Contributing

Hello, welcome and thank you for showing interest in contributing to Nova! Contributions are absolutely welcome, wanted, and dearly desired. That being said, before you being, you'll want to know a few things.

@aapoalas will likely leave a lot of PR comments. He's not doing it to be evil or anything, he's just an idiot.
Nova's code follows the ECMAScript specification. When in doubt, read, copy, implement the specification.
Testing is mainly based upon the test262 conformance suite. Updating conformance results needs to be done.

More information is found below.

Pull Request Code of Conduct

The following ground rules should be followed:

Be courteous.
Be respectful.
Be your best self, and assume the best of everyone.

Feel free to also assume Rust Code of Conduct.

Use Conventional Commits

The initial commit of a PR branch and the PR title should follow Conventional Commits. Consider also using the same logic in branch names. The commonly used prefixes here are fix, feat, and chore. Other recommended ones like build, perf, and refactor are of course good as well.

Scoping is also recommended, but is not currently clearly defined. Some examples are:

feat(ecmascript): This is an added feature to the spec-completeness of the engine, eg. a new abstract operation, heap data object or such.
fix(heap): This fixes something in the heap implementation, eg. maybe the heap garbage collection.
feat(vm): This adds to the interpreter.
chore(cli): This might bump a dependency in the nova_cli crate.

Use Conventional Comments

When reviewing PRs, use conventional comments to plainly describe your intention. Prefixing a comment with issue: means that you do not think the PR can be merged as-is and needs fixing. needs (nonblocking): means that you think there is a problem in the code, but it can (possibly should) be fixed as a followup or at a later date, and does not block merging. nitpick: is your personal hobby-horse, question: is pure curiosity or not understanding the code, etc.

Also: Whenever possible, give praise:! Praising the code and hard work of others makes you feel good, and probably makes them feel good as well. Even if it's a minor thing, like someone drive-by fixing that typo that bugged you the other day, or cleaning up a weird construct into something a bit nicer, or whatever: Even if it's not directly related to the focal point of the PR, praise the work others do.

By all this it goes to say: When someone gives you praise:, they mean it. When someone marks down an issue: they do not mean that your code is bad, they just mean that there's something there to improve.

Align with the ECMAScript specification

Nova's code and folder structure follows the ECMAScript specification as much as possible. This means that when you need to implement a new abstract operation from the specification, your best course of action is generally to copy the specification text as a comment into the file inside the nova_vm/src/ecmascript folder that is the closest match to the specification's header structure for that abstract operation. If a matching file clearly does not exist, you may create one. When in doubt, ask in Discord or open an issue on GitHub.

Once you've copied the specification text in as a comment, you can start turning the commented out abstract operation into a function. Start by changing the header and comment part of the abstract operation into a doc comment and make the header a level three header with a link to the original specification. Preferably, wrap the comment part into multiple lines (generally 80 is used as the line width point). Finally, remove the repetitive and redundant "It performs the following steps when called:" part.

As an example, this:

// 7.1.2 ToBoolean ( argument )
//
// The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:
//
// 1. If argument is a Boolean, return argument.
// 2. If argument is one of undefined, null, +0𝔽, -0𝔽, NaN, 0ℤ, or the empty String, return false.
// 3. NOTE: This step is replaced in section B.3.6.1.
// 4. Return true.

turns into

/// ### [7.1.2 ToBoolean ( argument )](https://tc39.es/ecma262/#sec-toboolean)
///
/// The abstract operation ToBoolean takes argument argument (an ECMAScript
/// language value) and returns a Boolean. It converts argument to a value of
/// type Boolean.
//
// 1. If argument is a Boolean, return argument.
// 2. If argument is one of undefined, null, +0𝔽, -0𝔽, NaN, 0ℤ, or the empty String, return false.
// 3. NOTE: This step is replaced in section B.3.6.1.
// 4. Return true.

Now after the doc comment part, add a function of the same name (using Rust's preferred casing) and move the abstract operation steps inside the function's body. Consider adding a todo!() at the end of the function to avoid Rust yelling at you a whole lot about your return value. Usually pub(crate) is the preferred publicity level, but go down to private if you can.

The first argument to Nova's functions is usually a &mut Agent or &Agent reference. A &mut Agent call can mutate the JavaScript heap, which also means it can call JavaScript. A &Agent call cannot mutate the heap and thus cannot call JavaScript, but can access data from the heap. A call that takes neither operates entirely on stack-local data. Other arguments should be named as the specification suggests.

In our example, from an overabundance of caution, we assume we'll need access to the heap and thus take an &Agent reference.

/// ### [7.1.2 ToBoolean ( argument )](https://tc39.es/ecma262/#sec-toboolean)
///
/// The abstract operation ToBoolean takes argument argument (an ECMAScript
/// language value) and returns a Boolean. It converts argument to a value of
/// type Boolean.
pub(crate) fn to_boolean(agent: &Agent, argument: Value) -> bool {
    // 1. If argument is a Boolean, return argument.
    // 2. If argument is one of undefined, null, +0𝔽, -0𝔽, NaN, 0ℤ, or the empty String, return false.
    // 3. NOTE: This step is replaced in section B.3.6.1.
    // 4. Return true.
    todo!()
}

After this, go step by step implementing the abstract operation. Note that the specification text is still not the bible, and you are allowed to take certain liberties with it. Specifically, you may do any and all of the following with some conditions:

Add fast-paths for common cases.
Move steps around.
Skip steps that are performed elsewhere or guaranteed to always do the same thing.

The condition for doing any of these is that the change must not be observable to a user. For example, if the specification states to initially set the values of a struct to all undefined, and then define them one at a time without calling any JavaScript (press the u key on the ECMAScript specification website to see where JavaScript may get called) then you may instead define the entire struct all-at-once and potentially avoid supporting undefined fields (Option<T>) entirely.

If the specification tells you to assert that a particular parameter is of a specification type T and Nova's implementation already guarantees that type through the function's type signature, you may ignore the assertion entirely. Consider adding a comment to explain that the type signature guarantees the assertion.

If the specification tells you to first get the value of an immutable type (string, boolean, number, symbol), or to prepare a value's data for creation (such as creating a new String), and then performs checks to see if all parameters are correct (throwing errors if incorrect), then you may freely reorder the value getter and/or the data creation to happen after the checks.

If the specification tells you to first check one parameter value (and throws an error if incorrect), then conditionally performs some JavaScript function call, and then checks another parameter value and throws an error if incorrect then you are not allowed to reorder the second check to happen together with the first, as that would be an observable change.

Tests in PRs

Nova mainly tests itself using the official ECMAScript test262 conformance suite. The expected results are saved into the tests/expectations.json file: All PR branches are tested to check that the test results match the expectations.

When making changes, you'll thus often need to update or at least check the expectations. You can do this with the following command:

cargo build --profile release && cargo run --bin test262 --profile release -- -u

This will build you a release version of Nova, and run the test262 conformance suite using that executable. At the end of the run, it will record the results in expectations.json.

You can run an individual test262 test case or a set of tests using

cargo build && cargo run --bin test262 eval-test internal/path/to/tests

Here the "internal/path/to/tests" matches a path or a subpath in the expectations.json. As an example:

cargo build && cargo run --bin test262 eval-test built-ins/Array/from/from-string.js

We also have some unit and integration test around using cargo's test harnesses. Adding to these is absolutely welcome, as they enable more Miri testing etc. These are also run on all PRs.

Custom quick-shot tests

Keep your own test.js in the nova folder and run it with

cargo run eval test.js

This is great for quick, simple things you want to test out in isolation.

Performance considerations

The engine is not at the point where performance is a big consideration. That being said, we do not want to write slow-by-construction code. Heap data clones should be kept to a minimum where reasonable, and fast-paths for the most common cases are highly recommended in abstract operations.

That being said, we do not have performance metrics at present. Therefore, it is also not a supremely important or reasonable thing to require code to be supremely optimal since we cannot prove it one way or the other.

List of active development ideas

Here are some good ideas on what you can contribute to.

Internal methods of exotic objects

ECMAScript spec has a ton of exotic objects. Most of these just have some extra internal slots while others change how they interact with user actions like get-by-identifier or get-by-value etc.

You can easily find exotic objects' internal methods by searching for "fn internal_get_prototype_of(" in the code base. Many of these matches will be in files that contain a lot of todo!() points. As an example, proxy.rs is currently entirely unimplemented. The internal methods of Proxies can be found here: These abstract internal methods would need to be translated into Nova Rust code in the proxy.rs file.

