This is my second post in a weekly series, where I attempt to distill my week down into some lessons or facts that I’ve picked up. Let’s get to it!

ES6 – what’s safe to use in browser development?

As of March 27, 2015, ES6 classes are still not yet safe for use in production browser code. There’s code to support them in Firefox, but they’re Nightly-only behind a build-time pref.

Array.prototype.includes and ArrayBuffer.transfer are also Nightly only at this time.

However, any of the rest of the ES6 Harmony work currently implemented by Nightly is fair-game for use, according to jorendorff. The JS team is also working on a Wiki page to tell us Firefox developers what ES6 stuff is safe for use and what is not.

Getting a profile from a hung process

Update [January 23rd, 2017]: Markus Stange contacted me to let me know that the function mozilla_sampler_save_profile_to_file has been changed to profiler_save_profile_to_file. This happened in this bug. I’ve updated the post to reflect this.

According to mstange, it is possible to get profiles from hung Firefox processes using lldb¹.

1. After the process has hung, attach lldb.
2. Type in², :

   p (void)profiler_save_profile_to_file("somepath/profile.txt")

3. Clone mstange’s handy profile analysis repository.
4. Run:

   python symbolicate_profile.py somepath/profile.txt

   To graft symbols into the profile. mstange’s scripts do some fairly clever things to get those symbols – if your Firefox was built by Mozilla, then it will retrieve the symbols from the Mozilla symbol server. If you built Firefox yourself, it will attempt to use some cleverness³ to grab the symbols from your binary.

   Your profile will now, hopefully, be updated with symbols.

   Then, load up Cleopatra, and upload the profile.

   I haven’t yet had the opportunity to try this, but I hope to next week. I’d be eager to hear people’s experience giving this a go – it might be a great tool in determining what’s going on in Firefox when it’s hung⁴!

Parameter vs. Argument

I noticed that when I talked about “things that I passed to functions⁵”, I would use “arguments” and “parameters” interchangeably. I recently learned that there is more to those terms than I had originally thought.

According to this MSDN article, an argument is what is passed in to a function by a caller. To the function, it has received parameters. It’s like two sides of a coin. Or, as the article puts it, like cars and parking spaces:

You can think of the parameter as a parking space and the argument as an automobile. Just as different automobiles can park in a parking space at different times, the calling code can pass a different argument to the same parameter every time that it calls the procedure.⁶

Not that it really makes much difference, but I like knowing the details.

1. Unfortunately, this technique will not work for Windows. 🙁  ↩

2. Assuming you’re running a build after this revision landed. ↩

3. A binary called dump_syms_mac in mstange’s toolkit, and nm on Linux ↩

4. I’m particularly interested in knowing if we can get Javascript stacks via this technique – I can see that being particularly useful with hung content processes. ↩

5. Or methods. ↩

6. Source ↩