Boa release v0.21 – a new release of Boa, a JavaScript engine written in Rust

4 days ago 1

Summary​

Boa v0.21 is now available! After 9 months of development we are very happy to present you the latest release of the Boa JavaScript engine. Boa makes it easy to embed a JS engine in your projects, and you can even use it from WebAssembly. See the about page for more info.

In this release, our conformance has grown from 89.92% to 94.12% in the official ECMAScript Test Suite (Test262). Our growth in conformance is driven by increased conformance for Temporal (discussed further below) with the rest of the development effort being focused on performance, internal improvements, and runtime features. We will continue to implement more of the specification; however, as of the current moment, Boa's conformance aligns with the major browser engine's conformance, so future increases in conformance will be minor or bound to the feature size going forward.

You can check the full list of changes here, and the full information on conformance here.

Feature Highlights​

Temporal​

There has been a lot of progress made on Temporal, the new Stage 3 date/time proposal. With this release, Boa's conformance on Temporal grew from 40.67% to ~97%. This implementation is backed by the temporal_rs date/time Rust library, which we went over in our announcement blog post. Give the post a read if you are interested in temporal_rs and its development history.

Span nodes and error backtraces​

We added support for storing spans in our AST nodes, which allows determining the exact location of an AST node on its original file. We already kind of supported this feature in our lexer, but we did not store the spans after parsing.

Why is this important? Well, as a direct result from this, Boa now supports error backtraces when an exception is thrown!

backtrace-example

As an additional plus, you can enable the native-backtrace feature to include "native" functions on a backtrace.

native-backtrace-example

This feature has been one of the most requested ones for years, and we hope it will greatly help with debugging errors when using Boa.

Utility macros​

We introduced a new set of proc macros that make it much easier to create JsValues, JsObjects, Classes and even Modules. Let's see them in action!

js_value​


assert_eq!(js_value!( 1 ), JsValue::from(1));
assert_eq!(js_value!( false ), JsValue::from(false));


assert_eq!(js_value!([ 1, 2, 3 ], context).display().to_string(), "[ 1, 2, 3 ]");
assert_eq!(
 js_value!({
"key": (js_string!("value"))
 }, context).display().to_string(),
 "{\n key: \"value\"\n}",
);

js_object​

let object = js_object!({
"key": "value",
alsoKey: 1,

[1 + 2]: (),
}, context);

assert_eq!(
 JsValue::from(object).display().to_string(),
 "{\n 3: null,\n key: \"value\",\n alsoKey: 1\n}"
);

boa_class​




#[derive(Clone, Trace, Finalize, JsData)]
enum AnimalType {
 Cat,
 Dog,
 Other,
}

#[derive(Clone, Trace, Finalize, JsData)]
struct Animal {
 ty: AnimalType,
 age: i32,
}

#[boa_class]
impl Animal {
#[boa(constructor)]
 fn new(name: String, age: i32) -> Self {
 let ty = match name.as_str() {
 "cat" => AnimalType::Cat,
 "dog" => AnimalType::Dog,
 _ => AnimalType::Other,
 };

 Self { ty, age }
 }
fn speak(#[boa(error = "`this` was not an animal")] &self) -> JsString {
 match self.ty {
 AnimalType::Cat => js_string!("meow"),
 AnimalType::Dog => js_string!("woof"),
 AnimalType::Other => js_string!(r"¯\_(ツ)_/¯"),
 }
 }

#[boa(method)]
 #[boa(length = 11)]
 fn method(context: &mut Context) -> JsObject {
 let obj = JsObject::with_null_proto();
 obj.set(js_string!("key"), 43, false, context).unwrap();
 obj
 }

#[boa(getter)]
 fn age(&self) -> i32 {
 self.age
 }

#[boa(setter)]
 #[boa(method)]
 #[boa(rename = "age")]
 fn set_age(&mut self, age: i32) {
 self.age = age;
 }

#[boa(static)]
 fn marked_static_method() -> i32 {
 123
 }
fn static_method() -> i32 {
 42
 }
}

let mut context = Context::default();


context.register_global_class::<Animal>().unwrap();

context
 .eval(Source::from_bytes(
 r#"
 let pet = new Animal("dog", 3);
 console.log(pet.age) // 3
 console.log(Animal.staticMethod()) // 42
 console.log(Animal.markedStaticMethod()) // 123

 v = pet.method();
 console.log(v.key) // 43

 pet.age = 4;
 console.log(pet.age) // 4
 "#,
 )).unwrap();

boa_module​


#[boa_module]
mod hello {
 use boa_engine::{JsString, js_string};
fn world() -> JsString {
 js_string!("hello world")
 }

type Animal = super::Animal;
const SOME_LITERAL_NUMBER: i32 = 1234;
#[boa(rename = "this_is_different")]
 const SOME_OTHER_LITERAL: i32 = 5678;
}



let module_loader = Rc::new(MapModuleLoader::new());
let mut context = Context::builder()
 .module_loader(module_loader.clone())
 .build()
 .unwrap();


module_loader.insert("/hello.js", hello::boa_module(None, &mut context));

let module = Module::parse(
 Source::from_bytes(
 r#"
 import * as m from '/hello.js';

 console.log(m.someLiteralNumber) // 1234
 console.log(m.this_is_different) // 5678

 console.log(m.world()) // "hello world"

 let pet = new m.Animal("dog", 8);
 console.log(pet.age) // 8
 console.log(pet.speak()) // "woof"
 "#,
 ),
 None,
 &mut context,
).unwrap();

let result = module
 .load_link_evaluate(&mut context)
 .await_blocking(&mut context)
 .unwrap();

Async APIs enhancements​

Historically, hooking functions returning a Future into Boa has been one of the biggest pain points of our API. This was mostly caused by how we defined FutureJob:

pub struct NativeJob {
 f: Box<dyn FnOnce(&mut Context) -> JsResult<JsValue>>,
 realm: Option<Realm>,
}

pub type FutureJob = Pin<Box<dyn Future<Output = NativeJob> + 'static>>;

With this definition, it was pretty much impossible to capture the Context inside the Future, and functions that needed to interweave engine operations with awaiting Futures needed to be split into multiple parts:

let fetch = async move {
 let body: Result<_, isahc::Error> = async {
 let mut response = Request::get(&url)
 .body(())?
 .send_async()
 .await?;

 Ok(response.text().await?)
 }
 .await;

NativeJob::new(move |context| -> JsResult<JsValue> {
 parse(body).await;
Ok(JsValue::undefined())
 })
};



context
 .job_queue()
 .enqueue_future_job(Box::pin(fetch), context);

We wanted to improve this API, and the solution we thought about was to make Context shareable by wrapping it using RefCell. However, this proved to be very difficult for two reasons:

  1. We needed to change all definitions to take &RefCell<Context> instead of &mut Context, which meant changing pretty much the whole codebase.
  2. Some of our VM code was reentrant, and that would cause panics in the reentrant parts of the code when calling RefCell::borrow_mut; we would need to patch up the engine to remove the reentrancy.

After putting a lot of thought on this, we came up with a really nice solution; instead of wrapping Context with RefCell, we would wrap &mut Context with RefCell, and only on the async-related APIs. This would allow not only capturing the context on Future-related functions, but also doing this without having to refactor big parts of the code. Thus, we ditched FutureJob and introduced a new type of job: NativeAsyncJob.





#[allow(clippy::type_complexity)]
pub struct NativeAsyncJob {
 f: Box<dyn for<'a> FnOnce(&'a RefCell<&mut Context>) -> BoxedFuture<'a>>,
 realm: Option<Realm>,
}

With this change, any API that integrates with Future can additionally capture the &RefCell<&mut Context> to run engine-related operations after awaiting on a Future.

