Home | SpiderMonkey JavaScript/WebAssembly Engine

Welcome! SpiderMonkey is Mozilla’s JavaScript and WebAssembly Engine, used in Firefox, Servo and various other 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 Jan 15, 2026 Flipping Responsibility for Jobs in SpiderMonkey 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) 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…