Debugging tricks in the browser | Hacker News Reader

Debugging tricks in the browser

alan.norbauer.com

646 pointsbkudria122 comments

The debugging tools built within the browsers have come a long way in the last couple of decades. I'm a JS veteran and I'm deeply grateful to all the people putting in such efforts to make debugging code in the browser so intuitive.

Whenever I go to a different zone of development, like backend or a different language, I miss this ecosystem of debugging tools that modern browsers have by default.

Absolutely. It's not perfect but it sure beats the debugging nodejs with vscode experience a fellow developer was pushing. I would take the console over this hell.

I don’t know if you are making the implication that you could use chrome devtools to debug nodejs, so I’m making it explicitly clear you can! https://nodejs.org/en/docs/guides/debugging-getting-started

Shout outs to the people behind the Firebug extension which really kicked browser devtools into high gear.

One of things about Firebug in particular was how you had one thing like a DOM node and you could inspect it in different ways. Like the element panel or an object inspector in console. I worked hard to open this to extensions as well. And it worked for subpanels as well. Maybe I’ll offer up a patch for chrome devtools, but it’s been a while since I’ve made a PR there and it was pretty modest.

What in particular do you feel is so special about the JS debugging experience?

I've done a lot of debugging on Python, Java and C++ over the years and never felt like I was missing tools (although I certainly met plenty of people that were ignorant of their options in this regard...)

I switched to c# dev from web a couple years ago and am baffled there’s no built in way to see the network requests my app is making when debugging

Lol been waiting patiently for that for 20 years in .Net. Such an obvious gap.

Right...but how would this work tho?

Since on a more open platform like the CLR, JVM Python interpreter or bare metal (as opposed to a controlled sandbox like the browser) there are many different network traffic libraries dealing with all kinds of different ways of communicating (everything from basic HTTP to binary protocols for communicating with hardware like DNP3).

How would the standard debugger hook into all these different libraries? I'm assuming they'd have to implement hooks and maintain them too, nevermind if there's more than one debugger available to be supported.

Seems like a lot of effort compared to just using a tool like Wireshark or, for HTTP stuff, a MITM proxy...

Inspect Element

Java’s/Spring’s hot swapping is clearly inferior in my experience, but of course the main benefit comes from that it takes less than a sec to “restart a page”, while a full blown ApplicationContext init usually takes way more. Also action history of Redux is something which I’ve never seen in Java, but of course there is usually no central store like in Redux. Another easier thing is to rewrite UI on the fly at any time with zero restrictions. Compared to Groovy for example, or any other things which generate code (e.g Lombok, JSP pages), it’s way easier to debug non native code with sourcemap files, which is defacto standard in JS code generation. Also for JS you have all of the code which is running in the browser, for standalone containers like Wildfly you still need to add their code, and sometimes many other things like extensions.

Sure, those are pretty interesting points but I feel like there is a bit of difference between the environment of a browser and that of a runtime like the JVM which tends to explain why certain debugging tools exist in one environment and not the other.

For the same reason it doesn't seem that useful to bring in very environment-specific stuff like Redux.

There are website view frameworks for Java, so at the end, many times, it’s not that different. Hotswapping Thymeleaf templates for example works quite well, but the only option is refreshing, while it’s not an inherent restriction.

I had an example for Redux, because one of my pain is that there is no larger view framework for Java, which can’t access JNDI. In other words, nothing stops anybody to create something like Redux, just the view layer shouldn’t be able to access anything else except the model. Even now, it is an antipattern in my opinion to reach anything from the view which has a state and it’s not in the model. Jakarta MVC, Spring MVC, and Struts are close to this, JSF’s model is the JNDI itself, so it’s even easier to write way worse spaghetti code.

>>setTimeout(function() { debugger; }, 5000);

This is clever; after all, the only way to beat the recursive turtle stack of chrome debuggers debugging themselves is with the debugger statement.

sam.pl, of the infamous myspace Sammy worm, used debugging gotcha's to prevent visitors from de-mystifying his obfuscated html homepage.

Nice, I never knew this trick existed

This is what happens when websites prevent an open Console/DevTools side panel. They basically have a main loop running, inserting a debugger; statement in various places where they're annoying, and they do that at 30FPS / 32ms so that the DevTools become useless because there's no way to "ignore" debugger statements.

Why would this be a problem? I have a debugger. If you conveniently invoke it for me I can simply delete the offending code.

