That said, today Boa has a whole team of maintainers who I’m sure will answer some questions here.
Yes the name does invoke the sense it’s a Python project but I liked it and stuck with it, I saw a Boa snake at a zoo once and knew I wanted to name my next project after it, I was also inspired by Mozilla at the time who named their projects after animals.
Speaking of Mozilla, Boa’s existence came to be because at the time I was working on Servo and wanted to include an all-rust JS engine, one didn’t really exist so I set about making one as a learning exercise, after around 2 years more joined me on that journey and today Boa is around 8 years old. It is not browser grade (although at 94.12% it is more compliant than some browser engines) but that doesn’t matter, plenty of Rust projects have found good use for it as they find it easy to embed and use, so we’re happy.
Another recent thing which we’re very proud is seeing our implementation of Temporal be used in V8 and other engines, so we’re also helping the wider ecosystem and raising all ships! (More here: https://boajs.dev/blog/2025/09/24/temporal-release)
We do hope to improve performance over the next year or so, hopefully that answers some of the Qs here.
Do you see catching up on performance with v8-jitless as a goal or is conformance the primary goal right now? Any plans on doing a JIT? I was always impressed by the idea of Truffle where you implement the language semantics once and you get both interpreter and JIT safely out of it which is a huge source of vulnerabilities in traditional JIT systems
Hi, I'm another one of the maintainers on the project.
In general, we are shifting more to performance now than conformance. We currently sit at around 94% conformance, so there's not really that much more to go conformance-wise. The remaining conformance gains are a couple of the newer specification features and Intl related features. Our current conformance can be found at https://boajs.dev/conformance.
Regarding performance, we are already making some gains, with hopefully more to come. The best example of this was probably the updates to script-bench-rs with our most recent release (which can be found at this commit https://github.com/khvzak/script-bench-rs/commit/d9635de77d2...). We still obviously have more to improve on, but we have already made some pretty steady progress from where we were.
EDIT: I forgot to answer your question about v8-jitless. Obviously in the future it would be nice to be able to be more competitive with v8-jitless, but at least for me, I'd just like to focus on improving the Boa overall.
Long answer: first, `fetch` is a runtime feature, and Boa is first and foremost an engine. So `boa_engine` -- the core project crate -- does not support `fetch` out of the box.
That being said, we do have a `boa_runtime` crate. This crate is not currently a full runtime, but it is a collection of runtime features that have been implemented and can be registered onto the context. `fetch` is one of the features that has an implementation completed in `boa_runtime`, and it does use reqwest if I'm remembering correctly. If you're interested to see some example code of registering features, you can look at our CLI code as an example :)
The past year has been huge for conformance for us, not only we caught up with the top engines but we surpassed them when working on Temporal and having all tests pass for that.
We hope to wind down some of the conformance priority now and focus on performance, we need to work on a new GC, refactor some parts of the engine, and improve various areas.
The idea of a JIT has been raised and we’re not against it, but it’s not on our plans right now (because of the above), that being said there is an open discussion.
Right now, we use a forked and modified version of the `gc`. We definitely need to update it. Admittedly, I've been hoping to work on it but got a little distracted with temporal ...
I don't think I've actually heard of rsgc before. It's definitely interesting to see playXE put it together. I know that they'd been working on starlight at one point, so it'd be interesting to see takeaways from it.
To get to your question on the existing GCs, so far the answer is we don't truly know. We really need to do some GC experiments and test different options in Boa with JavaScript. There are not really that many GC crates out there of which I'm aware. There rust-gc's `gc` crate, dumpster, and `arena-gc` (tack on rsgc). But of those, the `gc` crate truly has the best API that we've used in Boa, but the performance is not ideal amongst other optimizations. It would be nice to preserve that API while improving the performance as well. But that remains to be seen.
We're looking into the next release potentially being a v1.0, so maybe that option is a bit more possible after that release. But at the same time, I think Servo spends a decent amount of time maintaining the `mozjs` crate, so who knows.
