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Welcome!

SpiderMonkey is Mozilla’s JavaScript and WebAssembly Engine, used inFirefox, Servoand variousother projects. It is written in C++, Rust and JavaScript. You can embed it into C++ and Rust projects, and it can be run as a stand-alone shell. It can also be compiled to WASI; see our online demo.

What's New

• Oct 28, 2025

Who needs Graphviz when you can build it yourself?

• Apr 23, 2025

5 Things You Might Not Know about Developing Self-Hosted Code

• Apr 11, 2025

Shipping Temporal

• Mar 17, 2025

SpiderMonkey Newsletter (Firefox 135-137)

• Mar 5, 2025

Implementing Iterator.range in SpiderMonkey

Read more in the SpiderMonkey Blog

SpiderMonkey Internals

Understanding the engine

• Overview of SpiderMonkey
• Browse the code
• SMDOC Source Comments

In-depth Articles:

• Warp: Improved JS Performance
• Compiler Compiler: A Twitch series
• A New RegExp Engine
• Future-proofing the Debugger Implementation
• The Baseline Interpreter

Building and Contributing

• Build Documentation
• Running tests
• File a bug
• Good first bugs

Embedding SpiderMonkey

• Examples and Documentation
• mozilla.dev.tech.js-engine (ARCHIVED)
• mozilla.dev.tech.js-engine.internals (ARCHIVED)

Where to find us

• Matrix chat: #spidermonkey:mozilla.org
• Discourse: SpiderMonkey
• Mastodon: SpiderMonkey@mastodon.social

While we strongly recommend using Matrix chat if you have questions, we also maintain a non-exhaustive list of people who have volunteered their expertise on specific components of SpiderMonkey. For example, you might need to know who to needinfo about IonMonkey in Bugzilla…