Also both Chrome and Firefox have a "disable breakpoints" button that would instantly defeat this issue.

This button was added to specifically combat that, and even then it means you can't use breakpoints at all. IMO the missing link is a way to specifically ignore a single debugger call.

TFA has instructions on adding conditional breakpoint with 'false' inside. Wouldn't this work?

Yes, this will work. Sometimes you do it to yourself and you want to ignore the line.

The debugger lets you edit the code in the VM, so if there's a debugger call you don't want you can simply delete it and continue and it'll not show up again until you refresh

Wait, that's it? This is too simple to be so good.

That's one of my most used bookmarklets: https://darekkay.com/blog/debugging-dynamic-content/

Woah, “Emulate a focused page” is a great tip, too.

You can just hit F8 and the debugger will pause at the next instruction. Works in both Chrome/FF.

Neither the bookmarklet or F8 is ideal if you want to retain focus and any other state that you may effect by going and clicking or pressing a key.

F8 first causes focus to be lost from the document body. Just tested in Chrome.

Another way to do this, if you don't want to wait 5 seconds, is just to use the "pause script execution" shortcut, which is cmd-\. It'll freeze the page on whatever line of JS was currently executing, and then you can inspect the problematic element like normal.

Browser makers: please ignore 'debugger' statements by default with an opt-in in the dev tools. Almost all legitimate uses of the debugger statement can be replaced with a simple breakpoint if you're working on the code.

`queryObjects` is notably missing. It is a crazy API which returns a list of all objects created by a particular constructor. One can for example get a list of all functions on the heap by doing `queryObjects(Function)`.

This will return even functions contained in some module that are “private”.

To be honest, this might be one they’d be justified in not wanting you to know. Crazy indeed. Chromium-only, I presume.

Haha, I agree.

They do seem to have some strange restrictions on this. E.g. When you evaluate it the function returns undefined but it also outputs the array underneath. You can right click and save it to variable.

I think the point of this so that you cannot assign the output programmatically, there has to be a person who saves it to a variable by right clicking.

I think it’ll be because for architectural reasons it can’t return a value synchronously, combined with historical ergonomic reasons. I don’t know when it was introduced, it’s possible that originally it could be synchronous and only subsequent V8 changes prevented that. Probably it landed before you could use `await` in the console, and they decided that made the ergonomics of Promising it too bad for the typical use case (though now you could write `await queryObjects(Function)` if it worked that way). All I know is that the documentation at https://developer.chrome.com/docs/devtools/console/utilities... says it returns an array of objects, which is patently false.

I can’t see any reason for preventing assigning the output programmatically.

I work on Chrome DevTools.

The reason is absolutely security concerns. You don't want to leak a function that can expose a list of all objects on the JS heap.

I don’t understand. The page can’t access these things, only dev tools, so any action to expose it would still have to be mediated by user action; and even then, what’s so bad about exposing this? Everything in it is scoped to the document, and if it can expose things you don’t want exposed, then so can getEventListeners(), right? Yet getEventListeners returns an actual value. What’s the actual security problem of being able to list all objects on the JS heap?

I can never get watched variables to work. The scoping and updating rules for it are a mystery to me. I assume only global variables can be watched, but even then it never works as I expect, so I end up just flooding the log with values when testing.

I've thought for years the console should add Data.gui [1] style UI for viewing/testing variable and settings values. You can see it action on this CodePen [2].

1. https://github.com/dataarts/dat.gui

2. https://codepen.io/russellbeattie/full/kGxaqM

I've had the same frustration. The browsers have such great debugging features, in theory, but they never seem to work reliably. I can't even get all of my breakpoints to reliably hit.

Everything seems to work ok when the code is unrolled but as soon as it gets bundled, even if not minified, it seems that a lot of debugger features get broken, at least that's been my experience.

Note: I'm not a front-end engineer and I'm probably doing something wrong.

Even though minified variables appear under the correct name in the sidebar panel in chrome I still get an error that they're undefined in a watchpoint, which is annoying.

Disable source maps in the debugger. They are likely the source of your frustration and frankly I still do not understand why they are on by default given how bad the experience is of actively debugging with them on.

Your comment made me want to scratch this long-standing itch and write-up why you should disable source maps. Check this out:

https://alan.norbauer.com/articles/disable-source-maps

I’d like to see a way to access local variables of an IIFE, without breaking into code in the IIFE’s scope. Is there some way to convince the debugger to do this?

If you're entering a breakpoint from a function called by the IIFE, you can just walk up the stack. If you're outside the IIFE entirely, I don't think it's possible The variables you're looking for may not even exist in memory, either before or after execution. Javascript doesn't have static variables like other languages do, so each time the IIFE is called, the variables inside it are thrown out. Javascript is also very much single-threaded (unless you use web workers and such, which come with limited interactivity with their parent pages) so unless you're trying to race-condition yourself with an async/await call, I don't think there's even a way to conceptually have these variables around in memory outside the IIFE scope.

You could (ab)use `var` to initialize the variable outside the IIFE scope so you can see the values produced by the last IIFE call.

IIFE: immediately-invoked function expression

https://en.m.wikipedia.org/wiki/Immediately_invoked_function...

I wrote a tool to do that about 12 years ago. You still need to modify it a bit but you get a big bang for your buck

https://github.com/kristopolous/_inject

You can basically wander around any function context at any arbitrary time and see what happened. It exploits the reference counter to keep the contexts from being destroyed. It was really great back when I did a lot of client side js

The killer app version of this would be to open a repl at any context. As it stands it requires a good bit of competency to do it well.

Are you looking for logpoints?

https://firefox-source-docs.mozilla.org/devtools-user/debugg...

Edit: just realized this is literally the first thing mentioned in the linked article.

Not really. Suppose I have access to a closure that was created by the invocation of an IIFE. I would like to access variables that are in scope as seen from inside the closure, and I’d like to do this without executing the closure.

Why use IIFEs in JS now that we have block-scoping with let/const and modules to isolate data? (open question)

In Chrome you can inspect your closure (as you clarified in https://news.ycombinator.com/item?id=38226743#38231705) using the "Watch" pane, and then look at its "[[Scopes]]" pseudo-property. I don't think there is a way in Firefox.

For the sake of completeness, I can recommend Werkzeug. I use it for Django backend development and it's incredibly useful. It allows me to have "PDB" shell right in the browser whenever and wherever an exception is met.

+1. It's easy to drop a 1/0 wherever I want Werkzeug to show up in the browser.

lol next time try “assert False” but I guess it’s same same

does it still requires to have django-extension/runserver_plus ?

idk if it's a hard requirement, probably not... but I am with it... at a dev-specific settings.py just like Django debug toolbar, to avoid adding a dependency to production ;)

Aight, I was just asking because we hit some tiny road bumps with runserver_plus long ago, but it might be worth using it again to shorten debugging time.

I really want to have that experience everywhere, including nose.js and the browser. It is so simple and intuitive

One trick i use all the time is debugging by searching through loaded scripts by UI string:

1) Go to Network panel, start recording network requests

2) Open left sidebar and invoke search to type in the code/ui string you want to find

3) It'll usually find it in some weird bundled js chunk file, click on the result

4) It opens the network request for that file, now right click anywhere in file and pick "Open in Sources" or something along that line, that jumps to debugger

5) Now place your debugger statement, this will probably load sourcemaps too

I've been a Python/Elixir programmer for a long time and I make heavy use of pdb.set_trace()/IEx.pry().

Lately, I've inherited a very messy NodeJS backend and have been pulling my remaining hair out working without proper debugging tools. I've gone back to 'console.log' debugging, but it makes me feel like a caveman.

I can't believe that this whole popular ecosystem doesn't have a proper debugging REPL - can anybody point me in the right direction?

You can do some solid debugging with NodeJS if you use the VSCode dev tools. Just add it as a debugging option and you can add breakpoints and step through functions.

Works pretty great, JavaScript is pretty nice to debug since everything is incapsulated inside an object for the most part.

It does. `node --inspect-brk`. You can connect to it with VS Code or Chrome dev tools. The tricky part is whether there's build tooling infront of just the nodejs command, like converting typescript or something like that. But if you run `node`, then it's pretty easy.

Its debugging REPL is just the javascript console itself.

I looked into that a little bit, but I've got this TS transpiling crap to deal with as well, and I don't want to lose my live code reloading.

I guess I'll try again when I find the time. Was hoping there was some 3rd party package I could use to just drop in and set a trace. Thanks.

That should work fine in the vscode debugger, you just want to make sure that the transpiler you're using is generating sourcemaps. Generally they do by default. If you have issues, open a github issue and I'll fix it :)

As somebody else pointed out, —inspect or —inspect-brk is what you’re after.