I can't speak exactly for the other maintainers, but for me, I mostly enjoy working on Boa and Boa related things (I've spent the better part of the last year and a half on the temporal implementation). I think it would be cool to have a highly performant and conformant Rust JavaScript engine. So that's my goal :)
Congrats in general! The Temporal in rust release happening in Chrome & others is so cool! So exiting to see such a core piece of work being done & then shared widely among browsers. Speaks well to Rust, the trust folks have in it, and the ability to use Rust from a variety of projects.
There's also a very lovely talk
Cross-Engine Contributions at Scale: How newcomers accelerated Temporal and Upsert in SpiderMonkey, V8, and Boa that goes deep in depth in how technically Temporal was used across engines.
https://www.youtube.com/watch?v=WieD_9BswAE
Thanks! Temporal_rs has been a really fun project to work on, and it's been great to see that it's useful for other engines!
Hopefully, there will be more chances in the future for projects like temporal_rs. Beyond just temporal_rs, I think the Temporal integration in V8 and Kiesel was a good proof of concept for Rust based libraries over FFI using Diplomat.
Nice to see another contender in this space. If OP is here, can you comment on runtime sandboxing and interop support? Can I selectively disable (or intercept) certain features like network support?
Just to note, Boa is an JavaScript engine / interpreter. We're not a runtime, but we do already have some runtime features implemented in the `boa_runtime` crate, but I don't believe that includes any sandboxing as of right now. It's not even a full fledged runtime ... yet (there's been some discussion about expanding it to a full fledged runtime if anyone has a general interest to work on it). For now, `boa_runtime` is just a collection of features for users to potentially use and if they'd like to implement more, then they are totally welcome to.
I am not meant to be harsh, but note that it fails on a small number of test cases, on v0.21 that is ~900 out of ~50k. Strictly speaking it cannot be described as standard-comforming unless there is some reason behind every failed test. A better way to strive on standard conformance, like QuickJS takes, is to pin down the ecma262 revision and make it 100% compliant.
That may be fair. The better wording may even be specification compliant, but none of the Boa maintainers made this post (we found out about it after the fact).
We, the maintainers, could probably sit down some day and try to triage the failures into a documented txt file, but the overhead on that might make it hard to maintain for an open source project. Plus a large amount of the failures are just new specification features that nobody has implemented yet.
There's typically a pretty big difference between an interpreter meant to be embeddable and one that's not. Trying to embed V8 and keep up with V8 API changes would be a huge amount of work. I could see myself using something like this instead of Lua for some projects where V8 would be too much.
My first thought was that this could be interesting for yt-dlp?
Likely similar to something like https://github.com/mlua-rs/mlua - but wanting to execute javascript (instead of lua, and no static libs?) in the context of native rust.
This mad man had the courage to present BOA a rust project at JS Conf. The project had it's spotlight taken by Bun and Deno. I also think the project was progressing pretty slowly from what people were expecting.
Well the first two are runtimes built on top of JavaScriptCore and V8, respectively. So we're definitely in a different space.
QuickJS/QuickJS-NG might be a better comparison, but I think they are limited in specification conformance or at least selective in specification conformance in favor of remaining in a single file and fast. For instance, I'm not entirely sure whether they will be supporting Temporal once it goes Stage 4 because of the size of the feature, and I don't think they support Intl. But I also can't speak for QuickJS.
With projects like this competing against well known massive competitors (eg. the browser JS engines), not seeing their main competitors in a benchmark is a massive red flag to me: https://boajs.dev/benchmarks
Not seeing V8, SpiderMonkey JavaScriptCore is very strange...
Personally, I'm more impressed with https://github.com/Hans-Halverson/brimstone - it is faster, nearly just as full featured (almost full ES2025) and last but not least, a single person project.
Well, it is, because V8 is definitely an embeddable JS engine. For many people they might want to make a choice between V8 and Boa and for them the performance numbers are important information!
People who need an extremely high performance JavaScript engine, where the extra performance is worth using an engine that's hard to embed, has an unstable API, and an absolutely massive unwieldy Google-style C++ code base with all the pain that entails, plus a JIT and all the limitations that entails, JITed V8 is the right choice.
People who just want to run JavaScript code where performance isn't such a big concern would prefer something like Boa (or the other engines listed on the comparison benchmark page).
