SpiderMonkey: The JavaScript Engine


It is the first JavaScript engine. It runs JavaScript! It is written in C/C++, and contains an interpreter, a garbage collector, and a JIT (just-in-time) compiler. The JIT compiler was implemented by Mozilla in Firefox 3.5 (2009) [https://hacks.mozilla.org/2009/07/tracemonkey-overview/]. Over the years, this JIT compiler has been upgraded to become faster.

Who Made It and Maintains It:

It was first written by Brendan Eich in 1995. In 2011, Eich passed off management of the code to Dave Mandelin, who works for Mozilla(https://blog.mozilla.org/dmandelin/, https://twitter.com/dmandelin).

ChangeLog is at this url: http://hg.mozilla.org/mozilla-central/summary

You can see people’s names who have fixed bugs, or otherwise done work on it, here.

SpiderMonkey Source Code:


Forked: 32 times

Core contributors:



SpiderMonkey is run by Mozilla, and so Mozilla takes care of what licensing it has.

In the SpiderMonkey package, the licensing info. for SpiderMonkey is under ./toolkit/content/license.html. The license is really long, but at the top it says “most of the source code is available under the Mozilla Public License 2.0 (MPL).” It then says “The remainder of the software which is not under the MPL is available under one of a variety of other free and open source licenses.”

Nevertheless, the MPL2 (https://www.mozilla.org/MPL/2.0/) seems to be the main license for this code. This license grants anyone to use, reproduce, make available, modify, display, perform, distribute, and otherwise exploit” it, and to “make, use, sell, offer for sale, have made, import, and otherwise transfer” it.

Where is it?

SpiderMonkey Source Code:


Official Repo:


How to download.

Go to url: http://hg.mozilla.org/mozilla-central/summary

Click “zip” at the top of the page.

This downloads a 258 MB zip file.

On Windows 7:

Download MozillaBuild


The Community:

Since the project is managed by Mozilla, I would consider the community to be the open-source community in general. Anyone who was worked on the project is part of this community. Based on this, the community is huge. To give an idea of how big it is, just take a look at the change logs (http://hg.mozilla.org/mozilla-central/summary). Notice that you can go back in the logs by at least 100,000 logs.

Community Processes:

I found this website on how to write your first patch for SpiderMonkey: https://wiki.mozilla.org/JavaScript:New_to_SpiderMonkey. Basically, it tells you to install the code, and then make a small change. Testing for problems, and then efficiency is the next steps. Finally, fixing an actual problem would be where to go from there.

This means that the community takes testing to be an important matter, as well as efficiency in the program.

Tools Used By The Community:


This website is a forum for bugs and fixes in the Mozilla project. The topics are very specific, so be warned that it may be difficult to find answers here.


I have seen questions and answers on stack overflow, for problems installing and building.

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