In VsCode you can also use the “open JavaScript debug terminal” command: it opens a terminal in which any node command automatically starts with a debugger attached

What I like about the Elixir repl, which I assume is similar to the Erlang repl is its ability to connect to a live process, a feature I haven't seen in the Node JS ecosystem.

Maybe not sufficient, but I just found out about python snoop

https://pypi.org/project/snoop/

deep tracer decorator

Under the section "Debugging Property Reads": how would you convert `{configOption: true}` to `{get configOption() { debugger; return true; }}` using a conditional breakpoint?

That’s not under the conditional breakpoint heading. You would just override the value to be a getter in the console, or you could even change it in your source code if you have write access.

Thanks, I know I could do it in the console or the original source. But I was referring to the fact that the sentence in the post says to convert it to a getter "either in the original source code or using a conditional breakpoint."

Oh! I understand the question now.

You can put any expression into a conditional breakpoint, so anything you can do in the console you can do in a conditional breakpoint.

So, if you're doing this sort of thing once, you can just type it into the console and you're golden. But if you want to modify a stack local variable over and over again every time it is initialized, it's much easier to do in a conditional breakpoint because then it will happen every time that line of code runs, and your debugger never has to pause. (see https://alan.norbauer.com/articles/browser-debugging-tricks#...)

Got it! Thank you for the explanation.

This is something I really need to pick up / is there a dedicated book or study for this or is it just web dev / front end all the way down?

I can really recommend following the "What's new in DevTools" series by the chrome team. Clicking a link to read release notes when you're in the middle of something may not seem appealing, but spending 5 minutes to skim through it when a new version is released is well worth your time. There are also digestible videos that are just a few minutes long and will give you a brief overview. While their purpose is to show new features, in my experience you will often gain understanding of the current limitations of the tools as well.

Don't any browsers support re-style reverse debugging yet?

I haven't had the need to use it but I've been thinking if something like the last one `Monitor Events for Element` exists. Glad it does. Although according to the article, it's a Chrome-only feature. I wonder if there are any alternatives for Firefox?

I've noticed that a lot of websites will try to prevent you from using the debugger. They use various techniques ranging from calling `debugger` every second to entrapped sourcemaps to make debugger features work against you!

Take a look at disable-devtool [1], it surprises me just how many methods can be used to detect usage of an invaluable tool that should be a user's right to use. These "exploits" should really be patched browser-side, but I don't see any active efforts by browsers to fix this.

I've created a simple anti-anti-debug extension [2] that monkey-patches my way around these anti-debug scripts. It works fine for now, but I can't imagine it working consistently in the long term once the inevitable arms race begins.

How can we get Google, Mozilla, etc... to care about dev tool accessibility?

[1]: https://github.com/theajack/disable-devtool

[2]: https://github.com/Andrews54757/Anti-Anti-Debug

A simple way to solve this issue is to just deprecate the debugger statement and have people rely on setting up breakpoints manually, or request an explicit opt in into the debugger statement so that random websites don’t hijack it.

That would be helpful, but there are also other methods that don't involve using the debugger. For example, one technique involves periodically printing a custom object to the console with a toString getter. This is programmatically called by the browser only when the devtools are opened. This allows the website to know when you've opened devtools and they will redirect/block/crash your browser in response.

They don't need to deprecate it. I remember reading a blogpost here that was about bypassing these kinds of checks. They recompiled SpiderMonkey with the debugger() method renamed to something else.

I've pondered why websites who are that afraid of reverse engineering don't simply feed the browser mock data or kill a user's session (with a five minute IP block) when the browser requests a source map. Almost all browsers request them by default the moment you open the debugger and normal users very rarely hit the debugger.

Sure, it'd be trivial to circumvent such a block, but it'd easily inconvenience most of the low-hanging fruit enough that things like community maintained ad blockers could become ineffective. Surely simply never serving ads to people who open the dev tools would prevent the 99.9% of normal users from using effective ad blockers in their browsers.

I've seen some websites do this in the wild, it's why I recommend turning the sourcemaps off in the troubleshooting section of my anti-anti-debug tool.

It's pretty easy to circumvent this method, but honestly the user shouldn't have to configure their browsers to be resistant to anti-debugging. From the side of the website, it should be impossible to know if the debug tools are just open.

I haven't seen any anti reverse engineering on sites with significant technical expertise. For example Facebook just prints a very reasonable warning. I've seen anti-debugger stuff only on relatively shady and cheap websites. I suspect the issue with your suggestion is it would require replacing static file serving with a smarter backend.

Imagine if browser developer tools were reasonably architected so e.g. opening the DOM inspector didn't provide a vector for anti-tamper code targeting the JS debugger to DOS your attempts to poke at the CSS or read network requests. Or get this: for non-hostile content—like something you yourself have written—maybe you could have the debugger and the inspector open in separate windows, or even have multiple object inspectors open on different objects at the same time. Gee. Fuckin' novel idea. Maybe this is something we'll be able to look forward to once the year 2000 rolls around.

The ultimate way is process everything on the server with a sockets to webapi adaptor on the client. Then all you see is commands coming from the server to pull various strings.

Even then some sites detect the DevTools connection and prevent you from normal (on-site) actions

None of these are weird or something the browser is trying to hide from you, just things that an experienced front-end developer would probably know, although I was not aware of the monitor() command.

That being said, I am pleasantly surprised at the debugging and development tooling that is built right into most modern browsers. It really does make the UI development experience very powerful. I wish more back-end languages had this experience.

It's a joke title.

> I wish more back-end languages had this experience.

the jvm debugging experience is pretty good imho.

It's the compiled languages that have a poor experience with debugging - try injecting code to execute in a c debugger! It's hard as hell to do!

You know the tooling leaves something to be desired when editing instruction bytes is the best option to NOP out an assert - for example.

Generally a conditional breakpoint is good enough to do this (though perhaps not very fast).

> try injecting code to execute in a c debugger! It's hard as hell to do!

I mean, theoretically, not really? Allocate a page RW, put in some compiled code, remap as R+X, redirect execution there, return code execution to where it was. Jumping through those hoops will be more expensive when you use the debugger like that (you'd need to hot patch some kind of jump statement to circumvent that) but it's not exactly impossible.

Things become difficult when the compiler starts optimizing out code, because the debugger would need to keep track of everything, but I don't see why it'd be technically impossible to do so.

Executing code in a C debugger is basically how modern reverse engineering works, it's a lot harder than in other languages but it's certainly possible.

Yeah, both GDB and LLDB know how to do this.

> I wish more back-end languages had this experience.

I know what that feels like. I have that same feeling about a whole bunch of languages that has been available in .NET for at least 15 years now.

When I got into programming, I moved from VB6 to VB.NET to C#, the latter two using a cracked version of MS Visual Studio I downloaded at school. I only started seriously using the debugger somewhere around the time I taught myself C#.

Because of this, I got used to the idea that in a normal debugger, you could just click and drag back the point of execution, modify the code, and hit resume. The VS time traveling debugger will actually revert variable assignments, recompile the modified code, insert it in its place, and continue execution. This wouldn't work with P/Invoke calls, but it worked great for what I was trying to do. Debugging and fixing mistakes was absolutely trivial.

Imagine my betrayal when I found out that there was nothing like this in C++, or Java, or Javascript. Java has gotten a similar feature, but I've never worked with an IDE that integrated it even close to how well VS 2008 integrated it.

Even today, this seemingly basic feature that I took for granted just isn't available in many modern IDEs. I understand natively compiled languages like C/C++/Rust/Go not supporting the inline code replacement feature, but the hot reload in languages like Kotlin, Java, and Javascript just don't work like it did for me back in 2008.

As for all these tricks: I think most of them are available in most IDEs. Anything involving conditional breakpoints will work regardless of the language you use (although languages like Rust can have some restrictions). Tracing callers should work in any decent debugger. Timing individual functions may require writing out long package names to get to the right methods, but they should be available. The monitor() trick can be replaced by a logging method breakpoint.

How well DOM-related debugging works, will depend on whether or not your GUI framework uses a DOM and how it's constructed. If you're composing a DOM from multiple individual functions (like in React/Flutter/Kotlin Compose) then you can add breakpoints to all the relevant state alterations. If your tooling uses a more C-inspired windowing mechanism, you're probably going to have to get creative.

When you get down into the weeds of it, I think you'll find that more of these features are available for backend languages than you might expect. I would recommend any developer to occasionally read the changelogs of their IDEs (or to grab an IDE if you normally don't use any) and take a few moments to explore the possibilities of modern debuggers.

I love your description of undoing code execution, modifying the code and continuing. I think too often these days because of the UNIX philosophy of doing one thing well, the debugger and the compiler and even the linker are different projects, making it unnecessarily complicated to combine these tools other than passing blobs of output into another tool's input. We should aim higher.