Both have their uses, and their use case is almost entirely non-overlapping. You wouldn't choose Boa for a competitive web browser engine or as the runtime for your back-end server software. You would consider it for a plug-in system, or maybe a game's scripting system.
It isn't, v8 will have anywhere between 2-1000x performance depending on the exact code it's jitting. Boa absolutely destroys v8 here though:
use boa_engine::{Context, Source, JsResult};
fn main() -> JsResult<()> {
let js_code = r#"
let two = 1 + 1;
let definitely_not_four = two + "2";
definitely_not_four
"#;
// Instantiate the execution context
let mut context = Context::default();
// Parse the source code
let result = context.eval(Source::from_bytes(js_code))?;
println!("{}", result.display());
Ok(())
}
Huh, I just noticed thanks to this that the comment needs to be updated in the example lol it should probably say "Parse and evaluate the source code".
Is it though? V8 and spider monkey both have jits so performance numbers of the form “wow V8 is vastly faster than any of these other ones!” (similarly for sm). Does that really have any value?
I’m the creator of Boa, you can catch my talk about it at JS Conf EU 2019 https://www.youtube.com/watch?v=_uD2pijcSi4
That said, today Boa has a whole team of maintainers who I’m sure will answer some questions here.
Yes the name does invoke the sense it’s a Python project but I liked it and stuck with it, I saw a Boa snake at a zoo once and knew I wanted to name my next project after it, I was also inspired by Mozilla at the time who named their projects after animals.
Speaking of Mozilla, Boa’s existence came to be because at the time I was working on Servo and wanted to include an all-rust JS engine, one didn’t really exist so I set about making one as a learning exercise, after around 2 years more joined me on that journey and today Boa is around 8 years old. It is not browser grade (although at 94.12% it is more compliant than some browser engines) but that doesn’t matter, plenty of Rust projects have found good use for it as they find it easy to embed and use, so we’re happy.
One recent example is Biome who use it for their plugin infrastructure. https://github.com/biomejs/biome/pull/7300
Another recent thing which we’re very proud is seeing our implementation of Temporal be used in V8 and other engines, so we’re also helping the wider ecosystem and raising all ships! (More here: https://boajs.dev/blog/2025/09/24/temporal-release)
We do hope to improve performance over the next year or so, hopefully that answers some of the Qs here.
In general, we are shifting more to performance now than conformance. We currently sit at around 94% conformance, so there's not really that much more to go conformance-wise. The remaining conformance gains are a couple of the newer specification features and Intl related features. Our current conformance can be found at https://boajs.dev/conformance.
Regarding performance, we are already making some gains, with hopefully more to come. The best example of this was probably the updates to script-bench-rs with our most recent release (which can be found at this commit https://github.com/khvzak/script-bench-rs/commit/d9635de77d2...). We still obviously have more to improve on, but we have already made some pretty steady progress from where we were.
EDIT: I forgot to answer your question about v8-jitless. Obviously in the future it would be nice to be able to be more competitive with v8-jitless, but at least for me, I'd just like to focus on improving the Boa overall.
I immediately have use for this if y'all have async fetch support. I can use JavaScript as an extension language for our app.
I love how supremely easy it looks to embed in normal Rust apps without a complicated build script. That, to me, is a killer feature.
Really awesome work on this!
[1] Wreq is starting to get popular for its ability to bypass Cloudflare and look like a normal browser
Long answer: first, `fetch` is a runtime feature, and Boa is first and foremost an engine. So `boa_engine` -- the core project crate -- does not support `fetch` out of the box.
That being said, we do have a `boa_runtime` crate. This crate is not currently a full runtime, but it is a collection of runtime features that have been implemented and can be registered onto the context. `fetch` is one of the features that has an implementation completed in `boa_runtime`, and it does use reqwest if I'm remembering correctly. If you're interested to see some example code of registering features, you can look at our CLI code as an example :)
Thank you!
We hope to wind down some of the conformance priority now and focus on performance, we need to work on a new GC, refactor some parts of the engine, and improve various areas.
The idea of a JIT has been raised and we’re not against it, but it’s not on our plans right now (because of the above), that being said there is an open discussion.
