Implement Async Generators (#2200)

This Pull Request fixes #1560.

It changes the following:

- Implement `AsyncGeneratorFunction` builtin object.
- Implement `AsyncGenerator` builtin object.
- Implement async generator execution.
- Add some parse errors for async generators.

The `AsyncGenerator.prototype.return()` method currently does not work correctly with `finally` blocks, but I think we should merge this first to not increase the complexity of the pr too much.

boa_engine/src/builtins/async_generator/mod.rs

@@ -0,0 +1,754 @@
+//! This module implements the global `AsyncGenerator` object.
+//!
+//! More information:
+//!  - [ECMAScript reference][spec]
+//!
+//! [spec]: https://tc39.es/ecma262/#sec-asyncgenerator-objects
+
+use crate::{
+    builtins::{
+        generator::GeneratorContext, iterable::create_iter_result_object,
+        promise::if_abrupt_reject_promise, promise::PromiseCapability, BuiltIn, JsArgs, Promise,
+    },
+    object::{ConstructorBuilder, FunctionBuilder, JsObject, ObjectData},
+    property::{Attribute, PropertyDescriptor},
+    symbol::WellKnownSymbols,
+    value::JsValue,
+    vm::GeneratorResumeKind,
+    Context, JsResult,
+};
+use boa_gc::{Cell, Finalize, Gc, Trace};
+use boa_profiler::Profiler;
+use std::collections::VecDeque;
+
+/// Indicates the state of an async generator.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub(crate) enum AsyncGeneratorState {
+    Undefined,
+    SuspendedStart,
+    SuspendedYield,
+    Executing,
+    AwaitingReturn,
+    Completed,
+}
+
+/// `AsyncGeneratorRequest Records`
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorrequest-records
+#[derive(Debug, Clone, Finalize, Trace)]
+pub(crate) struct AsyncGeneratorRequest {
+    /// The `[[Completion]]` slot.
+    pub(crate) completion: (JsResult<JsValue>, bool),
+
+    /// The `[[Capability]]` slot.
+    capability: PromiseCapability,
+}
+
+/// The internal representation on an `AsyncGenerator` object.
+#[derive(Debug, Clone, Finalize, Trace)]
+pub struct AsyncGenerator {
+    /// The `[[AsyncGeneratorState]]` internal slot.
+    #[unsafe_ignore_trace]
+    pub(crate) state: AsyncGeneratorState,
+
+    /// The `[[AsyncGeneratorContext]]` internal slot.
+    pub(crate) context: Option<Gc<Cell<GeneratorContext>>>,
+
+    /// The `[[AsyncGeneratorQueue]]` internal slot.
+    pub(crate) queue: VecDeque<AsyncGeneratorRequest>,
+}
+
+impl BuiltIn for AsyncGenerator {
+    const NAME: &'static str = "AsyncGenerator";
+
+    fn init(context: &mut Context) -> Option<JsValue> {
+        let _timer = Profiler::global().start_event(Self::NAME, "init");
+
+        let iterator_prototype = context
+            .intrinsics()
+            .objects()
+            .iterator_prototypes()
+            .async_iterator_prototype();
+
+        let generator_function_prototype = context
+            .intrinsics()
+            .constructors()
+            .async_generator_function()
+            .prototype();
+
+        ConstructorBuilder::with_standard_constructor(
+            context,
+            Self::constructor,
+            context
+                .intrinsics()
+                .constructors()
+                .async_generator()
+                .clone(),
+        )
+        .name(Self::NAME)
+        .length(0)
+        .property(
+            WellKnownSymbols::to_string_tag(),
+            Self::NAME,
+            Attribute::READONLY | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
+        )
+        .method(Self::next, "next", 1)
+        .method(Self::r#return, "return", 1)
+        .method(Self::throw, "throw", 1)
+        .inherit(iterator_prototype)
+        .build();
+
+        context
+            .intrinsics()
+            .constructors()
+            .async_generator()
+            .prototype
+            .insert_property(
+                "constructor",
+                PropertyDescriptor::builder()
+                    .value(generator_function_prototype)
+                    .writable(false)
+                    .enumerable(false)
+                    .configurable(true),
+            );
+
+        None
+    }
+}
+
+impl AsyncGenerator {
+    #[allow(clippy::unnecessary_wraps)]
+    pub(crate) fn constructor(
+        _: &JsValue,
+        _: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        let prototype = context
+            .intrinsics()
+            .constructors()
+            .async_generator()
+            .prototype();
+
+        let this = JsObject::from_proto_and_data(
+            prototype,
+            ObjectData::async_generator(Self {
+                state: AsyncGeneratorState::Undefined,
+                context: None,
+                queue: VecDeque::new(),
+            }),
+        );
+
+        Ok(this.into())
+    }
+
+    /// `AsyncGenerator.prototype.next ( value )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgenerator-prototype-next
+    pub(crate) fn next(
+        this: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        // 1. Let generator be the this value.
+        let generator = this;
+
+        // 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
+        let promise_capability = PromiseCapability::new(
+            &context
+                .intrinsics()
+                .constructors()
+                .promise()
+                .constructor()
+                .into(),
+            context,
+        )
+        .expect("cannot fail with promise constructor");
+
+        // 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
+        // 4. IfAbruptRejectPromise(result, promiseCapability).
+        let generator_object = generator.as_object().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator_object, promise_capability, context);
+        let mut generator_obj_mut = generator_object.borrow_mut();
+        let generator = generator_obj_mut.as_async_generator_mut().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator, promise_capability, context);
+
+        // 5. Let state be generator.[[AsyncGeneratorState]].
+        let state = generator.state;
+
+        // 6. If state is completed, then
+        if state == AsyncGeneratorState::Completed {
+            drop(generator_obj_mut);
+
+            // a. Let iteratorResult be CreateIterResultObject(undefined, true).
+            let iterator_result = create_iter_result_object(JsValue::undefined(), true, context);
+
+            // b. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iteratorResult »).
+            promise_capability
+                .resolve()
+                .call(&JsValue::undefined(), &[iterator_result], context)
+                .expect("cannot fail per spec");
+
+            // c. Return promiseCapability.[[Promise]].
+            return Ok(promise_capability.promise().clone().into());
+        }
+
+        // 7. Let completion be NormalCompletion(value).
+        let completion = (Ok(args.get_or_undefined(0).clone()), false);
+
+        // 8. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
+        generator.enqueue(completion.clone(), promise_capability.clone());
+
+        // 9. If state is either suspendedStart or suspendedYield, then
+        if state == AsyncGeneratorState::SuspendedStart
+            || state == AsyncGeneratorState::SuspendedYield
+        {
+            // a. Perform AsyncGeneratorResume(generator, completion).
+            let generator_context = generator
+                .context
+                .clone()
+                .expect("generator context cannot be empty here");
+
+            drop(generator_obj_mut);
+
+            Self::resume(
+                generator_object,
+                state,
+                &generator_context,
+                completion,
+                context,
+            );
+        }
+
+        // 11. Return promiseCapability.[[Promise]].
+        Ok(promise_capability.promise().clone().into())
+    }
+
+    /// `AsyncGenerator.prototype.return ( value )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgenerator-prototype-return
+    pub(crate) fn r#return(
+        this: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        // 1. Let generator be the this value.
+        let generator = this;
+
+        // 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
+        let promise_capability = PromiseCapability::new(
+            &context
+                .intrinsics()
+                .constructors()
+                .promise()
+                .constructor()
+                .into(),
+            context,
+        )
+        .expect("cannot fail with promise constructor");
+
+        // 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
+        // 4. IfAbruptRejectPromise(result, promiseCapability).
+        let generator_object = generator.as_object().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator_object, promise_capability, context);
+        let mut generator_obj_mut = generator_object.borrow_mut();
+        let generator = generator_obj_mut.as_async_generator_mut().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator, promise_capability, context);
+
+        // 5. Let completion be Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
+        let completion = (Ok(args.get_or_undefined(0).clone()), true);
+
+        // 6. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
+        generator.enqueue(completion.clone(), promise_capability.clone());
+
+        // 7. Let state be generator.[[AsyncGeneratorState]].
+        let state = generator.state;
+
+        // 8. If state is either suspendedStart or completed, then
+        if state == AsyncGeneratorState::SuspendedStart || state == AsyncGeneratorState::Completed {
+            // a. Set generator.[[AsyncGeneratorState]] to awaiting-return.
+            generator.state = AsyncGeneratorState::AwaitingReturn;
+
+            // b. Perform ! AsyncGeneratorAwaitReturn(generator).
+            let next = generator
+                .queue
+                .front()
+                .cloned()
+                .expect("queue cannot be empty here");
+            drop(generator_obj_mut);
+            let (completion, _) = &next.completion;
+            Self::await_return(generator_object.clone(), completion.clone(), context);
+        }
+        // 9. Else if state is suspendedYield, then
+        else if state == AsyncGeneratorState::SuspendedYield {
+            // a. Perform AsyncGeneratorResume(generator, completion).
+            let generator_context = generator
+                .context
+                .clone()
+                .expect("generator context cannot be empty here");
+
+            drop(generator_obj_mut);
+            Self::resume(
+                generator_object,
+                state,
+                &generator_context,
+                completion,
+                context,
+            );
+        }
+
+        // 11. Return promiseCapability.[[Promise]].
+        Ok(promise_capability.promise().clone().into())
+    }
+
+    /// `AsyncGenerator.prototype.throw ( exception )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgenerator-prototype-throw
+    pub(crate) fn throw(
+        this: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        // 1. Let generator be the this value.
+        let generator = this;
+
+        // 2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
+        let promise_capability = PromiseCapability::new(
+            &context
+                .intrinsics()
+                .constructors()
+                .promise()
+                .constructor()
+                .into(),
+            context,
+        )
+        .expect("cannot fail with promise constructor");
+
+        // 3. Let result be Completion(AsyncGeneratorValidate(generator, empty)).
+        // 4. IfAbruptRejectPromise(result, promiseCapability).
+        let generator_object = generator.as_object().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator_object, promise_capability, context);
+        let mut generator_obj_mut = generator_object.borrow_mut();
+        let generator = generator_obj_mut.as_async_generator_mut().ok_or_else(|| {
+            context.construct_type_error("generator resumed on non generator object")
+        });
+        if_abrupt_reject_promise!(generator, promise_capability, context);
+
+        // 5. Let state be generator.[[AsyncGeneratorState]].
+        let mut state = generator.state;
+
+        // 6. If state is suspendedStart, then
+        if state == AsyncGeneratorState::SuspendedStart {
+            // a. Set generator.[[AsyncGeneratorState]] to completed.
+            generator.state = AsyncGeneratorState::Completed;
+
+            // b. Set state to completed.
+            state = AsyncGeneratorState::Completed;
+        }
+
+        // 7. If state is completed, then
+        if state == AsyncGeneratorState::Completed {
+            drop(generator_obj_mut);
+
+            // a. Perform ! Call(promiseCapability.[[Reject]], undefined, « exception »).
+            promise_capability
+                .reject()
+                .call(
+                    &JsValue::undefined(),
+                    &[args.get_or_undefined(0).clone()],
+                    context,
+                )
+                .expect("cannot fail per spec");
+
+            // b. Return promiseCapability.[[Promise]].
+            return Ok(promise_capability.promise().clone().into());
+        }
+
+        // 8. Let completion be ThrowCompletion(exception).
+        let completion = (Err(args.get_or_undefined(0).clone()), false);
+
+        // 9. Perform AsyncGeneratorEnqueue(generator, completion, promiseCapability).
+        generator.enqueue(completion.clone(), promise_capability.clone());
+
+        // 10. If state is suspendedYield, then
+        if state == AsyncGeneratorState::SuspendedYield {
+            let generator_context = generator
+                .context
+                .clone()
