Brimstone: ES2025 JavaScript engine written in Rust
39 comments
·November 16, 2025bcardarella
Just a small comparison, compiled for release:
Boa: 23M Brimstone: 6.3M
I don't know if closing the gap on features with Boa and hardening for production use will also bloat the compilation size. Regardless, for passing 97% of the spec at this size is pretty impressive.
jerf
It looks like Boa has Unicode tables compiled inside of itself: https://github.com/boa-dev/boa/tree/main/core/icu_provider
Brimstone does not appear to.
That covers the vast bulk of the difference. The ICU data is about 10.7MB in the source (boa/core/icu_provider) and may grow or shrink by some amount in the compiling.
I'm not saying it's all the difference, just the bulk.
There's a few reasons why svelte little executables with small library backings aren't possible anymore, and it isn't just ambient undefined "bloat". Unicode is a big one. Correct handling of unicode involves megabytes of tables and data that have to live somewhere, whether it's a linked library, compiled in, tables on disks, whatever. If a program touches text and it needs to handle it correctly rather than just passing it through, there's a minimum size for that now.
rixed
I was currious to see what that data consisted of and aparently that's a lot of translations, like the name of all possible calendar formats in all possible languages, etc. This seems useless in the vast majority of use cases, including that of a JS interpreter. Looks to me like the typical output of a comitee that's looking too hard to extend its domain.
Disclaimer: I never liked unicode specs.
twoodfin
As well-defined as Unicode is, surprising that no one has tried to replace ICU with a better mousetrap.
Not to say ICU isn’t a nice bit of engineering. The table builds in particular I recall having some great hacks.
embedding-shape
Is that with any other size optimizations? I think by default, most of them (like codegen-units=1, remove panic handling, etc) are tuned for performance, not binary size, so might want to look into if the results are different if you change them.
LtdJorge
Stripping can save a huge amount of binary size, there’s lots of formatting code added for println! and family, stacktrace printing, etc. However, you lose those niceties if stripping at that level.
boianmihailov
"Compacting garbage collector, written in very unsafe Rust" got me cracking.
varispeed
Sorry for the offtop, but I really miss the cracktros. Imagine having Ikari intro before you boot into your OS.
vardump
Sorry also for being offtopic, but "cracking" in this case most likely refers to cracking [with laughter].
BoredPositron
Plymouth let's you do it on Linux without hacking around like osx or windows.
maxloh
Could you compare it with Boa? It is written in Rust too.
ivankra
I have some benchmark results here: https://ivankra.github.io/javascript-zoo/?v8=true
It's impressively compliant, considering it's just a one man project! Almost as fully featured as Boa, plus or minus a few things. And generally faster too, almost double the speed of Boa on some benchmarks.
lucideer
First time seeing a few of the engines listed here - based on this table I'm surprised Samsung's Escargot hasn't gotten more attention. LGPL, 100% ES2016+ compliance, top 10 perf ranking, 25% the size of V8 & only 318 Github stars.
A quick HN search shows 0 comments for Escargot - is there some hidden problem with this engine not covered in this table?
mrec
Surprised at the lack of a license though.
nicoburns
Interesting. Hermes and QuickJS both come out looking very good in these (in terms of performance vs. binary size)
383toast
how does this compare to existing JS engines?
namanau
[flagged]
fgallih
[flagged]
chiffaa
> who in the fuck would write a garbage collector using garbage collected Rust?
Rust is not garbage collected unless you explicitly opt into using Rc/Arc
oconnor663
I still wouldn't call it GC in that case. It's pretty much exactly the same as std::shared_ptr in C++, and we don't usually call that GC. I don't know about the academic definition, but I draw the line at a cycle collector. (So e.g. Python is GC'd, but Rust/C++/Swift are not.)
cmrdporcupine
I consider reference to be garbage collection, and so do most CS textbooks. However Rc/Arc/shared_ptr are GC facilities used (often sparingly) inside predominantly non-GC'd languages, so, yeah, I wouldn't say Rust "is" or "has" GC. It has facilities for coping with cleanup, both RAII and GC.
cluckindan
Memory safety is one of Rust’s biggest selling points. It’s a bit baffling that this engine would choose to implement unsafe garbage collection.
scratcheee
The obvious use-case for unsafe is to implement alternative memory regimes that don’t exist in rust already, so you can write safe abstractions over them.
Rust doesn’t have the kind of high performance garbage collection you’d want for this, so starting with unsafe makes perfect sense to me. Hopefully they keep the unsafe layer small to minimise mistakes, but it seems reasonable to me.
jeroenhd
Rust also has some nice language features. Even unsafe rust doesn't have the huge "undefined behaviour" surface that languages like C++ still contain.
If I were to write a toy JS runtime in Rust, I'd try to make it as safe as possible and deal with unsafe only when optimization starts to become necessary, but it's not like that's the only way to use Rust.
LtdJorge
That’s the philosophy. Use the less constrained (but still somewhat constrained and borrow checked) unsafe to wrap/build the low level stuff, and expose a safe public API. That way you limit the exposure of human errors in unsafe code to a few key parts that can be well understood and tested.
nicoburns
Even using something as simple as Vec means using `unsafe` code (from the std library). The idea isn't to have no `unsafe` code (which is impossible). It's to limit it to small sections of your code that are much more easily verifiable.
For some use cases, that means that "user code" can have no `unsafe`. But implementing a GC is very much not one of those.
swiftcoder
The whole point of unsafe is to be able to circumvent the guardrails where the developer knows something the compiler isn't (yet) smart enough to understand. It's likely that implementing a high-performance GC runs afoul of quite a few of those edge cases.
the__alchemist
IMO the memory safety aspect is overblown by enthusiasts and purists. Rust is an overall nice fast imperative language.
phplovesong
Rust WAS really nice before it got mangled with syntax like we never seen before. Graydon did not imagine rust as what it is today. Rust core wo. async is ok, but in practice rust projects tend to have hundreds of deps and really slow compiles. Its just like javascript with npm.
Why is stuff written in rust always promoted as "written in rust" like its some magic thing?