Revamped JobQueue​

After introducing the new job type, changes had to be made on JobQueue to better support new types of jobs. Thus, JobQueue was revamped and renamed to be the new JobExecutor:






pub trait JobExecutor: Any {





fn enqueue_job(self: Rc<Self>, job: Job, context: &mut Context);
fn run_jobs(self: Rc<Self>, context: &mut Context) -> JsResult<()>;



async fn run_jobs_async(self: Rc<Self>, context: &RefCell<&mut Context>) -> JsResult<()>
 where
 Self: Sized,
 {
 self.run_jobs(&mut context.borrow_mut())
 }
}

As you can probably tell, we made a lot of changes on JobExecutor:

  • All methods now take Rc<Self> as their receiver, making it consistent with how the Context itself stores the JobExecutor.
  • enqueue_promise_job and enqueue_future_job now are unified in a single enqueue_job, where Job is an enum containing the type of job that needs to be scheduled. This makes it much simpler to extend the engine with newer job types in the future, such as the newly introduced TimeoutJob and GenericJob types.
  • run_jobs_async was converted to a proper async function, and excluded from JobExecutor's VTable. Additionally, this method now takes a &RefCell<&mut Context> as its context, which is the missing piece that enables sharing the Context between multiple Futures at the same time. This, however, means that we cannot provide a convenient wrapper such as Context::run_jobs for it anymore, which is one of the reasons why we decided to exclude that method from JobExecutor's VTable.

These changes not only made JobExecutor much simpler, but it also expanded the places where we could use its async capabilities to handle "special" features of ECMAScript that are more suited to an async way of doing things. ModuleLoader is one of those places.

Asyncified ModuleLoader​

Looking at the previous definition of ModuleLoader:

pub trait ModuleLoader {
fn load_imported_module(
 &self,
 referrer: Referrer,
 specifier: JsString,
 finish_load: Box<dyn FnOnce(JsResult<Module>, &mut Context)>,
 context: &mut Context,
 );
fn register_module(&self, _specifier: JsString, _module: Module) { ... }
 fn get_module(&self, _specifier: JsString) -> Option<Module> { ... }
 fn init_import_meta(
 &self,
 _import_meta: &JsObject,
 _module: &Module,
 _context: &mut Context,
 ) { ... }
}

... the weird finish_load on load_imported_module immediately pops up as an anomaly. In this case, finish_load is Boa's equivalent to HostLoadImportedModule ( referrer, moduleRequest, hostDefined, payload ), which is an abstract operation that is primarily used to define how an application will load and resolve a "module request"; think of it as a function that takes the "module-name" from import * as name from "module-name", then does "things" to load the module that corresponds to "module_name".

The peculiarity about this abstract operation is that it doesn't return anything! Instead, it just has a special requirement:

The host environment must perform FinishLoadingImportedModule(referrer, moduleRequest, payload, result), where result is either a normal completion containing the loaded Module Record or a throw completion, either synchronously or asynchronously.

Why expose the hook this way? Well, there is a clue in the previous requirement:

... either synchronously or asynchronously.

Aha! Directly returning from the hook makes it very hard to enable use cases where an application wants to load multiple modules asynchronously. Thus, the specification instead exposes a hook to pass the name of the module that needs to be loaded, and delegates the task of running the "post-load" phase to the host, which enables fetching modules synchronously or asynchronously, depending on the specific requirements of each application.

One downside of this definition, however, is that any data that is required by the engine to properly process the returned module would need to be transparently passed to the FinishLoadingImportedModule abstract operation, which is why the hook also has an additional requirement:

The operation must treat payload as an opaque value to be passed through to FinishLoadingImportedModule.

payload is precisely that data, and it may change depending on how the module is imported in the code; import "module" and import("module") are two examples of this.

We could expose this as an opaque *const () pointer argument and call it a day, but we're using Rust, dang it! and we like statically guaranteed safety! So, instead, we exposed FinishLoadingImportedModule as finish_load, which is a "closure" that captures payload on its stack, and can be called anywhere (like inside a Future) on the application with a proper Module and Context to further continue processing the module loaded by the ModuleLoader.

...
finish_load: Box<dyn FnOnce(JsResult<Module>, &mut Context)>,
...

Unfortunately, this API has downsides:

Fast forward a couple of years and we're now changing big parts of JobExecutor: adding new job types, tinkering with JobExecutor, changing API signatures, etc. Then, while looking at the definition of ModuleLoader, we thought...

Huh, can't we make load_imported_module async now?

And that's exactly what we did. Behold, the new ModuleLoader!

pub trait ModuleLoader: Any {
 async fn load_imported_module(
 self: Rc<Self>,
 referrer: Referrer,
 specifier: JsString,
 context: &RefCell<&mut Context>,
 ) -> JsResult<Module>;

 fn init_import_meta(
 self: Rc<Self>,
 _import_meta: &JsObject,
 _module: &Module,
 _context: &mut Context,
 ) {
 }
}

Then, the code snippet we mentioned before nicely simplifies to:

async fn load_imported_module(
 self: Rc<Self>,
 _referrer: boa_engine::module::Referrer,
 specifier: JsString,
 context: &RefCell<&mut Context>,
) -> JsResult<Module> {
 let url = specifier.to_std_string_escaped();

 let response = async {
 let request = Request::get(&url)
 .redirect_policy(RedirectPolicy::Limit(5))
 .body(())?;
 let response = request.send_async().await?.text().await?;
 Ok(response)
 }
 .await
 .map_err(|err: isahc::Error| JsNativeError::typ().with_message(err.to_string()))?;

 let source = Source::from_bytes(&response);

 Module::parse(source, None, &mut context.borrow_mut())
}

What about synchronous applications?

The advantage of having JobExecutor be the main entry point for any Rust Futures that are enqueued by the engine is that an application can decide how to handle all Futures received by the implementation of JobExecutor. Thus, an application that doesn't want to deal with async Rust executors can implement a completely synchronous ModuleLoader and poll on all futures received by JobExecutor using something like futures_lite::poll_once.

Why not just block on each Future one by one instead?

Well, there is one new built-in that was introduced on this release which heavily depends on "properly" running Futures, and by "properly" we mean "not blocking the whole thread waiting on a future to finish". More on that in a bit.

Built-ins updates​

Atomics.waitAsync​

This release adds support for the Atomics.waitAsync method introduced in ECMAScript's 2024 specification. This method allows doing thread synchronization just like Atomics.wait, but with the big difference that it will return a Promise that will resolve when the thread gets notified with the Atomics.notify method, instead of blocking until that happens.



const sab = new SharedArrayBuffer(1024);
const int32 = new Int32Array(sab);



const result = Atomics.waitAsync(int32, 0, 0, 1000);
result.value.then(() => console.log("waited!"));


Atomics.notify(int32, 0);


Note that this built-in requires having a "proper" implementation of a JobExecutor; again, "proper" in the sense of "not blocking the whole thread waiting on a future to finish", which can be accomplished with FutureGroup and futures_lite::poll_once if an async executor is not required (see SimpleJobExecutor's implementation). This is because it heavily relies on TimeoutJob to timeout if a notification doesn't arrive, and in NativeAsyncJob to communicate with the notifier threads using an async channel. This is the reason why we don't recommend just blocking on each received Future; that could cause TimeoutJobs to run much later than required, or even make it so that they don't run at all!

Set methods​

This release adds support for the new set methods added in ECMAScript's 2025 specification.

The new methods are:

Thanks to @Hemenguelbindi for their work on this feature.

