JavaScript:New to SpiderMonkey

From MozillaWiki
Jump to: navigation, search

Tutorial: your first patch

The first step to getting involved with SpiderMonkey is to make your first patch. This guides you through it, and at the end you should have learnt a lot of the procedures and formalisms involved in getting things done here.

We'll assume you're on a Unix-y platform, and that you know what you're doing. We'll ignore nearly all details.

Get the code

Spidermonkey development happens in the "mozilla-central" mercurial repository:

hg clone spidermonkey

See Mercurial bundles for information about downloading a single large file instead of using "hg clone". Recent versions of Mercurial will do this automatically.

Build the js shell

Most of the time, you'll be working with the JavaScript shell, instead of the full Firefox browser. See the SpiderMonkey build documentation for how to build the shell.

Fix something

At this point, you're ready to make your first fix.

TODO: something useless. Maybe adding a datemicrosecond function.

Building your changes

Having made the change, build the shell again.

./mach build

Testing your changes

It builds. Hurray. Time to run the tests to check you haven't broken anything.

./mach jit-test

The jit-tests are pretty quick, and should give you an idea if you've gotten something wrong. Assuming nothing goes wrong, try the ref-tests:

./mach jstests

Benchmark your changes

Tests all pass. Congrats, you didn't break anything. Now, did you make it faster or slower? We benchmark using a number of different benchmark suites.

  • v8
  • SunSpider: One of the original js benchmarks. No real value left.
  • Octane: Another js benchmark that we heavily optimized for in the past. Isn't representative of current web workloads.
  • speedometer: A full browser benchmark. Testing react, ember, etc. Intended to be a better measure of "responsiveness". We are currently optimizing for version 2 it.

Get the benchmarks:

svn checkout

And now run them:

cd SunSpider 
./sunspider --shell=<path-to-shell>/dist/bin/js --run=30 --suite=sunspider-0.9.1 
cd ..
Optimized build

Whoops, we benchmarked the debug version. Let's make an optimized build. Follow the build instructions and make sure your MOZCONFIG file has:

ac_add_options --disable-debug
ac_add_options --enable-optimize

Repeat the sunspider steps above, and take note of the line that looks like:

Results are located at sunspider-0.9.1-results/sunspider-results-2010-08-07-17.56.48.js

You'll need that later.


Build again and rerun SunSpider again. You should now have two files like:


Compare them using the compare script:

cd SunSpider 
./sunspider-compare-results --shell=<path-to-shell>/dist/bin/js --suite=sunspider-0.9.1 FILE1-withoutPatch FILE2-withPatch

Get a real bug

At this point, you've seen nearly everything you need to do hack on SpiderMonkey. So it's time to get a real bug to work on. You can get a bug on Bugs Ahoy.

Fix the bug, updating the bug report with your progress, and asking questions as you go (either in the bug comments, or in the SpiderMonkey room on Matrix). When it's done, repeat all the steps above. Then it's time to get your patch into the tree.

Submit a patch

To get the patch from mercurial, use:

hg diff  # if you're not

Add push it using moz-phab. See for more information.

Get a review

Nothing gets into the tree without a review, so you'll need one. The SpiderMonkey hackers list is a good place to start: if your patch changes something listed as an area of expertise for someone there, that's a good person to ask for a review.

Alternatively run hg blame on the files you've changed, and check who has been changing related code recently. They're likely to be good candidates. An irc bot can automate this process for a single file if you enter "/msg mrgiggles who can review something.cpp?" into your irc client.

The review will consist of comments on your changes, suggesting or requesting alternative ways to do something and asking you to make changes where needed. They might also request additional changes, for example tests. Fix what they ask, resubmit the patch to bugzilla, and ask for another review. After you repeat this step a few times, they will mark the patch as "r+" meaning it's now good to commit.


You can't commit to mozilla-central / mozilla-inbound until you have "level 3" access, so you'll need someone to do this for you. Try asking in #jsapi, or add the checkin-needed keyword to the bug. After you have been contributing for a while, you can get level 3 access by applying for it.