This PR can perhaps also serve as a good guide into how internal methods are implemented: Especially check the first and third commits. One important thing for internal method implementations is that whenever a special implementation exists in the spec, our internal method should link to it. Another thing is that if you cannot figure out what you should be calling in the method or the method you should be calling doesn't exist yet and you think implementing it would be too much work, it is perfectly fine to simply add a todo!() call to punt on the issue.

Builtin functions

Even more than internal methods, the ECMAScript spec defines builtin functions. The Nova engine already includes bindings for nearly all of them (only some Annex B functions should be missing) but the bindings are mostly just todo!() calls.

Implementing missing builtin functions, or at least the easy and commonly used parts of them, is a massive and important effort. You can find a mostly exhaustive list of these (by constructor or prototype, or combined) in the GitHub issue tracker.

Heap evolution

The heap needs much more work before it can be considered complete. Technical work items like the heap evolution works can be found from the GitHub issues with the technical label.

Parallel, single-mutator-multiple-reader vectors

The heap will need to be concurrently marked at some point. Additionally, we'll want to split some heap data structures into two or more parts; only the commonly used parts should be loaded into L1 cache during common engine operations.

For this purpose we'll need our own Vec, Vec2, Vec3 and possibly other vector types. The first order of business is to get the length and capacity to be stored as a u32. The second will be enabling the splitting of heap data structures; this sbould work in a way similar to ParallelVec so that the size of Vec2 and Vec3 stays equal to Vec.

Then finally, at some point we'll also want to make the whole heap thread-safe. Heap vectors (Vec, Vec2, ...) will become RCU-based, so when they expand (on push) they will return a None or Some(droppable_vec) which can either be dropped immediately (if concurrent heap marking is not currently ongoing) or pushed into a "graveyard" UnsafeCell<Vec<(*mut (), fn(*mut ()))>> that gets dropped at the end of a mark-and-sweep iteration.

Interleaved garbage collection

Currently Nova's garbage collection can only happen when no JavaScript is running. This means that for instance a dumb live loop like this will eventually exhaust all memory:

while (true) {
  ({});
}

We want to interleave garbage collection together with running JavaScript, but this is not a trivial piece of work. Right now calls in the engine look like this:

fn call(agent: &mut Agent, mut gc: Gc<'_, '_>,  value: Value) -> Value {
    // ...
}

For interleaved work, we'd want to ensure that either:

Value is safe to keep on stack. This means changing these to be Local<'a, Value>s that have an extra level of indirection between the stack and the heap, and the garbage collection can access that extra indirection.
Value cannot be used after a potential garbage collection point. This would mean adding a lifetime to Value that is bound to the &'a mut lifetime. The Rust borrow checker will stand in opposition to us here.

Some additional thoughts on the two approaches is found below.

`Local<'a, Value>`

Any problem can always be fixed by adding an extra level of indirection. In this case the problem of "where did you put that Value, is it still needed, and can I mutate it during garbage collection?" can be solved by adding a level of indirection. In V8 this would be the HandleScope. The garbage collector would be given access to the HandleScope's memory so that it can trace items "on the stack" and fix them to point to the proper items after garbage collection.

This would be the easiest solution, as this could optionally even be made to work in terms of actual pointers to Values. That would be a pretty poor solution, honestly, but it would work. One of the biggest downsides with this approach would be that we'd need not only Local<'a, Value> but also Local<'a, Object> and Local<'a, Array> and so on and so forth. This is effectively a second layer of Value definitions on top of the first.

If the Local<'a, Value> is not pointer based, then another downside is that we cannot drop the Local values automatically once they're no longer needed using impl Drop because we'd need access to the HandleScope inside the Drop. Something called linear types could fix this issue.