> Imagine my betrayal when I found out that there was nothing like this in C++, or Java, or Javascript.

For compiled languages (C, C++, Go, Rust etc.), check out https://rr-project.org/

This looks rather neat, and there seems to be ample IDE support as well! It looks absolutely perfect, but makes me wonder why this isn't included in VS Code and IntelliJ by default when you install a (supported) language's debugger.

What pre-historic Java IDE were you using ? Intellij has supported time travel debugging in Java for nearly a decade.

https://www.jetbrains.com/help/idea/altering-the-program-s-e...

Back then, Eclipse was what every guide and book told me to use, so that was where I started. These days I'm mostly on Jetbrains products. Microsoft made VS a weirdly slow, bloated piece of kit, but I still miss some of the features they have had for ages.

I don't get automatically reloaded classes in IntelliJ, though I think there's a button to manually reload code manually? The whole ease of development came from not having to manually pick what code to reload while debugging. Java also seems to have a bunch of limitations that C# doesn't have (and didn't have back when I tried it on Windows). For example, you can't add a method, or alter the return type of a method.

As for time travel debugging, I'm not sure how you do that in IntelliJ? I can't even see any way to undo the previous line of execution, let alone step through code in two directions. It looks like there are plugins for a bunch of languages, though, but those are all external tools built by third parties.

Rolling back executed code seems to work in Rider, but I don't seem to get any Edit and Continue functionality on Linux, which is probably a Microsoft thing.

It looks like I was mistaken though, "IntelliTrace" was a VS2010 feature, not 2008: https://learn.microsoft.com/en-us/archive/msdn-magazine/2010...

I think the author was just having a bit of fun with the title

I'm going to put in a very relevant self-plug for the tool that I work on.

I work at Replay.io, and we're building a true "time traveling debugger" for JS. Our app is meant to help simplify debugging scenarios by making it easy to record, reproduce and investigate your code.

The basic idea of Replay: Use our fork of Firefox or Chrome to make a recording of your app, load the recording in our debugger UI, and you can pause at _any_ point in the recording. In fact, you can add print statements to any line of code, and it will show you what it _would_ have printed _every time that line of code ran_!

From there, you can jump to any of those print statement hits, and do typical step debugging and inspection of variables. So, it's the best of both worlds - you can use print statements and step debugging, together, at any point in time in the recording. It also lets you inspect the DOM and the React component tree at any point as well.

I honestly wish I'd had Replay available much earlier in my career. I can think of quite a few bugs that I spent hours on that would have been _much_ easier to solve with a Replay recording. And as an OSS maintainer for Redux, there's been a number of bugs that I was _only_ able to solve myself in the last year because I was able to make a recording of a repro and investigate it further (like a tough subscription timing issue in RTK Query, or a transpilation issue in the RTK listener middleware).

If anyone would like to try it out, see https://replay.io/record-bugs for the getting started steps to use Replay (although FYI we're in the middle of a transition from Firefox to Chromium as our primary recording browser fork).

I also did a "Learn with Jason" episode where we talked about debugging concepts in general, looked at browser devtools UI features specifically, and then did an example of recording and debugging with Replay: https://www.learnwithjason.dev/travel-through-time-to-debug-...

If you've got any questions, please come by our Discord and ask! https://replay.io/discord

> (although FYI we're in the middle of a transition from Firefox to Chromium as our primary recording browser fork)

What's the goal of this transition?

Replay's founders originally worked as engineers on the Firefox DevTools (and in fact our debugger client UI started as a fork of the FF Devtools codebase, although at this point we've rewritten basically every single feature over the last year and a half). So, the original Replay implementation started as a feature built into Firefox, and thus the current Replay recording browser you'd download has been our fork of Firefox with all the recording capabilities built in.

But, Chromium is the dominant browser today. It's what consumers use, it's devs use for daily development, and it's what testing tools like Cypress and Playwright default to running your tests in. So, we're in the process of getting our Chromium fork up to parity with Firefox.

Currently, our Chromium for Linux fork is fully stable in terms of actual recording capability, and we use it extensively for recording E2E tests for ourselves and for customers. (in fact, if you want to, all the E2E recordings for our own PRs are public - you could pop open any of the recordings from this PR I merged yesterday [0] and debug how the tests ran in CI.)

But, our Chromium fork does not yet have the UI in place to let a user manually log in and hit "Record" themselves, the way the Firefox fork does. It actually automatically records each tab you open, saves the recordings locally, and then you use our CLI tool to upload them to your account. We're actually working on this "Record" button _right now_ and hope to have that available in the next few weeks.