Float16 support for TypedArrays, Dataview and Math built-ins​

This release adds support for f16 types for the TypedArray, Dataview, and Math built-ins.

const x = new Float16Array([37, 42.123456]);
console.log(x[1]);

Error.isError​

This release adds support for Error.isError, which will be introduced in ECMAScript's 2026 specification.

console.log(Error.isError(new Error()));
console.log(Error.isError({ __proto__: Error.prototype }));

Math.sumPrecise​

This release adds support for Math.sumPrecise, which will be introduced in ECMAScript's 2026 specification.

We've opted for using the new xsum summation algorithm for the underlying implementation.

let sum = Math.sumPrecise([1e20, 0.1, -1e20]);
console.log(sum);

Array.fromAsync​

This release adds support for Array.fromAsync, which will be introduced in ECMAScript's 2026 specification.

Array.fromAsync allows to conveniently create a array from an async iterable by awaiting all of the items consecutively.


async function toArray(asyncIterator) {
 const arr = [];
 for await (const i of asyncIterator) arr.push(i);
 return arr;
}

async function* asyncIterable() {
 for (let i = 0; i < 5; i++) {
 await new Promise((resolve) => setTimeout(resolve, 10 * i));
 yield i;
 }
}

Array.fromAsync(asyncIterable()).then((array) => console.log(array));

toArray(asyncIterable()).then((array) => console.log(array));

Boa Runtime​

Work on Boa's runtime crate has continued with additional APIs added.

Additional APIs​

Additional APIs added the the Runtime crate include:

Conformance testing​

We've added some support for conformance testing runtime features against the Web Platform Tests (WPT).

Performance​

NaN Boxing​

With this release, Boa's JsValue will use nan-boxing by default. The NaN boxing of JsValue increased memory and runtime performance over the older enum.

As a note, the current implementation is not compatible with all platforms. While we hope to address this in the future, the legacy enum JsValue will be available via the jsvalue-enum feature flag.

Unfamiliar with NaN Boxing? We won't go over it in depth here, but we recommend this article to learn more.

Register VM​

Boa's virtual machine (VM) moved from a stack based VM to a register based VM in PR #3798.

New Contributors​

Looking Forward​

Garbage collector rewrite​

This has been long overdue. Boa's garbage collector is a forked and modified version of rust-gc, and we have long been pushing our forked gc to its limits.

We have seen some evidence from previous pull requests that simply swapping allocators from Rust's global allocator can increase Boa's performance, and work on this will hopefully resume soon. If you're interested in garbage collectors and/or would be interested in helping out, feel free to join our GC room on Matrix.

Runtime functionality​

The boa_runtime crate was initially meant to contain functionality that was not meant to exist in the core ECMAScript implementation, for instance the console implementation. Noticeably, we have since added more runtime features to the crate with even more features expected in the next release.

Our current plan is if there is enough interest and the crate becomes too large, we will split boa_runtime into it's own repository. If you are interested in implementing any runtime features or contributing to a runtime, feel free to reach out.

Continuing performance improvements​

As mentioned on previous release posts, we will continue to further work on improving Boa's overall performance.

Intl and ECMA402 conformance​

We currently have some general support for the ECMA402 and ECMAScript's Intl object. We will continue to some general work on ECMA402 conformance to allow Boa to be fully usable for internationalization use cases.

How can you support Boa?​

Boa is an independent JavaScript engine implementing the ECMAScript specification, and we rely on the support of the community to keep it going. If you want to support us, you can do so by donating to our open collective. Proceeeds here go towards this very website, the domain name, and remunerating members of the team who have worked on the features released.

If financial contribution is not your strength, you can contribute by asking to be assigned to one of our open issues, and asking for mentoring if you don't know your way around the engine. Our contribution guide should help you here. If you are more used to working with JavaScript or frontend web development, we also welcome help to improve our web presence, either in our website, or in our testing representation page or benchmarks page. You can also contribute to our Criterion benchmark comparison GitHub action.

We are also looking to improve the documentation of the engine, both for developers of the engine itself and for users of the engine. Feel free to contact us in Matrix.

Thank You​

Once again, big thanks to all the contributors of this release!!!

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