`Value` lifetime bound to garbage collection safepoints

Any problem can always be fixed by adding an extra lifetime. In this case the problem of "you're not allowed to keep that Value on stack, I would need to mutate it during garbage collection" can be solved by using a lifetime to make sure that Values are never on the stack when garbage collection might happen. This isn't too hard, really, it just means calls change to be:

fn call<'a>(agent: &'a mut Agent, value: Value<'a>) -> Value<'a> {
    // ...
}

This works perfectly well, except for the fact that it cannot be called. Why? Because the Value<'a> borrows the exclusively owned &'a mut Agent lifetime; this is called a reborrow and it's fine within a function but it cannot be done intra-procedurally. What we could do is this:

fn call(agent: &mut Agent, mut gc: Gc<'_, '_>,  value: Register<Value<'_>>) -> Register<Value<'_>> {
    // SAFETY: We've not called any methods that take `&mut Agent` before this.
    // `Value` is thus still a valid reference.
    let value = unsafe { value.bind(agent) };
    // ...
    result.into_register()
}

Now we can at least call the function, and lifetimes would protect us from keeping Value<'a> on the stack unsafely. They would not help us with making sure that Register<Value<'a>> is used properly and even if it did, the whole Register<Value<'a>> system is fairly painful to use as each function call would need to start with this unsafe {} song and dance.

But what about when we call some mutable function and need to keep a reference to a stack value past that call? This is how that would look:

fn call(agent: &mut Agent, mut gc: Gc<'_, '_>,  value: Register<Value<'_>>) -> JsResult<Register<Value<'_>>> {
    let value = unsafe { value.bind(agent) };
    let kept_value: Global<Value<'static>> = value.make_global(value);
    other_call(agent, gc.reborrow(), value.into_register())?;
    let value = kept_value.take(agent);
    // ...
}

We'd need to make the Value temporarily a Global (which introduces an extra level of indirection), and then "unwrap" that Global after the call. Globals do currently exist in Nova, but they are "leaky" in that dropping them on the stack does not clear their memory on the heap, and is effectively a heap memory leak. In this case we can see that if other_call returns early with an error, then we accidentally leak kept_value's data. This is again not good.

So we'd need a Local<'a, Value<'_>> type of indirection in this case as well. Whether or not the whole Register<Value<'_>> system makes any sense with that added in is then very much up for debate.

Other things

This list serves as a "this is where you were" for returning developers as well as a potential easy jumping-into point for newcomers.

Write implementations of more abstract operations
- See nova_vm/src/ecmascript/abstract_operations
- Specifically eg. operations_on_objects.rs is missing multiple operations, even stubs.

Some more long-term prospects and/or wild ideas:

Reintroduce lifetimes to Heap if possible
- Value<'gen> lifetime that is "controlled" by a Heap generation number: Heap Values are valid while we can guarantee that the Heap generation number isn't mutably borrowed. This is basically completely equal to a scope based Local<'a, Value> lifetime but the intended benefit is that the Value<'gen> lifetimes can also be used during Heap compaction: When Heap GC and compaction occurs it can be written as a transformation from Heap<'old> to Heap<'new> and the borrow checker would then help to make sure that any and all T<'new> structs within the heap are properly transformed to T<'new>.
Add a Local<'a, Value> enum that acts as our GC-safe, indirected Value storage. See above for more discussion on this under "Heap evolution".
- A plain Value won't be safe to keep on the stack when calling into the engine if the engine is free to perform garbage collection at (effectively) any (safe)point within the engine code. The Value's internal number might need to be readjusted due to GC, which then breaks the Value's identity in a sense.
- A Local<'a, Value>s would not point directly to the heap but would instead point to an intermediate storage (this is also exactly how V8 does it) where identities never change. A nice benefit here is that if we make Local itself an equivalent enum to Value, just with a different index type inside, then we can have the intermediate storage store only heap value references with 5 bytes each: struct Storage<const N: usize> { types: [u8; N]; values: [u32; N]; }. We cannot drop the types array as it is needed for marking and sweeping the storage.
Add DISCRIMINANT + 0x80 variants that work as thrown values of type DISCRIMINANT
- As a result, eg. a thrown String would be just a String with the top bit set true. This would stop Result usage which is a darn shame (can be fixed with Nightly features). But it would mean that returning a (sort of) Result<Value> would fit in a register.
Consider a register based VM instead of going stack based

Running the test262 suite

Clone this repository with submodules:

git clone --recurse-submodules git@github.com:trynova/nova.git
Execute the test262 runner:

cargo build -p nova_cli && cargo run --bin test262

Important: The test runner executes the compiled nova_cli directly. If you have made changes to CLI or VM, ensure that the nova_cli target is rebuilt before executing the test runner.