Meanwhile, our Chrome for Mac and Windows forks are in early alpha, and the runtime team is focusing on stability and performance.

Our goal is to get the manual recording capabilities in place ASAP so we can switch over and make Chromium the default browser you'd download to make recordings as an individual developer. It's already the default for configuring E2E test setups to record replays, since the interactive UI piece isn't necessary there.

Also, many of the new time-travel-powered features that we're building rely on capabilities exposed by our Chromium fork, which the Firefox fork doesn't have. That includes the improved React DevTools support I've built over the last year, which relies on our time-travel backend API to extract React component tree data, and then does post-processing to enable nifty things like sourcemapping original component names even if you recorded a production app. I did a talk just a couple weeks ago at React Advanced about how I built that feature [1]. Meanwhile, my teammate Brian Vaughn, who was formerly on the React core team and built most of the current React DevTools browser extension UI, has just rebuilt our React DevTools UI components and started to integrate time-travel capabilities. He just got a working example of highlighting which props/hooks/state changed for a selected component, and we've got some other neat features like jumping between each time a component rendered coming soon. All that relies on data extracted from Chromium-based recordings.

[0] https://github.com/replayio/devtools/pull/9885#issuecomment-...

[1] https://blog.isquaredsoftware.com/2023/10/presentations-reac...

Do you plan to maintain the FF Replay fork?

Great work Replay team! Awesome tech.

Thanks! Yeah, the browser runtime team is doing incredible work to make this possible, backend has made great progress speeding up requests and improving scaling, and us frontend folks are getting to build on top of that.

We're not currently doing active dev work on the FF fork due to the focus on Chromium, and I think the goal is to fully try to move over to Chromium as the user recording browser. So, we probably _won't_ be maintaining the FF fork going forward due to limited resources. I think our next runtime focus after Chromium parity will be Node. We've already got an alpha-level Node for Linux fork that actually works decently - I've used it myself to record Jest or Vitest tests or Node scripts, and debug them. But that does need a lot of work to flesh it out.

I'm the author of the article and my advice is that you stop reading it and go learn about Replay.io instead. It's the most slept on web tech and will up your debugging game immensely. Seriously, go check it out.

Is replay.io able to deterministically replay multiple WebWorkers interacting with a SharedArrayBuffer?

We might be able to replay the app as a whole, but we don't currently support debugging workers that I know of. I know we don't support recording WebGL at the moment, and there's probably a couple similar recording limitations I can't think of off the top of my head.

We've got some articles linked in our docs that talk about how the recording/replay mechanism uses "effective determinism" that's close enough to the original to be accurate:

https://docs.replay.io/learn-more/contribute/how-replay-work...

>> you can use print statements and step debugging, together, at any point in time in the recording

I am working on something similar. I am building an IDE that allows to jump from the line printed by a console.log call to a corresponding code location, and observe variables and intermediate expressions.

It also displays a dynamic calltree of a program, allowing to navigate it in a time-travel manner.

Currently it only supports pure functional subset of JavaScript, but I am working on support for imperative programming (mutating data in place).

https://leporello.tech/

Once I can replicate an issue locally, I don’t normally find it too difficult to understand what’s happening.

Harder is reproducing a bug based on what I’ve got to go on in Sentry. Mostly it’s easy enough to figure out but the traces you get in JS are very light in information compared to what you get in Python for example.

Anyone have any tips on getting more information in the initial report?

> Once you launch the browser, you will be prompted to sign in with your Google account.

What is the reason/need for this?

Replay takes data privacy seriously. Recordings can be shared, but you control who has access to them, and we need to know who owns each recording. Our user accounts are currently based on Google auth, so you have to log in before recording so we can track ownership.

I realise now the linked instructions are for end users filing bug reports. Can I debug things with it by myself without a google account?

Afraid not, sorry. The recording and debugging process is the same whether it's an end user or QA submitting a report, or a dev making the recording yourself. All recordings have to be uploaded in order for our backend to do the replaying work. It takes some pretty massive EC2 instances to make the replaying process happen - it's not something you'd be able to run locally.

Probably a good place to ask a specific question about debugging here.

A few years ago, I was hacking a web bookreader.

It has a function that is used to decode images (they're encrypted in some way) into canvas, and I want to find it so I can call it directly in my user script to batch download decoded images.