I don't think I've actually heard of rsgc before. It's definitely interesting to see playXE put it together. I know that they'd been working on starlight at one point, so it'd be interesting to see takeaways from it.
To get to your question on the existing GCs, so far the answer is we don't truly know. We really need to do some GC experiments and test different options in Boa with JavaScript. There are not really that many GC crates out there of which I'm aware. There rust-gc's `gc` crate, dumpster, and `arena-gc` (tack on rsgc). But of those, the `gc` crate truly has the best API that we've used in Boa, but the performance is not ideal amongst other optimizations. It would be nice to preserve that API while improving the performance as well. But that remains to be seen.
Someone on our last release thread on Reddit (https://www.reddit.com/r/rust/comments/1odnore/boa_0210_rele...) mentioned adding Boa to Blitz, which I think would be interesting to see as well.
We're looking into the next release potentially being a v1.0, so maybe that option is a bit more possible after that release. But at the same time, I think Servo spends a decent amount of time maintaining the `mozjs` crate, so who knows.
I can't speak exactly for the other maintainers, but for me, I mostly enjoy working on Boa and Boa related things (I've spent the better part of the last year and a half on the temporal implementation). I think it would be cool to have a highly performant and conformant Rust JavaScript engine. So that's my goal :)
I also really enjoyed your earlier deep dive blog posts on implementation of Temporal, https://boajs.dev/blog/2025/06/15/temporal-impl-1
There's also a very lovely talk Cross-Engine Contributions at Scale: How newcomers accelerated Temporal and Upsert in SpiderMonkey, V8, and Boa that goes deep in depth in how technically Temporal was used across engines. https://www.youtube.com/watch?v=WieD_9BswAE
Hopefully, there will be more chances in the future for projects like temporal_rs. Beyond just temporal_rs, I think the Temporal integration in V8 and Kiesel was a good proof of concept for Rust based libraries over FFI using Diplomat.
Just to note, Boa is an JavaScript engine / interpreter. We're not a runtime, but we do already have some runtime features implemented in the `boa_runtime` crate, but I don't believe that includes any sandboxing as of right now. It's not even a full fledged runtime ... yet (there's been some discussion about expanding it to a full fledged runtime if anyone has a general interest to work on it). For now, `boa_runtime` is just a collection of features for users to potentially use and if they'd like to implement more, then they are totally welcome to.
We, the maintainers, could probably sit down some day and try to triage the failures into a documented txt file, but the overhead on that might make it hard to maintain for an open source project. Plus a large amount of the failures are just new specification features that nobody has implemented yet.
My first thought was that this could be interesting for yt-dlp?
The alternative might be https://github.com/denoland/rusty_v8 but without needing C++ V8.
(this is the first I'm hearing of Boa)
QuickJS/QuickJS-NG might be a better comparison, but I think they are limited in specification conformance or at least selective in specification conformance in favor of remaining in a single file and fast. For instance, I'm not entirely sure whether they will be supporting Temporal once it goes Stage 4 because of the size of the feature, and I don't think they support Intl. But I also can't speak for QuickJS.
I'm always on the lookout for embeddable JS engines.
How hard would it be to make Boa deterministic?
Like, with seeded randomness, etc.
Not seeing V8, SpiderMonkey JavaScriptCore is very strange...
They do compare with JIT-less V8 and SpiderMonkey there, just JSC is missing.
I recently did my own benchmarking with a lot more engines, including other Rust engines: https://ivankra.github.io/javascript-zoo/?v8=true
Personally, I'm more impressed with https://github.com/Hans-Halverson/brimstone - it is faster, nearly just as full featured (almost full ES2025) and last but not least, a single person project.
Though they aren't really competing on anything as far as I can tell, so maybe calling it a "similar project" is more fitting.
People who just want to run JavaScript code where performance isn't such a big concern would prefer something like Boa (or the other engines listed on the comparison benchmark page).
Both have their uses, and their use case is almost entirely non-overlapping. You wouldn't choose Boa for a competitive web browser engine or as the runtime for your back-end server software. You would consider it for a plug-in system, or maybe a game's scripting system.
I have to check this out.
This is awesome. You literally just sold me on it.
That is useful information! Benchmarks can show you this useful information.