+                .expect("generator context cannot be empty here");
+            drop(generator_obj_mut);
+
+            // a. Perform AsyncGeneratorResume(generator, completion).
+            Self::resume(
+                generator_object,
+                state,
+                &generator_context,
+                completion,
+                context,
+            );
+        }
+
+        // 12. Return promiseCapability.[[Promise]].
+        Ok(promise_capability.promise().clone().into())
+    }
+
+    /// `AsyncGeneratorEnqueue ( generator, completion, promiseCapability )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorenqueue
+    pub(crate) fn enqueue(
+        &mut self,
+        completion: (JsResult<JsValue>, bool),
+        promise_capability: PromiseCapability,
+    ) {
+        // 1. Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]: promiseCapability }.
+        let request = AsyncGeneratorRequest {
+            completion,
+            capability: promise_capability,
+        };
+
+        // 2. Append request to the end of generator.[[AsyncGeneratorQueue]].
+        self.queue.push_back(request);
+    }
+
+    /// `AsyncGeneratorCompleteStep ( generator, completion, done [ , realm ] )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorcompletestep
+    pub(crate) fn complete_step(
+        next: &AsyncGeneratorRequest,
+        completion: JsResult<JsValue>,
+        done: bool,
+        context: &mut Context,
+    ) {
+        // 1. Let queue be generator.[[AsyncGeneratorQueue]].
+        // 2. Assert: queue is not empty.
+        // 3. Let next be the first element of queue.
+        // 4. Remove the first element from queue.
+        // 5. Let promiseCapability be next.[[Capability]].
+        let promise_capability = &next.capability;
+
+        // 6. Let value be completion.[[Value]].
+        match completion {
+            // 7. If completion.[[Type]] is throw, then
+            Err(value) => {
+                // a. Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
+                promise_capability
+                    .reject()
+                    .call(&JsValue::undefined(), &[value], context)
+                    .expect("cannot fail per spec");
+            }
+            // 8. Else,
+            Ok(value) => {
+                // a. Assert: completion.[[Type]] is normal.
+
+                // TODO: Realm handling not implemented yet.
+                // b. If realm is present, then
+                // i. Let oldRealm be the running execution context's Realm.
+                // ii. Set the running execution context's Realm to realm.
+                // iii. Let iteratorResult be CreateIterResultObject(value, done).
+                // iv. Set the running execution context's Realm to oldRealm.
+                // c. Else,
+                // i. Let iteratorResult be CreateIterResultObject(value, done).
+                let iterator_result = create_iter_result_object(value, done, context);
+
+                // d. Perform ! Call(promiseCapability.[[Resolve]], undefined, « iteratorResult »).
+                promise_capability
+                    .resolve()
+                    .call(&JsValue::undefined(), &[iterator_result], context)
+                    .expect("cannot fail per spec");
+            }
+        }
+    }
+
+    /// `AsyncGeneratorResume ( generator, completion )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorresume
+    pub(crate) fn resume(
+        generator: &JsObject,
+        state: AsyncGeneratorState,
+        generator_context: &Gc<Cell<GeneratorContext>>,
+        completion: (JsResult<JsValue>, bool),
+        context: &mut Context,
+    ) {
+        // 1. Assert: generator.[[AsyncGeneratorState]] is either suspendedStart or suspendedYield.
+        assert!(
+            state == AsyncGeneratorState::SuspendedStart
+                || state == AsyncGeneratorState::SuspendedYield
+        );
+
+        // 2. Let genContext be generator.[[AsyncGeneratorContext]].
+        let mut generator_context_mut = generator_context.borrow_mut();
+
+        // 3. Let callerContext be the running execution context.
+        // 4. Suspend callerContext.
+
+        // 5. Set generator.[[AsyncGeneratorState]] to executing.
+        generator
+            .borrow_mut()
+            .as_async_generator_mut()
+            .expect("already checked before")
+            .state = AsyncGeneratorState::Executing;
+
+        // 6. Push genContext onto the execution context stack; genContext is now the running execution context.
+        std::mem::swap(
+            &mut context.realm.environments,
+            &mut generator_context_mut.environments,
+        );
+        std::mem::swap(&mut context.vm.stack, &mut generator_context_mut.stack);
+        context
+            .vm
+            .push_frame(generator_context_mut.call_frame.clone());
+
+        // 7. Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
+        match completion {
+            (Ok(value), r#return) => {
+                context.vm.push(value);
+                if r#return {
+                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Return;
+                } else {
+                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Normal;
+                }
+            }
+            (Err(value), _) => {
+                context.vm.push(value);
+                context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Throw;
+            }
+        }
+        drop(generator_context_mut);
+        let result = context.run();
+
+        let mut generator_context_mut = generator_context.borrow_mut();
+        std::mem::swap(
+            &mut context.realm.environments,
+            &mut generator_context_mut.environments,
+        );
+        std::mem::swap(&mut context.vm.stack, &mut generator_context_mut.stack);
+        generator_context_mut.call_frame =
+            context.vm.pop_frame().expect("generator frame must exist");
+        drop(generator_context_mut);
+
+        // 8. Assert: result is never an abrupt completion.
+        assert!(result.is_ok());
+
+        // 9. Assert: When we return here, genContext has already been removed from the execution context stack and callerContext is the currently running execution context.
+        // 10. Return unused.
+    }
+
+    /// `AsyncGeneratorAwaitReturn ( generator )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorawaitreturn
+    pub(crate) fn await_return(
+        generator: JsObject,
+        completion: JsResult<JsValue>,
+        context: &mut Context,
+    ) {
+        // 1. Let queue be generator.[[AsyncGeneratorQueue]].
+        // 2. Assert: queue is not empty.
+        // 3. Let next be the first element of queue.
+        // 4. Let completion be Completion(next.[[Completion]]).
+
+        // 5. Assert: completion.[[Type]] is return.
+        let value = completion.expect("completion must be a return completion");
+
+        // Note: The spec is currently broken here.
+        // See: https://github.com/tc39/ecma262/pull/2683
+
+        // 6. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
+        let promise_completion = Promise::promise_resolve(
+            context.intrinsics().constructors().promise().constructor(),
+            value,
+            context,
+        );
+
+        let promise = match promise_completion {
+            Ok(value) => value,
+            Err(value) => {
+                let mut generator_borrow_mut = generator.borrow_mut();
+                let gen = generator_borrow_mut
+                    .as_async_generator_mut()
+                    .expect("already checked before");
+                gen.state = AsyncGeneratorState::Completed;
+                let next = gen.queue.pop_front().expect("queue must not be empty");
+                drop(generator_borrow_mut);
+                Self::complete_step(&next, Err(value), true, context);
+                Self::drain_queue(&generator, context);
+                return;
+            }
+        };
+
+        // 7. Let fulfilledClosure be a new Abstract Closure with parameters (value) that captures generator and performs the following steps when called:
+        // 8. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
+        let on_fulfilled = FunctionBuilder::closure_with_captures(
+            context,
+            |_this, args, generator, context| {
+                let mut generator_borrow_mut = generator.borrow_mut();
+                let gen = generator_borrow_mut
+                    .as_async_generator_mut()
+                    .expect("already checked before");
+
+                // a. Set generator.[[AsyncGeneratorState]] to completed.
+                gen.state = AsyncGeneratorState::Completed;
+
+                // b. Let result be NormalCompletion(value).
+                let result = Ok(args.get_or_undefined(0).clone());
+
+                // c. Perform AsyncGeneratorCompleteStep(generator, result, true).
+                let next = gen.queue.pop_front().expect("must have one entry");
+                drop(generator_borrow_mut);
+                Self::complete_step(&next, result, true, context);
+
+                // d. Perform AsyncGeneratorDrainQueue(generator).
+                Self::drain_queue(generator, context);
+
+                // e. Return undefined.
+                Ok(JsValue::undefined())
+            },
+            generator.clone(),
+        )
+        .name("")
+        .length(1)
+        .build();
+
+        // 9. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures generator and performs the following steps when called:
+        // 10. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
+        let on_rejected = FunctionBuilder::closure_with_captures(
+            context,
+            |_this, args, generator, context| {
+                let mut generator_borrow_mut = generator.borrow_mut();
+                let gen = generator_borrow_mut
+                    .as_async_generator_mut()
+                    .expect("already checked before");
+
+                // a. Set generator.[[AsyncGeneratorState]] to completed.
+                gen.state = AsyncGeneratorState::Completed;
+
+                // b. Let result be ThrowCompletion(reason).
+                let result = Err(args.get_or_undefined(0).clone());
+
+                // c. Perform AsyncGeneratorCompleteStep(generator, result, true).
+                let next = gen.queue.pop_front().expect("must have one entry");
+                drop(generator_borrow_mut);
+                Self::complete_step(&next, result, true, context);
+
+                // d. Perform AsyncGeneratorDrainQueue(generator).
+                Self::drain_queue(generator, context);
+
+                // e. Return undefined.
+                Ok(JsValue::undefined())
+            },
+            generator,
+        )
+        .name("")
+        .length(1)
+        .build();
+
+        // 11. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
+        let promise_obj = promise
+            .as_object()
+            .expect("constructed promise must be a promise");
+        promise_obj
+            .borrow_mut()
+            .as_promise_mut()
+            .expect("constructed promise must be a promise")
+            .perform_promise_then(&on_fulfilled.into(), &on_rejected.into(), None, context);
+    }
+
+    /// `AsyncGeneratorDrainQueue ( generator )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratordrainqueue
+    pub(crate) fn drain_queue(generator: &JsObject, context: &mut Context) {
+        let mut generator_borrow_mut = generator.borrow_mut();
+        let gen = generator_borrow_mut
+            .as_async_generator_mut()
+            .expect("already checked before");
+
+        // 1. Assert: generator.[[AsyncGeneratorState]] is completed.
+        assert_eq!(gen.state, AsyncGeneratorState::Completed);
+
+        // 2. Let queue be generator.[[AsyncGeneratorQueue]].
+        let queue = &mut gen.queue;
+
+        // 3. If queue is empty, return unused.
+        if queue.is_empty() {
+            return;
+        }
+
+        // 4. Let done be false.
+        // 5. Repeat, while done is false,
+        loop {
+            // a. Let next be the first element of queue.
+            let next = queue.front().expect("must have entry");
+
+            // b. Let completion be Completion(next.[[Completion]]).
+            match &next.completion {
+                // c. If completion.[[Type]] is return, then
+                (completion, true) => {
+                    // i. Set generator.[[AsyncGeneratorState]] to awaiting-return.
+                    gen.state = AsyncGeneratorState::AwaitingReturn;
+
+                    // ii. Perform ! AsyncGeneratorAwaitReturn(generator).
+                    let completion = completion.clone();
+                    drop(generator_borrow_mut);
+                    Self::await_return(generator.clone(), completion, context);
+
+                    // iii. Set done to true.
+                    break;
+                }
+                // d. Else,
+                (completion, false) => {
+                    // i. If completion.[[Type]] is normal, then
+                    let completion = if completion.is_ok() {
+                        // 1. Set completion to NormalCompletion(undefined).
+                        Ok(JsValue::undefined())
+                    } else {
+                        completion.clone()
+                    };
+
+                    // ii. Perform AsyncGeneratorCompleteStep(generator, completion, true).
+                    let next = queue.pop_front().expect("must have entry");
+                    Self::complete_step(&next, completion, true, context);
+
+                    // iii. If queue is empty, set done to true.
+                    if queue.is_empty() {
+                        break;
+                    }
+                }
+            }
+        }
+    }
+}

boa_engine/src/builtins/async_generator_function/mod.rs

@@ -0,0 +1,129 @@
+//! This module implements the `AsyncGeneratorFunction` object.
+//!
+//! More information:
+//!  - [ECMAScript reference][spec]
+//!
+//! [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorfunction-objects
+
+use crate::{
+    builtins::{
+        function::{BuiltInFunctionObject, ConstructorKind, Function},
+        BuiltIn,
+    },
+    object::ObjectData,
+    property::PropertyDescriptor,
+    symbol::WellKnownSymbols,
+    value::JsValue,
+    Context, JsResult,
+};
+use boa_profiler::Profiler;
+
+/// The internal representation on a `AsyncGeneratorFunction` object.
+#[derive(Debug, Clone, Copy)]
+pub struct AsyncGeneratorFunction;
+
+impl BuiltIn for AsyncGeneratorFunction {
+    const NAME: &'static str = "AsyncGeneratorFunction";
+
+    fn init(context: &mut Context) -> Option<JsValue> {
+        let _timer = Profiler::global().start_event(Self::NAME, "init");
+
+        let prototype = &context
+            .intrinsics()
+            .constructors()
+            .async_generator_function()
+            .prototype;
+        let constructor = &context
+            .intrinsics()
+            .constructors()
+            .async_generator_function()
+            .constructor;
+
+        constructor.set_prototype(Some(
+            context.intrinsics().constructors().function().constructor(),
+        ));
+        let property = PropertyDescriptor::builder()
+            .value(1)
+            .writable(false)
+            .enumerable(false)
+            .configurable(true);
+        constructor.borrow_mut().insert("length", property);
+        let property = PropertyDescriptor::builder()
+            .value(Self::NAME)
+            .writable(false)
+            .enumerable(false)
+            .configurable(true);
+        constructor.borrow_mut().insert("name", property);
+        let property = PropertyDescriptor::builder()
+            .value(
+                context
+                    .intrinsics()
+                    .constructors()
+                    .async_generator_function()
+                    .prototype(),
+            )
+            .writable(false)
+            .enumerable(false)
+            .configurable(false);
+        constructor.borrow_mut().insert("prototype", property);
+        constructor.borrow_mut().data = ObjectData::function(Function::Native {
+            function: Self::constructor,
+            constructor: Some(ConstructorKind::Base),
+        });
+
+        prototype.set_prototype(Some(
+            context.intrinsics().constructors().function().prototype(),
+        ));
+        let property = PropertyDescriptor::builder()
+            .value(
+                context
+                    .intrinsics()
+                    .constructors()
+                    .async_generator_function()
+                    .constructor(),
+            )
+            .writable(false)
+            .enumerable(false)
+            .configurable(true);
+        prototype.borrow_mut().insert("constructor", property);
+        let property = PropertyDescriptor::builder()
+            .value(
+                context
+                    .intrinsics()
+                    .constructors()
+                    .async_generator()
+                    .prototype(),
+            )
+            .writable(false)
+            .enumerable(false)
+            .configurable(true);
+        prototype.borrow_mut().insert("prototype", property);
+        let property = PropertyDescriptor::builder()
+            .value("AsyncGeneratorFunction")
+            .writable(false)
+            .enumerable(false)
+            .configurable(true);
+        prototype
+            .borrow_mut()
+            .insert(WellKnownSymbols::to_string_tag(), property);
+
+        None
+    }
+}
+
+impl AsyncGeneratorFunction {
+    /// `AsyncGeneratorFunction ( p1, p2, … , pn, body )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-asyncgeneratorfunction
+    pub(crate) fn constructor(
+        new_target: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        BuiltInFunctionObject::create_dynamic_function(new_target, args, true, true, context)
+            .map(Into::into)
+    }
+}

boa_engine/src/builtins/function/mod.rs

@@ -13,6 +13,7 @@
 
 use crate::{
     builtins::{BuiltIn, JsArgs},
+    bytecompiler::{FunctionCompiler, FunctionKind},
     context::intrinsics::StandardConstructors,
     environments::DeclarativeEnvironmentStack,
     object::{
@@ -246,6 +247,10 @@ pub enum Function {
         code: Gc<crate::vm::CodeBlock>,
         environments: DeclarativeEnvironmentStack,
     },
+    AsyncGenerator {
+        code: Gc<crate::vm::CodeBlock>,
+        environments: DeclarativeEnvironmentStack,
+    },
 }
 
 unsafe impl Trace for Function {
@@ -267,7 +272,8 @@ unsafe impl Trace for Function {
                 mark(environments);
                 mark(promise_capability);
             }
-            Self::Generator { code, environments } => {
+            Self::Generator { code, environments }
+            | Self::AsyncGenerator { code, environments } => {
                 mark(code);
                 mark(environments);
             }
@@ -288,7 +294,7 @@ impl Function {
             Self::Native { constructor, .. } | Self::Closure { constructor, .. } => {
                 constructor.is_some()
             }
-            Self::Generator { .. } | Self::Async { .. } => false,
+            Self::Generator { .. } | Self::AsyncGenerator { .. } | Self::Async { .. } => false,
             Self::Ordinary { code, .. } => !(code.this_mode == ThisMode::Lexical),
         }
     }
@@ -476,7 +482,9 @@ impl BuiltInFunctionObject {
         generator: bool,
         context: &mut Context,
     ) -> JsResult<JsObject> {
-        let default = if r#async {
+        let default = if r#async && generator {
+            StandardConstructors::async_generator_function
+        } else if r#async {
             StandardConstructors::async_function
         } else if generator {
             StandardConstructors::generator_function
@@ -518,6 +526,20 @@ impl BuiltInFunctionObject {
                 parameters
             };
 
+            // It is a Syntax Error if FormalParameters Contains YieldExpression is true.
+            if generator && r#async && parameters.contains_yield_expression() {
+                return context.throw_syntax_error(
+                    "yield expression not allowed in async generator parameters",
+                );
+            }
+
+            // It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
+            if generator && r#async && parameters.contains_await_expression() {
+                return context.throw_syntax_error(
+                    "await expression not allowed in async generator parameters",
+                );
+            }
+
             let body_arg = body_arg.to_string(context)?;
 
             let body = match Parser::new(body_arg.as_bytes()).parse_function_body(
@@ -581,20 +603,17 @@ impl BuiltInFunctionObject {
                 }
             }
 
-            let code = crate::bytecompiler::ByteCompiler::compile_function_code(
+            let code = FunctionCompiler::new()
-                crate::bytecompiler::FunctionKind::Expression,
+                .name(Sym::ANONYMOUS)
-                Some(Sym::ANONYMOUS),
+                .generator(generator)
-                &parameters,
+                .r#async(r#async)
-                &body,
+                .kind(FunctionKind::Expression)
-                generator,
+                .compile(&parameters, &body, context)?;
-                false,
-                context,
-            )?;
 
             let environments = context.realm.environments.pop_to_global();
 
             let function_object = if generator {
-                crate::vm::create_generator_function_object(code, context)
+                crate::vm::create_generator_function_object(code, r#async, context)
             } else {
                 crate::vm::create_function_object(code, r#async, Some(prototype), context)
             };
@@ -603,29 +622,29 @@ impl BuiltInFunctionObject {
 
             Ok(function_object)
         } else if generator {
-            let code = crate::bytecompiler::ByteCompiler::compile_function_code(
+            let code = FunctionCompiler::new()
-                crate::bytecompiler::FunctionKind::Expression,
+                .name(Sym::ANONYMOUS)
-                Some(Sym::ANONYMOUS),
+                .generator(true)
+                .kind(FunctionKind::Expression)
+                .compile(
                     &FormalParameterList::empty(),
                     &StatementList::default(),
-                true,
-                false,
                     context,
                 )?;
 
             let environments = context.realm.environments.pop_to_global();
-            let function_object = crate::vm::create_generator_function_object(code, context);
+            let function_object =
+                crate::vm::create_generator_function_object(code, r#async, context);
             context.realm.environments.extend(environments);
 
             Ok(function_object)
         } else {
-            let code = crate::bytecompiler::ByteCompiler::compile_function_code(
+            let code = FunctionCompiler::new()
-                crate::bytecompiler::FunctionKind::Expression,
+                .name(Sym::ANONYMOUS)
-                Some(Sym::ANONYMOUS),
+                .kind(FunctionKind::Expression)
+                .compile(
                     &FormalParameterList::empty(),
                     &StatementList::default(),
-                false,
-                false,
                     context,
                 )?;
 

boa_engine/src/builtins/iterable/mod.rs

@@ -14,6 +14,8 @@ use boa_profiler::Profiler;
 pub struct IteratorPrototypes {
     /// %IteratorPrototype%
     iterator_prototype: JsObject,
+    /// %AsyncIteratorPrototype%
+    async_iterator_prototype: JsObject,
     /// %MapIteratorPrototype%
     array_iterator: JsObject,
     /// %SetIteratorPrototype%
@@ -33,6 +35,7 @@ impl IteratorPrototypes {
         let _timer = Profiler::global().start_event("IteratorPrototypes::init", "init");
 
         let iterator_prototype = create_iterator_prototype(context);
+        let async_iterator_prototype = create_async_iterator_prototype(context);
         Self {
             array_iterator: ArrayIterator::create_prototype(iterator_prototype.clone(), context),
             set_iterator: SetIterator::create_prototype(iterator_prototype.clone(), context),
@@ -44,6 +47,7 @@ impl IteratorPrototypes {
             map_iterator: MapIterator::create_prototype(iterator_prototype.clone(), context),
             for_in_iterator: ForInIterator::create_prototype(iterator_prototype.clone(), context),
             iterator_prototype,
+            async_iterator_prototype,
         }
     }
 
@@ -57,6 +61,11 @@ impl IteratorPrototypes {
         self.iterator_prototype.clone()
     }
 
+    #[inline]
+    pub fn async_iterator_prototype(&self) -> JsObject {
+        self.async_iterator_prototype.clone()
+    }
+
     #[inline]
     pub fn set_iterator(&self) -> JsObject {
         self.set_iterator.clone()
@@ -506,3 +515,24 @@ macro_rules! if_abrupt_close_iterator {
 
 // Export macro to crate level
 pub(crate) use if_abrupt_close_iterator;
+
+/// Create the `%AsyncIteratorPrototype%` object
+///
+/// More information:
+///  - [ECMA reference][spec]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-asynciteratorprototype
+#[inline]
+fn create_async_iterator_prototype(context: &mut Context) -> JsObject {
+    let _timer = Profiler::global().start_event("AsyncIteratorPrototype", "init");
+
+    let symbol_iterator = WellKnownSymbols::async_iterator();
+    let iterator_prototype = ObjectInitializer::new(context)
+        .function(
+            |v, _, _| Ok(v.clone()),
+            (symbol_iterator, "[Symbol.asyncIterator]"),
+            0,
+        )
+        .build();
+    iterator_prototype
+}

boa_engine/src/builtins/mod.rs

@@ -3,6 +3,8 @@
 pub mod array;
 pub mod array_buffer;
 pub mod async_function;
+pub mod async_generator;
+pub mod async_generator_function;
 pub mod bigint;
 pub mod boolean;
 pub mod dataview;
@@ -77,8 +79,9 @@ pub(crate) use self::{
 
 use crate::{
     builtins::{
-        array_buffer::ArrayBuffer, generator::Generator, generator_function::GeneratorFunction,
+        array_buffer::ArrayBuffer, async_generator::AsyncGenerator,
-        typed_array::TypedArray,
+        async_generator_function::AsyncGeneratorFunction, generator::Generator,
+        generator_function::GeneratorFunction, typed_array::TypedArray,
     },
     property::{Attribute, PropertyDescriptor},
     Context, JsValue,
@@ -188,7 +191,9 @@ pub fn init(context: &mut Context) {
         Generator,
         GeneratorFunction,
         Promise,
-        AsyncFunction
+        AsyncFunction,
+        AsyncGenerator,
+        AsyncGeneratorFunction
     };
 
     #[cfg(feature = "intl")]

boa_engine/src/builtins/promise/mod.rs

@@ -40,13 +40,13 @@ macro_rules! if_abrupt_reject_promise {
             Err(value) => {
                 // a. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »).
                 $context.call(
-                    &$capability.reject.clone().into(),
+                    &$capability.reject().clone().into(),
                     &JsValue::undefined(),
                     &[value],
                 )?;
 
                 // b. Return capability.[[Promise]].
-                return Ok($capability.promise.clone().into());
+                return Ok($capability.promise().clone().into());
             }
             // 2. Else if value is a Completion Record, set value to value.[[Value]].
             Ok(value) => value,
@@ -54,6 +54,8 @@ macro_rules! if_abrupt_reject_promise {
     };
 }
 
+pub(crate) use if_abrupt_reject_promise;
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum PromiseState {
     Pending,

boa_engine/src/bytecompiler/function.rs

@@ -0,0 +1,194 @@
+use crate::{
+    builtins::function::ThisMode,
+    bytecompiler::{ByteCompiler, FunctionKind},
+    syntax::ast::node::{Declaration, FormalParameterList, StatementList},
+    vm::{BindingOpcode, CodeBlock, Opcode},
+    Context, JsResult,
+};
+use boa_gc::Gc;
+use boa_interner::Sym;
+use rustc_hash::FxHashMap;
+
+/// `FunctionCompiler` is used to compile AST functions to bytecode.
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct FunctionCompiler {
+    name: Sym,
+    generator: bool,
+    r#async: bool,
+    strict: bool,
+    kind: FunctionKind,
+}
+
+impl FunctionCompiler {
+    /// Create a new `FunctionCompiler`.
+    #[inline]
+    pub(crate) fn new() -> Self {
+        Self {
+            name: Sym::EMPTY_STRING,
+            generator: false,
+            r#async: false,
+            strict: false,
+            kind: FunctionKind::Declaration,
+        }
+    }
+
+    /// Set the name of the function.
+    #[inline]
+    pub(crate) fn name<N>(mut self, name: N) -> Self
+    where
+        N: Into<Option<Sym>>,
+    {
+        let name = name.into();
+        if let Some(name) = name {
+            self.name = name;
+        }
+        self
+    }
+
+    /// Indicate if the function is a generator function.
+    #[inline]
+    pub(crate) fn generator(mut self, generator: bool) -> Self {
+        self.generator = generator;
+        self
+    }
+
+    /// Indicate if the function is an async function.
+    #[inline]
+    pub(crate) fn r#async(mut self, r#async: bool) -> Self {
+        self.r#async = r#async;
+        self
+    }
+
+    /// Indicate if the function is in a strict context.
+    #[inline]
+    pub(crate) fn strict(mut self, strict: bool) -> Self {
+        self.strict = strict;
+        self
+    }
+
+    /// Indicate if the function is a declaration, expression or arrow function.
+    #[inline]
+    pub(crate) fn kind(mut self, kind: FunctionKind) -> Self {
+        self.kind = kind;
+        self
+    }
+
+    /// Compile a function statement list and it's parameters into bytecode.
+    pub(crate) fn compile(
+        mut self,
+        parameters: &FormalParameterList,
+        body: &StatementList,
+        context: &mut Context,
+    ) -> JsResult<Gc<CodeBlock>> {
+        self.strict = self.strict || body.strict();
+
+        let length = parameters.length();
+        let mut code = CodeBlock::new(self.name, length, self.strict);
+
+        if self.kind == FunctionKind::Arrow {
+            code.this_mode = ThisMode::Lexical;
+        }
+
+        let mut compiler = ByteCompiler {
+            code_block: code,
+            literals_map: FxHashMap::default(),
+            names_map: FxHashMap::default(),
+            bindings_map: FxHashMap::default(),
+            jump_info: Vec::new(),
+            in_async_generator: self.generator && self.r#async,
+            context,
+        };
+
+        compiler.context.push_compile_time_environment(true);
+
+        // An arguments object is added when all of the following conditions are met
+        // - If not in an arrow function (10.2.11.16)
+        // - If the parameter list does not contain `arguments` (10.2.11.17)
+        // Note: This following just means, that we add an extra environment for the arguments.
+        // - If there are default parameters or if lexical names and function names do not contain `arguments` (10.2.11.18)
+        if !(self.kind == FunctionKind::Arrow) && !parameters.has_arguments() {
+            compiler
+                .context
+                .create_mutable_binding(Sym::ARGUMENTS, false);
+            compiler.code_block.arguments_binding = Some(
+                compiler
+                    .context
+                    .initialize_mutable_binding(Sym::ARGUMENTS, false),
+            );
+        }
+
+        for parameter in parameters.parameters.iter() {
+            if parameter.is_rest_param() {
+                compiler.emit_opcode(Opcode::RestParameterInit);
+            }
+
+            match parameter.declaration() {
+                Declaration::Identifier { ident, .. } => {
+                    compiler.context.create_mutable_binding(ident.sym(), false);
+                    if let Some(init) = parameter.declaration().init() {
+                        let skip = compiler.emit_opcode_with_operand(Opcode::JumpIfNotUndefined);
+                        compiler.compile_expr(init, true)?;
+                        compiler.patch_jump(skip);
+                    }
+                    compiler.emit_binding(BindingOpcode::InitArg, ident.sym());
+                }
+                Declaration::Pattern(pattern) => {
+                    for ident in pattern.idents() {
+                        compiler.context.create_mutable_binding(ident, false);
+                    }
+                    compiler.compile_declaration_pattern(pattern, BindingOpcode::InitArg)?;
+                }
+            }
+        }
+
+        if !parameters.has_rest_parameter() {
+            compiler.emit_opcode(Opcode::RestParameterPop);
+        }
+
+        let env_label = if parameters.has_expressions() {
+            compiler.code_block.num_bindings = compiler.context.get_binding_number();
+            compiler.context.push_compile_time_environment(true);
+            compiler.code_block.function_environment_push_location =
+                compiler.next_opcode_location();
+            Some(compiler.emit_opcode_with_two_operands(Opcode::PushFunctionEnvironment))
+        } else {
+            None
+        };
+
+        // When a generator object is created from a generator function, the generator executes until here to init parameters.
+        if self.generator {
+            compiler.emit_opcode(Opcode::PushUndefined);
+            compiler.emit_opcode(Opcode::Yield);
+        }
+
+        compiler.create_declarations(body.items())?;
+        compiler.compile_statement_list(body.items(), false)?;
+
+        if let Some(env_label) = env_label {
+            let (num_bindings, compile_environment) =
+                compiler.context.pop_compile_time_environment();
+            let index_compile_environment = compiler.push_compile_environment(compile_environment);
+            compiler.patch_jump_with_target(env_label.0, num_bindings as u32);
+            compiler.patch_jump_with_target(env_label.1, index_compile_environment as u32);
+
+            let (_, compile_environment) = compiler.context.pop_compile_time_environment();
+            compiler.push_compile_environment(compile_environment);
+        } else {
+            let (num_bindings, compile_environment) =
+                compiler.context.pop_compile_time_environment();
+            compiler
+                .code_block
+                .compile_environments
+                .push(compile_environment);
+            compiler.code_block.num_bindings = num_bindings;
+        }
+
+        compiler.code_block.params = parameters.clone();
+
+        // TODO These are redundant if a function returns so may need to check if a function returns and adding these if it doesn't
+        compiler.emit(Opcode::PushUndefined, &[]);
+        compiler.emit(Opcode::Return, &[]);
+
+        Ok(Gc::new(compiler.finish()))
+    }
+}

boa_engine/src/bytecompiler/mod.rs

@@ -1,5 +1,6 @@
+mod function;
+
 use crate::{
-    builtins::function::ThisMode,
     environments::{BindingLocator, CompileTimeEnvironment},
     syntax::ast::{
         node::{
@@ -11,8 +12,7 @@ use crate::{
             object::{MethodDefinition, PropertyDefinition, PropertyName},
             operator::assign::AssignTarget,
             template::TemplateElement,
-            Class, Declaration, FormalParameterList, GetConstField, GetField, GetSuperField,
+            Class, Declaration, GetConstField, GetField, GetSuperField,
-            StatementList,
         },
         op::{AssignOp, BinOp, BitOp, CompOp, LogOp, NumOp, UnaryOp},
         Const, Node,
@@ -25,6 +25,8 @@ use boa_interner::{Interner, Sym};
 use rustc_hash::FxHashMap;
 use std::mem::size_of;
 
+pub(crate) use function::FunctionCompiler;
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 enum Literal {
     String(JsString),
@@ -72,6 +74,7 @@ pub struct ByteCompiler<'b> {
     names_map: FxHashMap<Sym, u32>,
     bindings_map: FxHashMap<BindingLocator, u32>,
     jump_info: Vec<JumpControlInfo>,
+    in_async_generator: bool,
     context: &'b mut Context,
 }
 
@@ -87,6 +90,7 @@ impl<'b> ByteCompiler<'b> {
             names_map: FxHashMap::default(),
             bindings_map: FxHashMap::default(),
             jump_info: Vec::new(),
+            in_async_generator: false,
             context,
         }
     }
@@ -896,8 +900,7 @@ impl<'b> ByteCompiler<'b> {
                                 self.emit_opcode(Opcode::DefineOwnPropertyByValue);
                             }
                         },
-                        PropertyDefinition::MethodDefinition(kind, name) => {
+                        PropertyDefinition::MethodDefinition(kind, name) => match kind {
-                            match kind {
                             MethodDefinition::Get(expr) => match name {
                                 PropertyName::Literal(name) => {
                                     self.function(&expr.clone().into(), true)?;
@@ -968,12 +971,9 @@ impl<'b> ByteCompiler<'b> {
                                     self.emit_opcode(Opcode::DefineOwnPropertyByValue);
                                 }
                             },
-                                // TODO: Implement async generators
+                            MethodDefinition::AsyncGenerator(expr) => match name {
-                                MethodDefinition::AsyncGenerator(_) => {
-                                    // TODO: Implement async
-                                    match name {
                                 PropertyName::Literal(name) => {
-                                            self.emit_opcode(Opcode::PushUndefined);
+                                    self.function(&expr.clone().into(), true)?;
                                     self.emit_opcode(Opcode::Swap);
                                     let index = self.get_or_insert_name(*name);
                                     self.emit(Opcode::DefineOwnPropertyByName, &[index]);
@@ -981,13 +981,11 @@ impl<'b> ByteCompiler<'b> {
                                 PropertyName::Computed(name_node) => {
                                     self.compile_stmt(name_node, true)?;
                                     self.emit_opcode(Opcode::ToPropertyKey);
-                                            self.emit_opcode(Opcode::PushUndefined);
+                                    self.function(&expr.clone().into(), true)?;
                                     self.emit_opcode(Opcode::DefineOwnPropertyByValue);
                                 }
-                                    }
+                            },
-                                }
+                        },
-                            }
-                        }
                         PropertyDefinition::SpreadObject(expr) => {
                             self.compile_expr(expr, true)?;
                             self.emit_opcode(Opcode::Swap);
@@ -1144,11 +1142,9 @@ impl<'b> ByteCompiler<'b> {
                     self.emit_opcode(Opcode::Pop);
                 }
             }
-            // TODO: implement AsyncGeneratorExpr
+            Node::GeneratorExpr(_) | Node::AsyncFunctionExpr(_) | Node::AsyncGeneratorExpr(_) => {
-            Node::AsyncGeneratorExpr(_) => {
+                self.function(expr, use_expr)?;
-                self.emit_opcode(Opcode::PushUndefined);
             }
-            Node::GeneratorExpr(_) | Node::AsyncFunctionExpr(_) => self.function(expr, use_expr)?,
             Node::Yield(r#yield) => {
                 if let Some(expr) = r#yield.expr() {
                     self.compile_expr(expr, true)?;
@@ -1163,6 +1159,17 @@ impl<'b> ByteCompiler<'b> {
                     let start = self.emit_opcode_with_operand(Opcode::GeneratorNextDelegate);
                     self.emit(Opcode::Jump, &[start_address]);
                     self.patch_jump(start);
+                } else if self.in_async_generator {
+                    self.emit_opcode(Opcode::Await);
+                    self.emit_opcode(Opcode::AsyncGeneratorNext);
+                    let jump_return = self.emit_opcode_with_operand(Opcode::JumpIfFalse);
+                    let jump = self.emit_opcode_with_operand(Opcode::JumpIfFalse);
+                    self.emit_opcode(Opcode::Yield);
+                    self.emit_opcode(Opcode::GeneratorNext);
+                    self.patch_jump(jump);
+                    self.emit_opcode(Opcode::Await);
+                    self.emit_opcode(Opcode::GeneratorNext);
+                    self.patch_jump(jump_return);
                 } else {
                     self.emit_opcode(Opcode::Yield);
                     self.emit_opcode(Opcode::GeneratorNext);
@@ -1942,10 +1949,8 @@ impl<'b> ByteCompiler<'b> {
                     self.pop_try_control_info(None);
                 }
             }
-            Node::GeneratorDecl(_) | Node::AsyncFunctionDecl(_) => self.function(node, false)?,
+            Node::GeneratorDecl(_) | Node::AsyncFunctionDecl(_) | Node::AsyncGeneratorDecl(_) => {
-            // TODO: implement AsyncGeneratorDecl
+                self.function(node, false)?;
-            Node::AsyncGeneratorDecl(_) => {
-                self.emit_opcode(Opcode::PushUndefined);
             }
             Node::ClassDecl(class) => self.class(class, false)?,
             Node::Empty => {}
@@ -1954,126 +1959,6 @@ impl<'b> ByteCompiler<'b> {
         Ok(())
     }
 