After committing, a large series of tests will be run to make sure you didn't break anything. You will need to hang around to make sure you didn't break something. It is difficult to determine what failures are real and what are what we call "intermittent oranges" ("orange" because that is the color used on our continuous integration dashboard for test failures). If you do break something, a sheriff will probably let you know via IRC and will probably back your patch out. You can always ask for help determining what is going on. (Over time you'll get a feel for figuring out when breakage is real on your own.)

Overview of the JS engine

The JS engine is a swiftly moving target. The most detailed information is available at Here are some particularly interesting, mostly up-to-date resources:

High level overviews


Medium level documentation

jsapi.h: and the files in

Frequently used coding recipes and mappings from JS idioms to SpiderMonkey code:

Detailed documentation




Function reference:

Collaboration and teamwork

Communication (in descending order of information content)

Nearly all communication is handled through Bugzilla. All bugs, feature requests, issues, enhancements, etc, are all referred to as bugs. No code may enter the Mozilla repository without being a patch attached to a bugzilla bug first. To follow all SpiderMonkey related bugs:

Go to email preferences
Watch the user general@spidermonkey.bugs
Or watch one of the JS components in the Core product: JavaScript Engine, JavaScript Engine: JIT, JavaScript: GC, or JavaScript: Standard Library

SpiderMonkey contributors generally hang out in the very active #jsapi IRC channel.

The js-internals mailing list is used for communicating with other SpiderMonkey hackers. (Internals as in discussing the internals of the SpiderMonkey engine. All are welcome on the list.).

Reading Planet Mozilla is the best way to keep up with the Mozilla project, which includes some SpiderMonkey related blogs:

General JavaScript blog
Jason Orendorff
Nicholas Nethercote
Jeff Walden
Jim Blandy
Jan de Mooij
Hannes Verschore
Shu-yu Guo
Benjamin Bouvier

The js-engine mailing list is generally used for communicating with people who embed SpiderMonkey, such as asking questions and announcing API changes. No actual development happens on the list.

Code considerations


Most active work on SpiderMonkey is done in the mozilla-central branch of the mozilla repository.

Coding Style

SpiderMonkey uses the same coding style as Firefox. Please see for more details


Everything goes through bugzilla. We find a bug or have an idea, then submit it to bugzilla, then file patches to solve it. When it is solved, the patch is reviewed by team members, and is then committed to the mozilla-inbound repository. A link to the commit is then automatically added as a comment to the bug. mozilla-inbound is periodically merged to mozilla-central by a Sheriff. This exempts us from watching treeherder to check for failures caused by our patches, and commits are automatically backed out if errors are found.

Sample Workflows

Steve Fink: Mozilla workflow

I (Paul Bone) suggest stating with a unified repository checkout with the extensions and customization from Steve's workflow, using hg share to create workspaces, use bookmarks to track your new feature developments, and using hg histedit and hg rebase to prepare your patches for publication.


The following docs are for the Mozilla project, and differ ever so slightly from what you should do for SpiderMonkey. For example, you should find reviewers in #jsapi rather than #developers.

How to get commit access

How to make patches

Submitting a patch

Supported Platforms

.hgrc file

This .hgrc file contains a lot of the wisdom distilled through the wiki:

username = First Last <email@domain.tld>
qfulldiff = diff --rev qparent:.

diff = -p -U 8
qdiff = -p -U 8
qnew = -U
commit = -v

git = true
showfunc = true
unified = 8

try = ssh://

Try server

Often, you may be a little wary of breaking things. Mozilla has a Try Server where you can send a patch, and it'll be built and tested on tons of machines, which report to TreeHerder. Although you don't have access to TryServer just yet, you can get someone else to push it there for you. Just attach a patch to your bug, and ask in a comment, or on #jsapi. To be able to push to try-server yourself, you will need "level 1" access, which you can request once you've contributed a patch or two.

Example workflow

The workflow is detailed on the contribution page: