Top all-time · #2441
SpiderMonkey JavaScript/WebAssembly Engine
Company blog · spidermonkey.dev/blogprogrammingsystemsweb
SpiderMonkey JavaScript/WebAssembly Engine is the company blog at spidermonkey.dev, writing about programming, systems and web. Since 2020, 31 different Hacker News readers have linked it 36 times, most often “SpiderMonkey JavaScript/WebAssembly Engine”. It is still writing: the last post was on 2026-05-20, and you can follow it by feed.
Too few sampled comments (16) to say how often HN recommends it.?
Why HN links it
The blog of Mozilla's SpiderMonkey JavaScript and WebAssembly engine, with posts on engine performance, Memory64, asm.js, WASI and periodic newsletters. HN readers link it for technical detail on engine internals.
Summarised by Claude from a sample of 16 HN comments that link here. Each comment is marked recommending, critical or neutral (citing without judging); how this works.
SpiderMonkey is Mozilla’s JavaScript and WebAssembly Engine, used in Firefox, Servo and various other projects. It is written in C++ and Rust.
Most-linked posts?
- SpiderMonkey JavaScript/WebAssembly Engine6 people · 6 mentions · first linked 2021-05-05
- Is WebAssembly Memory64 worth using?4 people · 6 mentions · first linked 2025-01-16
- Optimizing the Ion compiler back end2 people · 2 mentions · first linked 2024-10-16
- Saying Goodbye to Asm.js2 people · 2 mentions · first linked 2026-05-20
- Who needs Graphviz when you can build it yourself?2 people · 2 mentions · first linked 2025-10-28
What Hacker News says?
- kg · 2025-09-17
> WebAssembly apps tend to run slower in 64-bit mode than they do in 32-bit mode. This performance penalty depends on the workload, but it can range from just 10% to over 100%—a 2x slowdown just from changing your pointer size. > This is not simply due to a lack of optimization. Instead, the performance of Memory64 is restricted by hardware, operating systems, and the design of WebAssembly itself. https://spidermonk…
Comment linking /blog/2025/01/15/is-memory64-actually-worth-using.html
on “WASM 3.0 Completed” Submitted Who needs Graphviz when you can build it yourself? as a story · 524 points
HN discussionSpidermonkey is quite embeddable by itself [1]. On Linux things like polkit, couchdb, mongodb use it. Still, the compilation process seems to download the whole firefox tarball and only compiles the JS related parts [2, 3]. [1] https://spidermonkey.dev [2] https://archlinux.org/packages/extra/x86_64/js78/ [3] https://www.linuxfromscratch.org/blfs/view/svn/general/js78....
Comment linking /
on “Ask HN: Why do new browsers use Chromium instead of Firefox as their base?”Submitted Optimizing the Ion compiler back end as a story · 164 points
HN discussion> They can use their internal render engine. What they can't use is their Javascript framework because a modern JIT requires taking compiled javascript code in memory and marking it as executable > You can even ship a custom Javascript engine on iOS if you disable JIT, it will just be slow. Wouldn't this mean that Mozilla could theoretically ship "real" Firefox (Gecko/XUL/SpiderMonkey) on iOS after the SpiderMonkey…
Comment linking /blog/2021/07/19/newsletter-firefox-90-91.html
on “So you want to write a GUI framework”Submitted SpiderMonkey JavaScript/WebAssembly Engine as a story · 90 points
HN discussionFor what it's worth, the Fastly team recently ported spidermonkey to WASI https://bugzilla.mozilla.org/show_bug.cgi?id=1701197 https://spidermonkey.dev/blog/2021/07/19/newsletter-firefox-... https://bytecodealliance.org/articles/making-javascript-run-...
Comment linking /blog/2021/07/19/newsletter-firefox-90-91.html
on “Firefox 92”Submitted Is WebAssembly Memory64 worth using? as a story · 39 points
HN discussionThis is probably referring to some work to use Stencil for self-hosted code. That reduced process overhead by 6%, which is quite a bit: https://bugzilla.mozilla.org/show_bug.cgi?id=1688794 There might have been some other Stencil work that landed in 92 to reduce memory. There's a description of what Stencil is, and some work related to it that landed in Firefox 90 and 91, here: https://spidermonkey.dev/blog/2021/07/…
Comment linking /blog/2021/07/19/newsletter-firefox-90-91.html
on “Firefox 92”Submitted New graph layout algorithm for SpiderMonkey's SSA compiler as a story · 14 points
HN discussionThis announcement has some links to articles with more details: https://hacks.mozilla.org/2024/03/improving-performance-in-f... The SpiderMonkey devs also post newsletters every few months detailing all the recent improvements they've made: https://spidermonkey.dev/
Comment linking /
on “Firefox 125”Submitted SpiderMonkey (Firefox JavaScript Engine) has a new website as a story · 4 points
HN discussionUnfortunately 64bit address suppport does the opposite, that comes with a non-trivial performance penalty because it breaks the tricks that were used to minimize sandboxing overhead in 32bit mode. https://spidermonkey.dev/blog/2025/01/15/is-memory64-actuall...
Comment linking /blog/2025/01/15/is-memory64-actually-worth-using.html
on “Head in the Zed Cloud”Submitted SpiderMonkey Newsletter 4 (Firefox 76-77) as a story · 4 points
HN discussionAny chance you could add a link for SpiderMonkey? It has its own site now: https://spidermonkey.dev/
Comment linking /
on “Enumerating and analyzing 40 non-V8 JavaScript implementations”Submitted Shipping Temporal in Firefox 139 as a story · 3 points
HN discussionThe tarot-style images of the monkeys in the docs are amazing :) https://spidermonkey.dev/docs/
Comment linking /docs/
on “SpiderMonkey JavaScript/WebAssembly Engine”Submitted Saying Goodbye to Asm.js as a story · 2 points
HN discussionHere is the original blog post that is more readable https://spidermonkey.dev/blog/2024/10/16/75x-faster-optimizi...
Comment linking /blog/2024/10/16/75x-faster-optimizing-the-ion-compiler-backend.html
on “Firefox Is Now "More Than 75X Faster" Running WebAssembly”Submitted SpiderMonkey Newsletter (Firefox 132-134) as a story · 2 points
HN discussionAn example: “Are We ESMified Yet?” is a Mozilla dashboard tracking an incremental Firefox code migration (1.5 years and counting) from a Mozilla-specific "JSM" JavaScript module system to the standard ECMAScript Module "ESM" system. Current ESMification: 96.69%. “Are We X Yet?” is a Mozilla meme for dashboards like this. https://spidermonkey.dev/areweesmifiedyet/
Comment linking /areweesmifiedyet/
on “Long Term Refactors”Submitted Ergonomic Brand Checks will ship with Firefox 90 as a story · 2 points
HN discussionEngines other than v8 exist. React Native uses Hermes or JavaScriptCore (Apple/Safari). [0] Other engines include SpiderMonkey (Mozilla/Firefox) [1] and QuickJS [2] [0]: https://reactnative.dev/docs/javascript-environment [1]: https://spidermonkey.dev/ [2]: https://bellard.org/quickjs/
Comment linking /
on “Microsoft forced me to switch to Linux”Submitted Private Fields and Methods ship with Firefox 90 as a story · 2 points
HN discussion> asm.js (that's been deprecated, hasn't it?) https://spidermonkey.dev/blog/2026/05/20/saying-goodbye-to-a... iirc, v8 never had any special compilation path for it to begin with.
Comment linking /blog/2026/05/20/saying-goodbye-to-asmjs.html
on “The Birth and Death of JavaScript (2014)”Submitted SpiderMonkey Newsletter 2 (Firefox 73) as a story · 2 points
HN discussionI already posted the link in another reply, but this is a good overview why wasm32 is usually the better choice over wasm64: https://spidermonkey.dev/blog/2025/01/15/is-memory64-actuall... TL;DR: wasm64 has slower memory load/store operation because it requires 'software bounds checking', so unless you absolutely need more than 4 GB RAM, wasm32 is the better choice.
Comment linking /blog/2025/01/15/is-memory64-actually-worth-using.html
on “Ported my C game to WASM, here's everybug that I hit”Submitted Flipping Responsibility for Jobs in SpiderMonkey as a story · 1 points
HN discussionBecause 64-bit WASM can be quite a bit slower than 32-bit WASM: https://spidermonkey.dev/blog/2025/01/15/is-memory64-actuall... TL;DR: wasm64 requires explicit heap bounds checks, while in wasm32 the memory mapping hardware does it for free. E.g. quote: "The only reason to use Memory64 is if you actually need more than 4GB of memory. Memory64 won’t make your code faster or more “modern”. 64-bit pointers in WebAssemb…
Comment linking /blog/2025/01/15/is-memory64-actually-worth-using.html
on “Ported my C game to WASM, here's everybug that I hit”Submitted Making Teleporting Smarter as a story · 1 points
HN discussion