-    /// Compile a function statement list and it's parameters into bytecode.
-    pub(crate) fn compile_function_code(
-        kind: FunctionKind,
-        name: Option<Sym>,
-        parameters: &FormalParameterList,
-        body: &StatementList,
-        generator: bool,
-        strict: bool,
-        context: &mut Context,
-    ) -> JsResult<Gc<CodeBlock>> {
-        let strict = strict || body.strict();
-        let length = parameters.length();
-        let mut code = CodeBlock::new(name.unwrap_or(Sym::EMPTY_STRING), length, strict);
-
-        if let FunctionKind::Arrow = kind {
-            code.this_mode = ThisMode::Lexical;
-        }
-
-        let mut compiler = ByteCompiler {
-            code_block: code,
-            literals_map: FxHashMap::default(),
-            names_map: FxHashMap::default(),
-            bindings_map: FxHashMap::default(),
-            jump_info: Vec::new(),
-            context,
-        };
-
-        compiler.context.push_compile_time_environment(true);
-
-        // An arguments object is added when all of the following conditions are met
-        // - If not in an arrow function (10.2.11.16)
-        // - If the parameter list does not contain `arguments` (10.2.11.17)
-        // Note: This following just means, that we add an extra environment for the arguments.
-        // - If there are default parameters or if lexical names and function names do not contain `arguments` (10.2.11.18)
-        if !(kind == FunctionKind::Arrow) && !parameters.has_arguments() {
-            compiler
-                .context
-                .create_mutable_binding(Sym::ARGUMENTS, false);
-            compiler.code_block.arguments_binding = Some(
-                compiler
-                    .context
-                    .initialize_mutable_binding(Sym::ARGUMENTS, false),
-            );
-        }
-
-        for parameter in parameters.parameters.iter() {
-            if parameter.is_rest_param() {
-                compiler.emit_opcode(Opcode::RestParameterInit);
-            }
-
-            match parameter.declaration() {
-                Declaration::Identifier { ident, .. } => {
-                    compiler.context.create_mutable_binding(ident.sym(), false);
-                    if let Some(init) = parameter.declaration().init() {
-                        let skip = compiler.emit_opcode_with_operand(Opcode::JumpIfNotUndefined);
-                        compiler.compile_expr(init, true)?;
-                        compiler.patch_jump(skip);
-                    }
-                    compiler.emit_binding(BindingOpcode::InitArg, ident.sym());
-                }
-                Declaration::Pattern(pattern) => {
-                    for ident in pattern.idents() {
-                        compiler.context.create_mutable_binding(ident, false);
-                    }
-                    compiler.compile_declaration_pattern(pattern, BindingOpcode::InitArg)?;
-                }
-            }
-        }
-
-        if !parameters.has_rest_parameter() {
-            compiler.emit_opcode(Opcode::RestParameterPop);
-        }
-
-        let env_label = if parameters.has_expressions() {
-            compiler.code_block.num_bindings = compiler.context.get_binding_number();
-            compiler.context.push_compile_time_environment(true);
-            compiler.code_block.function_environment_push_location =
-                compiler.next_opcode_location();
-            Some(compiler.emit_opcode_with_two_operands(Opcode::PushFunctionEnvironment))
-        } else {
-            None
-        };
-
-        // When a generator object is created from a generator function, the generator executes until here to init parameters.
-        if generator {
-            compiler.emit_opcode(Opcode::PushUndefined);
-            compiler.emit_opcode(Opcode::Yield);
-        }
-
-        compiler.create_declarations(body.items())?;
-        compiler.compile_statement_list(body.items(), false)?;
-
-        if let Some(env_label) = env_label {
-            let (num_bindings, compile_environment) =
-                compiler.context.pop_compile_time_environment();
-            let index_compile_environment = compiler.push_compile_environment(compile_environment);
-            compiler.patch_jump_with_target(env_label.0, num_bindings as u32);
-            compiler.patch_jump_with_target(env_label.1, index_compile_environment as u32);
-
-            let (_, compile_environment) = compiler.context.pop_compile_time_environment();
-            compiler.push_compile_environment(compile_environment);
-        } else {
-            let (num_bindings, compile_environment) =
-                compiler.context.pop_compile_time_environment();
-            compiler
-                .code_block
-                .compile_environments
-                .push(compile_environment);
-            compiler.code_block.num_bindings = num_bindings;
-        }
-
-        compiler.code_block.params = parameters.clone();
-
-        // TODO These are redundant if a function returns so may need to check if a function returns and adding these if it doesn't
-        compiler.emit(Opcode::PushUndefined, &[]);
-        compiler.emit(Opcode::Return, &[]);
-
-        Ok(Gc::new(compiler.finish()))
-    }
-
     /// Compile a function AST Node into bytecode.
     pub(crate) fn function(&mut self, function: &Node, use_expr: bool) -> JsResult<()> {
         let (kind, name, parameters, body, generator, r#async) = match function {
@@ -2101,6 +1986,14 @@ impl<'b> ByteCompiler<'b> {
                 true,
                 false,
             ),
+            Node::AsyncGeneratorDecl(generator) => (
+                FunctionKind::Declaration,
+                Some(generator.name()),
+                generator.parameters(),
+                generator.body(),
+                true,
+                true,
+            ),
             Node::FunctionExpr(function) => (
                 FunctionKind::Expression,
                 function.name(),
@@ -2125,6 +2018,14 @@ impl<'b> ByteCompiler<'b> {
                 true,
                 false,
             ),
+            Node::AsyncGeneratorExpr(generator) => (
+                FunctionKind::Expression,
+                generator.name(),
+                generator.parameters(),
+                generator.body(),
+                true,
+                true,
+            ),
             Node::ArrowFunctionDecl(function) => (
                 FunctionKind::Arrow,
                 function.name(),
@@ -2136,20 +2037,20 @@ impl<'b> ByteCompiler<'b> {
             _ => unreachable!(),
         };
 
-        let code = Self::compile_function_code(
+        let code = FunctionCompiler::new()
-            kind,
+            .name(name)
-            name,
+            .generator(generator)
-            parameters,
+            .r#async(r#async)
-            body,
+            .strict(self.code_block.strict)
-            generator,
+            .kind(kind)
-            self.code_block.strict,
+            .compile(parameters, body, self.context)?;
-            self.context,
-        )?;
 
         let index = self.code_block.functions.len() as u32;
         self.code_block.functions.push(code);
 
-        if generator {
+        if generator && r#async {
+            self.emit(Opcode::GetGeneratorAsync, &[index]);
+        } else if generator {
             self.emit(Opcode::GetGenerator, &[index]);
         } else if r#async {
             self.emit(Opcode::GetFunctionAsync, &[index]);
@@ -2627,6 +2528,7 @@ impl<'b> ByteCompiler<'b> {
             names_map: FxHashMap::default(),
             bindings_map: FxHashMap::default(),
             jump_info: Vec::new(),
+            in_async_generator: false,
             context: self.context,
         };
         compiler.context.push_compile_time_environment(true);
@@ -2806,8 +2708,20 @@ impl<'b> ByteCompiler<'b> {
                                 self.emit_opcode(Opcode::DefineClassMethodByValue);
                             }
                         },
-                        // TODO: implement async
+                        MethodDefinition::AsyncGenerator(expr) => match name {
-                        MethodDefinition::AsyncGenerator(_) => {}
+                            PropertyName::Literal(name) => {
+                                self.function(&expr.clone().into(), true)?;
+                                self.emit_opcode(Opcode::Swap);
+                                let index = self.get_or_insert_name(*name);
+                                self.emit(Opcode::DefineClassMethodByName, &[index]);
+                            }
+                            PropertyName::Computed(name_node) => {
+                                self.compile_stmt(name_node, true)?;
+                                self.emit_opcode(Opcode::ToPropertyKey);
+                                self.function(&expr.clone().into(), true)?;
+                                self.emit_opcode(Opcode::DefineClassMethodByValue);
+                            }
+                        },
                     }
                 }
                 ClassElement::PrivateStaticMethodDefinition(name, method_definition) => {
@@ -2838,8 +2752,11 @@ impl<'b> ByteCompiler<'b> {
                             let index = self.get_or_insert_name(*name);
                             self.emit(Opcode::SetPrivateMethod, &[index]);
                         }
-                        // TODO: implement async
+                        MethodDefinition::AsyncGenerator(expr) => {
-                        MethodDefinition::AsyncGenerator(_) => {}
+                            self.function(&expr.clone().into(), true)?;
+                            let index = self.get_or_insert_name(*name);
+                            self.emit(Opcode::SetPrivateMethod, &[index]);
+                        }
                     }
                 }
                 ClassElement::FieldDefinition(name, field) => {
@@ -2861,6 +2778,7 @@ impl<'b> ByteCompiler<'b> {
                         names_map: FxHashMap::default(),
                         bindings_map: FxHashMap::default(),
                         jump_info: Vec::new(),
+                        in_async_generator: false,
                         context: self.context,
                     };
                     field_compiler.context.push_compile_time_environment(true);
@@ -2894,6 +2812,7 @@ impl<'b> ByteCompiler<'b> {
                         names_map: FxHashMap::default(),
                         bindings_map: FxHashMap::default(),
                         jump_info: Vec::new(),
+                        in_async_generator: false,
                         context: self.context,
                     };
                     field_compiler.context.push_compile_time_environment(true);
@@ -3002,8 +2921,11 @@ impl<'b> ByteCompiler<'b> {
                             let index = self.get_or_insert_name(*name);
                             self.emit(Opcode::PushClassPrivateMethod, &[index]);
                         }
-                        // TODO: implement async
+                        MethodDefinition::AsyncGenerator(expr) => {
-                        MethodDefinition::AsyncGenerator(_) => {}
+                            self.function(&expr.clone().into(), true)?;
+                            let index = self.get_or_insert_name(*name);
+                            self.emit(Opcode::PushClassPrivateMethod, &[index]);
+                        }
                     }
                 }
                 ClassElement::MethodDefinition(..) => {}
@@ -3087,10 +3009,20 @@ impl<'b> ByteCompiler<'b> {
                                 self.emit_opcode(Opcode::DefineClassMethodByValue);
                             }
                         },
-                        // TODO: implement async
+                        MethodDefinition::AsyncGenerator(expr) => match name {
-                        MethodDefinition::AsyncGenerator(_) => {
+                            PropertyName::Literal(name) => {
-                            self.emit_opcode(Opcode::Pop);
+                                self.function(&expr.clone().into(), true)?;
+                                self.emit_opcode(Opcode::Swap);
+                                let index = self.get_or_insert_name(*name);
+                                self.emit(Opcode::DefineClassMethodByName, &[index]);
                             }
+                            PropertyName::Computed(name_node) => {
+                                self.compile_stmt(name_node, true)?;
+                                self.emit_opcode(Opcode::ToPropertyKey);
+                                self.function(&expr.clone().into(), true)?;
+                                self.emit_opcode(Opcode::DefineClassMethodByValue);
+                            }
+                        },
                     }
                 }
                 ClassElement::PrivateMethodDefinition(..)

boa_engine/src/context/intrinsics.rs

@@ -72,6 +72,8 @@ impl StandardConstructor {
 /// Cached core standard constructors.
 #[derive(Debug, Clone)]
 pub struct StandardConstructors {
+    async_generator_function: StandardConstructor,
+    async_generator: StandardConstructor,
     object: StandardConstructor,
     proxy: StandardConstructor,
     date: StandardConstructor,
@@ -117,6 +119,8 @@ pub struct StandardConstructors {
 impl Default for StandardConstructors {
     fn default() -> Self {
         let result = Self {
+            async_generator_function: StandardConstructor::default(),
+            async_generator: StandardConstructor::default(),
             object: StandardConstructor::default(),
             proxy: StandardConstructor::default(),
             date: StandardConstructor::default(),
@@ -187,6 +191,16 @@ impl Default for StandardConstructors {
 }
 
 impl StandardConstructors {
+    #[inline]
+    pub fn async_generator_function(&self) -> &StandardConstructor {
+        &self.async_generator_function
+    }
+
+    #[inline]
+    pub fn async_generator(&self) -> &StandardConstructor {
+        &self.async_generator
+    }
+
     #[inline]
     pub fn object(&self) -> &StandardConstructor {
         &self.object

boa_engine/src/context/mod.rs

@@ -729,6 +729,7 @@ impl Context {
             arg_count: 0,
             generator_resume_kind: GeneratorResumeKind::Normal,
             thrown: false,
+            async_generator: None,
         });
 
         self.realm.set_global_binding_number();

