@SpiderMonkey

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An Open Source, embeddable engine for JavaScript and WebAssembly. Maintained by the SpiderMonkey team at @mozilla.

Read about the latest SpiderMonkey changes here: https://spidermonkey.dev/blog/

Learn about the engine: https://spidermonkey.dev/docs/

Browse the code: https://searchfox.org/mozilla-central/source/js/src

Check out our online demo of SpiderMonkey compiled to WebAssembly! https://mozilla-spidermonkey.github.io/sm-wasi-demo/

Websitehttps://www.spidermonkey.dev
SpiderMonkeyFeb 26
Ryan HuntFeb 26

I wrote a thing! Here's my hot take on where WebAssembly should go next.

https://hacks.mozilla.org/2026/02/making-webassembly-a-first-class-language-on-the-web/

Why is WebAssembly a second-class language on the web? – Mozilla Hacks - the Web developer blog

This post is an expanded version of a presentation I gave at the recent WebAssembly CG meeting in Munich. WebAssembly has come a long way since its first release in 2017. The 1.0 version of WebAssembly was already a great fit for low-level languages like C and C++, and immediately enabled many new kinds of applications to efficiently target the web.

Mozilla Hacks – the Web developer blog
SpiderMonkeyDec 15
Firefox for Web DevelopersDec 15
JavaScript declarations in conditionals was discussed at TC39 recently, but consensus broke down over one key detail. How do you think it should behave?
SpiderMonkeyOct 28
Ben VisnessOct 28

The write-up of my new graph layout algorithm for SpiderMonkey is finally live.

We built a custom layout algorithm for JS and WASM that follows the structure of the source code. No more spaghetti nightmares from Graphviz, and thousands of times faster.

https://spidermonkey.dev/blog/2025/10/28/iongraph-web.html

Who needs Graphviz when you can build it yourself?

Exploring a new layout algorithm for control flow graphs.

SpiderMonkey JavaScript/WebAssembly Engine
SpiderMonkeyOct 20
FirefoxDevToolsOct 9

As an experiment, we (the Firefox team) wanted to try a new way to get feedback on which Interop proposals matter most.

So, here's a web app where you can rank the proposals you care about, giving us data we can use when reviewing which ones to champion.

https://interop-rank.jakearchibald.com/

Interop Feature Ranking

Rank the web platform features you care most about

SpiderMonkeyAug 1, 2025
MattStudiesJul 31, 2025

Hey.... wanna be my manager?

Come work on the SpiderMonkey team @ Mozilla!

Manage amazing engineers that are also fun :)

Currently listed for Remote Canada, Germany, France, UK

https://job-boards.greenhouse.io/mozilla/jobs/7074795?gh_src=ozo1nb921us

#getfedihired

Engineering Manager, DOM (Web Standards) or SpiderMonkey (Javascript)

Remote

SpiderMonkeyApr 14, 2025

Exciting news: we're shipping Temporal in Firefox 139! Temporal is a new API for working with dates and times in JS.

This is a very large feature and our implementation was contributed by a single volunteer.

Read about it in our blog post: https://spidermonkey.dev/blog/2025/04/11/shipping-temporal.html

Shipping Temporal

The Temporal proposal provides a replacement for Date, a long standing pain-point in the JavaScript language. This blog post describes some of the history and motivation behind the proposal. The Temporal API itself is well docmented on MDN.

SpiderMonkey JavaScript/WebAssembly Engine
SpiderMonkeyFeb 28, 2025
Show threadhytradboiFeb 26, 2025
Talk 25: "A quick ramp-up on ramping up quickly" by Iain Ireland (@iainireland)
SpiderMonkeyFeb 17, 2025

Temporal is now enabled by default in @firefoxnightly!

#Temporal #JS #Firefox

https://bugzilla.mozilla.org/show_bug.cgi?id=1946823

1946823 - Enable Temporal on Nightly

RESOLVED (dminor) in Core - JavaScript: Standard Library. Last updated 2025-02-15.

SpiderMonkeyJan 24, 2025

#JS #Temporal is coming.

It's already available in @firefoxnightly behind the `javascript.options.experimental.temporal` pref in about:config.

MDN blog post about Temporal: https://developer.mozilla.org/en-US/blog/javascript-temporal-is-coming/

JavaScript Temporal is coming | MDN Blog

A new way to handle dates and times is being added to JavaScript. Let's take a look at Temporal, what problems it solves, the current state, and what you'll find in the new documentation about it on MDN.

MDN Web Docs
SpiderMonkeyJan 16, 2025
Ben VisnessJan 15, 2025

After working hard to get 64-bit WebAssembly released in browsers, I would now like to discourage most people from using it 😛

(My first post on the SpiderMonkey website btw, which is a fun milestone)

https://spidermonkey.dev/blog/2025/01/15/is-memory64-actually-worth-using.html

Is Memory64 actually worth using?

After many long years, the Memory64 proposal for WebAssembly has finally been released in both Firefox 134 and Chrome 133. In short, this proposal adds 64-bit pointers to WebAssembly.

SpiderMonkey JavaScript/WebAssembly Engine