So I monkey patched `CanvasRenderingContext2D` function, added breakpoint, and found where the the function is defined in the 100k lines of obfuscated JS source code, easily.

The problem is.. once the page is rendered, the function would be nested in some objects, something like `window.abd.fdsfsd.r2323.fsdfs.fasf.xyy.myfunc`.

I don't know how exactly I can find the full "path" of the function so I can call it, despite I'm literally pausing inside of it. I eventually got it done, but it was manual and painful.

So I'm wandering: is there a better way to do it? The browser obviously knows it, it just lacks of a way to tell.

I would like to know this as well.

A tree overview of all nested functions and objects with a search function that quickly jumps to that place, and maybe even let you call that function with data from state?

You can use `trace` to trace the call stack but I don't know of any way to walk back up the call stack.

Do you want to have this in Tampermonkey script ONLY or not necessarily? You can use ResourceOverride addon to simply replace the 100k script with your own script that is almost the same but also assigns the function you need to window.myFn or something. There it can just be picked up by TM.

Thanks for the suggestion, but the goal is to just use this function in my own code (which has lots of other things going on).

I guess it technically can be done by modifying the existing JS in-place, but doing so with a very large, minified JS would be a nightmare in term of maintenance.

To parent off of GP, I've run into this recently while working on some webextension code and I'm thinking my solution is going to be to monkey-patch the script, but merely to expose a reference to the object/function in question.

So TLDR don't monkey-patch a giant minified source file with a bunch of your own code. Monkey-patch the function just enough to get it to be exposed globally with its context, and then have your code separately reference it.

This is still fragile (unless you have a stable entry point with regex or something, which is not completely certain to exist, it'll likely break whenever the script changes), and is still not ideal, and I'm still thinking a bit to see if I can do anything better, but I suspect it's more stable than trying to access the function directly by coding the path and should be a great deal simpler as well since it will work even if the function/context is in a closure.

----

Of course, you might be lucky and there might actually be a way to get at the function. For example, I am separately looking into seeing if I can figure out a clean way to reliably look for imports exposed through a webpack bundle, since those are exposed globally and if you can find the paths reliably you should be able to get access to any of the imports (although you still won't have access to the closures). I haven't made much progress on it though, mostly because it's kind of frustrating to work on.

----

There is also the possibility (although I suspect it would get very messy very quickly, and I'm not sure of current browser support) that you could theoretically maybe rig something together with `function.caller` (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...), possibly patching the minified source file merely to remove "use strict".

I have not messed with `caller` in probably close to a decade at this point, so I make no promises at to whether it still works at all :) but... in its heyday before we realized how wildly insecure it was, it did allow reconstructing the stack from a function in-code in some situations.

The browser does not know the path. Also, if the function (or one of its parents) is in a closure, there may not even be a path to the function from window.

If you're sure the function is reachable from window you can search for it recursively:

    (function () {
        function search(prefix, obj, fn, seen = null) {
            if (!seen) seen = new Set();

            // Prevent cycles.
            if (seen.has(obj)) return false;
            seen.add(obj);

            console.log('Looking in ' + prefix);

            for (let key of Object.keys(obj)) {
                let child = obj[key];

                if (child === null || child === undefined) { 
                } else if (child === fn) { 
                    console.log('Found it! ' + prefix + '.' + key); 
                    return prefix + '.' + key;
                } else if (typeof child === 'object') { 
                    // Search this child.
                    let res = search(prefix + '.' + key, child, fn, seen);
                    if (res) {
                        return res;
                    }
                }
            }

            return false;
        }

        // For example:
        let fn = function() { alert('hi'); }
        window.a = {};
        window.a.b = {};
        window.a.b.c = {};
        window.a.b.c.f = fn;
        return search('window', window, fn);
    })();

Yeah it's reachable (in unsafeWindow for user script), I eventually find it by something similar.

In your example, you already has a reference of `fn` to use, which isn't the case for my userscript (if I have a reference, I would just use it!).

I have to search based on the features in plain text of the function (using some regexes on `fn.toString()` to check how the arguments look like).

The idea is that you use a breakpoint somewhere where you have a reference to the function to see it then paste the search() function in the debugger console and call it to find it in window

Oh I got it now. Thanks! Will try next time.

Have you tried accessing `arguments.callee`[0]? It is unfortunately deprecated and might throw an error. I've never tried it myself as I rarely use breakpoints in the debugger.

  [0]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/callee

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