boa_engine/src/object/mod.rs

@@ -26,6 +26,7 @@ use crate::{
     builtins::{
         array::array_iterator::ArrayIterator,
         array_buffer::ArrayBuffer,
+        async_generator::AsyncGenerator,
         function::arguments::Arguments,
         function::{
             arguments::ParameterMap, BoundFunction, Captures, ConstructorKind, Function,
@@ -164,6 +165,8 @@ pub struct ObjectData {
 /// Defines the different types of objects.
 #[derive(Debug, Trace, Finalize)]
 pub enum ObjectKind {
+    AsyncGenerator(AsyncGenerator),
+    AsyncGeneratorFunction(Function),
     Array,
     ArrayIterator(ArrayIterator),
     ArrayBuffer(ArrayBuffer),
@@ -199,6 +202,26 @@ pub enum ObjectKind {
 }
 
 impl ObjectData {
+    /// Create the `AsyncGenerator` object data
+    pub fn async_generator(async_generator: AsyncGenerator) -> Self {
+        Self {
+            kind: ObjectKind::AsyncGenerator(async_generator),
+            internal_methods: &ORDINARY_INTERNAL_METHODS,
+        }
+    }
+
+    /// Create the `AsyncGeneratorFunction` object data
+    pub fn async_generator_function(function: Function) -> Self {
+        Self {
+            internal_methods: if function.is_constructor() {
+                &CONSTRUCTOR_INTERNAL_METHODS
+            } else {
+                &FUNCTION_INTERNAL_METHODS
+            },
+            kind: ObjectKind::GeneratorFunction(function),
+        }
+    }
+
     /// Create the `Array` object data and reference its exclusive internal methods
     pub fn array() -> Self {
         Self {
@@ -474,6 +497,8 @@ impl ObjectData {
 impl Display for ObjectKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(match self {
+            Self::AsyncGenerator(_) => "AsyncGenerator",
+            Self::AsyncGeneratorFunction(_) => "AsyncGeneratorFunction",
             Self::Array => "Array",
             Self::ArrayIterator(_) => "ArrayIterator",
             Self::ArrayBuffer(_) => "ArrayBuffer",
@@ -539,6 +564,42 @@ impl Object {
         &self.data.kind
     }
 
+    /// Checks if it's an `AsyncGenerator` object.
+    #[inline]
+    pub fn is_async_generator(&self) -> bool {
+        matches!(
+            self.data,
+            ObjectData {
+                kind: ObjectKind::AsyncGenerator(_),
+                ..
+            }
+        )
+    }
+
+    /// Returns a reference to the async generator data on the object.
+    #[inline]
+    pub fn as_async_generator(&self) -> Option<&AsyncGenerator> {
+        match self.data {
+            ObjectData {
+                kind: ObjectKind::AsyncGenerator(ref async_generator),
+                ..
+            } => Some(async_generator),
+            _ => None,
+        }
+    }
+
+    /// Returns a mutable reference to the async generator data on the object.
+    #[inline]
+    pub fn as_async_generator_mut(&mut self) -> Option<&mut AsyncGenerator> {
+        match self.data {
+            ObjectData {
+                kind: ObjectKind::AsyncGenerator(ref mut async_generator),
+                ..
+            } => Some(async_generator),
+            _ => None,
+        }
+    }
+
     /// Checks if it an `Array` object.
     #[inline]
     pub fn is_array(&self) -> bool {
@@ -1601,7 +1662,10 @@ impl<'context> FunctionBuilder<'context> {
             } => {
                 *constructor = yes.then(|| ConstructorKind::Base);
             }
-            Function::Ordinary { .. } | Function::Generator { .. } | Function::Async { .. } => {
+            Function::Ordinary { .. }
+            | Function::Generator { .. }
+            | Function::AsyncGenerator { .. }
+            | Function::Async { .. } => {
                 unreachable!("function must be native or closure");
             }
         }

boa_engine/src/syntax/ast/node/declaration/async_generator_decl/mod.rs

@@ -45,8 +45,8 @@ impl AsyncGeneratorDecl {
     }
 
     /// Gets the body of the async function declaration.
-    pub fn body(&self) -> &[Node] {
+    pub fn body(&self) -> &StatementList {
-        self.body.items()
+        &self.body
     }
 
     /// Implements the display formatting with indentation.
@@ -60,7 +60,7 @@ impl AsyncGeneratorDecl {
             interner.resolve_expect(self.name),
             join_nodes(interner, &self.parameters.parameters)
         );
-        if self.body().is_empty() {
+        if self.body().items().is_empty() {
             buf.push_str(") {}");
         } else {
             buf.push_str(&format!(

boa_engine/src/syntax/ast/node/mod.rs

@@ -945,6 +945,7 @@ impl Node {
                     return true;
                 }
             }
+            Node::AwaitExpr(_) if symbol == ContainsSymbol::AwaitExpression => return true,
             Node::AwaitExpr(expr) => {
                 if expr.expr().contains(symbol) {
                     return true;
@@ -1259,6 +1260,7 @@ pub(crate) enum ContainsSymbol {
     SuperProperty,
     SuperCall,
     YieldExpression,
+    AwaitExpression,
 }
 
 impl ToInternedString for Node {

boa_engine/src/syntax/ast/node/parameters.rs

@@ -114,6 +114,19 @@ impl FormalParameterList {
         }
         false
     }
+
+    /// Check if the any of the parameters contains a await expression.
+    pub(crate) fn contains_await_expression(&self) -> bool {
+        for parameter in self.parameters.iter() {
+            if parameter
+                .declaration()
+                .contains(ContainsSymbol::AwaitExpression)
+            {
+                return true;
+            }
+        }
+        false
+    }
 }
 
 impl From<Vec<FormalParameter>> for FormalParameterList {

boa_engine/src/syntax/parser/expression/primary/async_generator_expression/mod.rs

@@ -107,6 +107,22 @@ where
 
         let params = FormalParameters::new(true, true).parse(cursor, interner)?;
 
+        // It is a Syntax Error if FormalParameters Contains YieldExpression is true.
+        if params.contains_yield_expression() {
+            return Err(ParseError::lex(LexError::Syntax(
+                "yield expression not allowed in async generator expression parameters".into(),
+                params_start_position,
+            )));
+        }
+
+        // It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
+        if params.contains_await_expression() {
+            return Err(ParseError::lex(LexError::Syntax(
+                "await expression not allowed in async generator expression parameters".into(),
+                params_start_position,
+            )));
+        }
+
         cursor.expect(
             Punctuator::CloseParen,
             "async generator expression",

boa_engine/src/syntax/parser/expression/primary/object_initializer/mod.rs

@@ -780,8 +780,32 @@ where
         let name =
             ClassElementName::new(self.allow_yield, self.allow_await).parse(cursor, interner)?;
 
+        let params_start_position = cursor
+            .peek(0, interner)?
+            .ok_or(ParseError::AbruptEnd)?
+            .span()
+            .start();
+
         let params = UniqueFormalParameters::new(true, true).parse(cursor, interner)?;
 
+        // It is a Syntax Error if FormalParameters Contains YieldExpression is true.
+        if params.contains_yield_expression() {
+            return Err(ParseError::lex(LexError::Syntax(
+                "yield expression not allowed in async generator method definition parameters"
+                    .into(),
+                params_start_position,
+            )));
+        }
+
+        // It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
+        if params.contains_await_expression() {
+            return Err(ParseError::lex(LexError::Syntax(
+                "await expression not allowed in async generator method definition parameters"
+                    .into(),
+                params_start_position,
+            )));
+        }
+
         let body_start = cursor
             .expect(
                 TokenKind::Punctuator(Punctuator::OpenBlock),

boa_engine/src/vm/call_frame.rs

@@ -2,7 +2,7 @@
 //!
 //! This module will provides everything needed to implement the `CallFrame`
 
-use crate::vm::CodeBlock;
+use crate::{object::JsObject, vm::CodeBlock};
 use boa_gc::{Finalize, Gc, Trace};
 
 #[derive(Clone, Debug, Finalize, Trace)]
@@ -32,6 +32,10 @@ pub struct CallFrame {
 
     // Indicate that the last try block has thrown an exception.
     pub(crate) thrown: bool,
+
+    // When an async generator is resumed, the generator object is needed
+    // to fulfill the steps 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart).
+    pub(crate) async_generator: Option<JsObject>,
 }
 
 impl CallFrame {
@@ -111,7 +115,7 @@ pub(crate) enum FinallyReturn {
 }
 
 /// Indicates how a generator function that has been called/resumed should return.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub(crate) enum GeneratorResumeKind {
     Normal,
     Throw,

boa_engine/src/vm/code_block.rs

@@ -4,6 +4,7 @@
 
 use crate::{
     builtins::{
+        async_generator::{AsyncGenerator, AsyncGeneratorState},
         function::{
             arguments::Arguments, ClassFieldDefinition, ConstructorKind, Function, ThisMode,
         },
@@ -24,7 +25,7 @@ use crate::{
 use boa_gc::{Cell, Finalize, Gc, Trace};
 use boa_interner::{Interner, Sym, ToInternedString};
 use boa_profiler::Profiler;
-use std::{convert::TryInto, mem::size_of};
+use std::{collections::VecDeque, convert::TryInto, mem::size_of};
 
 /// This represents whether a value can be read from [`CodeBlock`] code.
 ///
@@ -223,7 +224,10 @@ impl CodeBlock {
                 *pc += size_of::<u32>();
                 format!("{operand1}, {operand2}")
             }
-            Opcode::GetFunction | Opcode::GetFunctionAsync | Opcode::GetGenerator => {
+            Opcode::GetFunction
+            | Opcode::GetFunctionAsync
+            | Opcode::GetGenerator
+            | Opcode::GetGeneratorAsync => {
                 let operand = self.read::<u32>(*pc);
                 *pc += size_of::<u32>();
                 format!(
@@ -364,6 +368,7 @@ impl CodeBlock {
             | Opcode::PopOnReturnSub
             | Opcode::Yield
             | Opcode::GeneratorNext
+            | Opcode::AsyncGeneratorNext
             | Opcode::PushClassField
             | Opcode::SuperCallDerived
             | Opcode::Await
@@ -546,13 +551,22 @@ pub(crate) fn create_function_object(
 /// Creates a new generator function object.
 pub(crate) fn create_generator_function_object(
     code: Gc<CodeBlock>,
+    r#async: bool,
     context: &mut Context,
 ) -> JsObject {
-    let function_prototype = context
+    let function_prototype = if r#async {
+        context
+            .intrinsics()
+            .constructors()
+            .async_generator_function()
+            .prototype()
+    } else {
+        context
             .intrinsics()
             .constructors()
             .generator_function()
-        .prototype();
+            .prototype()
+    };
 
     let name_property = PropertyDescriptor::builder()
         .value(context.interner().resolve_expect(code.name))
@@ -569,17 +583,34 @@ pub(crate) fn create_generator_function_object(
         .build();
 
     let prototype = JsObject::from_proto_and_data(
-        context.intrinsics().constructors().generator().prototype(),
+        if r#async {
+            context
+                .intrinsics()
+                .constructors()
+                .async_generator()
+                .prototype()
+        } else {
+            context.intrinsics().constructors().generator().prototype()
+        },
         ObjectData::ordinary(),
     );
 
