Protocol"
"URL Protocol"=""
[HKEY_CLASSES_ROOT\foo\shell]
[HKEY_CLASSES_ROOT\foo\shell\open]
[HKEY_CLASSES_ROOT\foo\shell\open\command]
#="\"C:\\Program Files (x86)\\Notepad++\\notepad++.exe\" \"%1\""

Not sure whether this is the right place for my answer, but as I found very few helpful threads and this was one of them, I am posting my solution here.
Problem: I wanted Linux Mint 19.2 Cinnamon to open Evolution when clicking on mailto links in Chromium. Gmail was registered as default handler in chrome://settings/handlers and I could not choose any other handler.
Solution:
Use the xdg-settings in the console
xdg-settings set default-url-scheme-handler mailto org.gnome.Evolution.desktop
Solution was found here https://alt.os.linux.ubuntu.narkive.com/U3Gy7inF/kubuntu-mailto-links-in-chrome-doesn-t-open-evolution and adapted for my case.

I've found the solution by Jun Hsieh and MuffinMan generally works when it comes to clicking links on pages in Chrome or pasting into the URL bar, but it doesn't seem to work in a specific case of passing the string on the command line.
For example, both of the following commands open a blank Chrome window which then does nothing.
"c:\Program Files (x86)\Google\Chrome\Application\chrome.exe" "foo://C:/test.txt"
"c:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --new-window "foo://C:/test.txt"
For comparison, feeding Chrome an http or https URL with either of these commands causes the web page to be opened.
This became apparent because one of our customers reported that clicking links for our product from a PDF being displayed within Adobe Reader fails to invoke our product when Chrome is the default browser. (It works fine with MSIE and Firefox as default, but not when either Chrome or Edge are default.)
I'm guessing that instead of just telling Windows to invoke the URL and letting Windows figure things out, the Adobe product is finding the default browser, which is Chrome in this case, and then passing the URL on the command line.
I'd be interested if anyone knows of Chrome security or other settings which might be relevant here so that Chrome will fully handle a protocol handler, even if it's provided via the command line. I've been looking but so far haven't found anything.
I've been testing this against Chrome 88.0.4324.182.

open
C:\Users\<Username>\AppData\Local\Google\Chrome\User Data\Default
open Preferences then search for excluded_schemes you will find it in 'protocol_handler' delete this excluded scheme(s) to reset chrome to open url with default application

How to debug Chrome extension example with code injection?

maybe this question is a bit noob style but I don't understand this JavaScript stuff.
My question: how do I debug injected code of the following
chrome extension example?
The file popup.js executes send_links.js (this is the injected file, if I understand this correct). I would like to debug send_links.js. I cannot set any breakpoint because I cannot see send_links.js in the Developer Tools of Chrome. I tried the command "debugger;" in send_links.js but it does not work. "console.log("blah");" commands are ignored too.
Thank you!

The debugger keyword will work if you open the Developer Tools for the current tab before you press the action button.
Also, if you want the script to display with its name, add the following line anywhere in send_links.js:
//# sourceURL=send_links.js
Then the script will show up in the 'Content Scripts' tab of the Developer Tools of the current tab. There you can set breakpoints and such. But you need always to open the Developer Tools for the tab before pressing the button for this to work.

All Injected Files or Content Scripts are Exposed to Page Developer Tools, You can set breakpoints and all regular stuff you do on regular Java Script Pages.
(source: google.com)
All your console log(s) appear in the page they are injected.
Ex:
If i inject console.log(document.getElementsByTagName('body')[0].style); to http://www.google.co.in/, then i need to open devtools of http://www.google.co.in/ page and look in its console.
The Output appeared is similar to regular debugging result.
EDIT 1
They are exposed through chrome.tabs.executeScript() but indirectly, when you set debugger; command you can inspect code.
Demonstration
If some sample code injects
chrome.tabs.executeScript(35, {
"code": "console.log('hi');debugger;//# sourceURL=send_links.js"
});
debugger of page shows the script being injected.

I guess it's because you open the debugger tool on the tab and not on the extension. Right click on the icon of your extension and choose the Inspect popup menu item. You can find more informations on this page http://developer.chrome.com/extensions/tut_debugging.html

In this case the script is not injected until you open the popup. Once the popup window loads, it injects send_links.js, which in turn sends a message as soon as it is done getting the links. You could reverse this messaging and inject the file in the manifest:
"content_scripts": [{
"matches": ["<all_urls>"],
"js": ["send_links.js"]
}],
add an on message handler to send_links.js with support to send a response
chrome.extension.onMessage.addListener(function(message,sender,sendResponse){
[...]
sendResponse(links);
});
and then replace the onMessage handler and executeScript in the popup with a sendMessage callback
chrome.windows.getCurrent(function (currentWindow) {
chrome.tabs.query({active: true, windowId: currentWindow.id},function(tab) {
chrome.tabs.sendMessage(tab[0].id, {method: "getlinks"},function(links){
for (var index in links) {
allLinks.push(links[index]);
}
allLinks.sort();
visibleLinks = allLinks;
showLinks();
});
});
});
This arrangement will put send_links.js into every page so that you can debug it more easily. Once it is bug free you can switch back to programmatic injection because in cases like these it is more efficient.
You can find the script under Sources > Content Scripts > One of them (plfdheimenpnchlahmhicnkejgmhjhom for example).
Edited source files

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