+    let constructor = if r#async {
+        let function = Function::AsyncGenerator {
+            code,
+            environments: context.realm.environments.clone(),
+        };
+        JsObject::from_proto_and_data(
+            function_prototype,
+            ObjectData::async_generator_function(function),
+        )
+    } else {
         let function = Function::Generator {
             code,
             environments: context.realm.environments.clone(),
         };
-
+        JsObject::from_proto_and_data(function_prototype, ObjectData::generator_function(function))
-    let constructor =
+    };
-        JsObject::from_proto_and_data(function_prototype, ObjectData::generator_function(function));
 
     let prototype_property = PropertyDescriptor::builder()
         .value(prototype)
@@ -748,6 +779,7 @@ impl JsObject {
                     arg_count,
                     generator_resume_kind: GeneratorResumeKind::Normal,
                     thrown: false,
+                    async_generator: None,
                 });
 
                 let result = context.run();
@@ -870,6 +902,7 @@ impl JsObject {
                     arg_count,
                     generator_resume_kind: GeneratorResumeKind::Normal,
                     thrown: false,
+                    async_generator: None,
                 });
 
                 let _result = context.run();
@@ -982,6 +1015,7 @@ impl JsObject {
                     arg_count,
                     generator_resume_kind: GeneratorResumeKind::Normal,
                     thrown: false,
+                    async_generator: None,
                 };
                 let mut stack = args;
 
@@ -1018,6 +1052,155 @@ impl JsObject {
 
                 init_result?;
 
+                Ok(generator.into())
+            }
+            Function::AsyncGenerator { code, environments } => {
+                let code = code.clone();
+                let mut environments = environments.clone();
+                drop(object);
+
+                std::mem::swap(&mut environments, &mut context.realm.environments);
+
+                let lexical_this_mode = code.this_mode == ThisMode::Lexical;
+
+                let this = if lexical_this_mode {
+                    None
+                } else if code.strict {
+                    Some(this.clone())
+                } else if this.is_null_or_undefined() {
+                    Some(context.global_object().clone().into())
+                } else {
+                    Some(
+                        this.to_object(context)
+                            .expect("conversion cannot fail")
+                            .into(),
+                    )
+                };
+
+                if code.params.has_expressions() {
+                    context.realm.environments.push_function(
+                        code.num_bindings,
+                        code.compile_environments[1].clone(),
+                        this,
+                        self.clone(),
+                        None,
+                        lexical_this_mode,
+                    );
+                } else {
+                    context.realm.environments.push_function(
+                        code.num_bindings,
+                        code.compile_environments[0].clone(),
+                        this,
+                        self.clone(),
+                        None,
+                        lexical_this_mode,
+                    );
+                }
+
+                if let Some(binding) = code.arguments_binding {
+                    let arguments_obj = if code.strict || !code.params.is_simple() {
+                        Arguments::create_unmapped_arguments_object(args, context)
+                    } else {
+                        let env = context.realm.environments.current();
+                        Arguments::create_mapped_arguments_object(
+                            &this_function_object,
+                            &code.params,
+                            args,
+                            &env,
+                            context,
+                        )
+                    };
+                    context.realm.environments.put_value(
+                        binding.environment_index(),
+                        binding.binding_index(),
+                        arguments_obj.into(),
+                    );
+                }
+
+                let arg_count = args.len();
+
+                // Push function arguments to the stack.
+                let mut args = if code.params.parameters.len() > args.len() {
+                    let mut v = args.to_vec();
+                    v.extend(vec![
+                        JsValue::Undefined;
+                        code.params.parameters.len() - args.len()
+                    ]);
+                    v
+                } else {
+                    args.to_vec()
+                };
+                args.reverse();
+
+                let param_count = code.params.parameters.len();
+
+                let call_frame = CallFrame {
+                    code,
+                    pc: 0,
+                    catch: Vec::new(),
+                    finally_return: FinallyReturn::None,
+                    finally_jump: Vec::new(),
+                    pop_on_return: 0,
+                    loop_env_stack: Vec::from([0]),
+                    try_env_stack: Vec::from([crate::vm::TryStackEntry {
+                        num_env: 0,
+                        num_loop_stack_entries: 0,
+                    }]),
+                    param_count,
+                    arg_count,
+                    generator_resume_kind: GeneratorResumeKind::Normal,
+                    thrown: false,
+                    async_generator: None,
+                };
+                let mut stack = args;
+
+                std::mem::swap(&mut context.vm.stack, &mut stack);
+                context.vm.push_frame(call_frame);
+
+                let init_result = context.run();
+
+                let call_frame = context.vm.pop_frame().expect("frame must exist");
+                std::mem::swap(&mut environments, &mut context.realm.environments);
+                std::mem::swap(&mut context.vm.stack, &mut stack);
+
+                let prototype = if let Some(prototype) = this_function_object
+                    .get("prototype", context)
+                    .expect("AsyncGeneratorFunction must have a prototype property")
+                    .as_object()
+                {
+                    prototype.clone()
+                } else {
+                    context
+                        .intrinsics()
+                        .constructors()
+                        .async_generator()
+                        .prototype()
+                };
+
+                let generator = Self::from_proto_and_data(
+                    prototype,
+                    ObjectData::async_generator(AsyncGenerator {
+                        state: AsyncGeneratorState::SuspendedStart,
+                        context: Some(Gc::new(Cell::new(GeneratorContext {
+                            environments,
+                            call_frame,
+                            stack,
+                        }))),
+                        queue: VecDeque::new(),
+                    }),
+                );
+
+                {
+                    let mut generator_mut = generator.borrow_mut();
+                    let gen = generator_mut
+                        .as_async_generator_mut()
+                        .expect("must be object here");
+                    let mut gen_context = gen.context.as_ref().expect("must exist").borrow_mut();
+                    gen_context.call_frame.async_generator = Some(generator.clone());
+                }
+
+                init_result?;
+
                 Ok(generator.into())
             }
         }
@@ -1215,6 +1398,7 @@ impl JsObject {
                     arg_count,
                     generator_resume_kind: GeneratorResumeKind::Normal,
                     thrown: false,
+                    async_generator: None,
                 });
 
                 let result = context.run();
@@ -1253,7 +1437,9 @@ impl JsObject {
                     }
                 }
             }
-            Function::Generator { .. } | Function::Async { .. } => {
+            Function::Generator { .. }
+            | Function::Async { .. }
+            | Function::AsyncGenerator { .. } => {
                 unreachable!("not a constructor")
             }
         }

boa_engine/src/vm/mod.rs

@@ -4,6 +4,7 @@
 
 use crate::{
     builtins::{
+        async_generator::{AsyncGenerator, AsyncGeneratorState},
         function::{ConstructorKind, Function},
         iterable::IteratorRecord,
         Array, ForInIterator, JsArgs, Number, Promise,
@@ -1695,7 +1696,13 @@ impl Context {
             Opcode::GetGenerator => {
                 let index = self.vm.read::<u32>();
                 let code = self.vm.frame().code.functions[index as usize].clone();
-                let function = create_generator_function_object(code, self);
+                let function = create_generator_function_object(code, false, self);
+                self.vm.push(function);
+            }
+            Opcode::GetGeneratorAsync => {
+                let index = self.vm.read::<u32>();
+                let code = self.vm.frame().code.functions[index as usize].clone();
+                let function = create_generator_function_object(code, true, self);
                 self.vm.push(function);
             }
             Opcode::CallEval => {
@@ -2179,6 +2186,54 @@ impl Context {
                     return Ok(ShouldExit::True);
                 }
             },
+            Opcode::AsyncGeneratorNext => {
+                let value = self.vm.pop();
+
+                if self.vm.frame().generator_resume_kind == GeneratorResumeKind::Throw {
+                    return Err(value);
+                }
+
+                let completion = Ok(value);
+                let generator_object = self
+                    .vm
+                    .frame()
+                    .async_generator
+                    .as_ref()
+                    .expect("must be in generator context here")
+                    .clone();
+                let next = generator_object
+                    .borrow_mut()
+                    .as_async_generator_mut()
+                    .expect("must be async generator object")
+                    .queue
+                    .pop_front()
+                    .expect("must have item in queue");
+                AsyncGenerator::complete_step(&next, completion, false, self);
+
+                let mut generator_object_mut = generator_object.borrow_mut();
+                let gen = generator_object_mut
+                    .as_async_generator_mut()
+                    .expect("must be async generator object");
+
+                if let Some(next) = gen.queue.front() {
+                    let (completion, r#return) = &next.completion;
+                    if *r#return {
+                        match completion {
+                            Ok(value) | Err(value) => self.vm.push(value),
+                        }
+                        self.vm.push(true);
+                    } else {
+                        self.vm.push(completion.clone()?);
+                        self.vm.push(false);
+                    }
+
+                    self.vm.push(false);
+                } else {
+                    gen.state = AsyncGeneratorState::SuspendedYield;
+                    self.vm.push(true);
+                    self.vm.push(true);
+                }
+            }
             Opcode::GeneratorNextDelegate => {
                 let done_address = self.vm.read::<u32>();
                 let received = self.vm.pop();
@@ -2483,6 +2538,24 @@ impl Context {
                             .call(&JsValue::undefined(), &[result.clone()], self)
                             .expect("cannot fail per spec");
                     }
+                    // Step 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
+                    else if let Some(generator_object) = self.vm.frame().async_generator.clone() {
+                        let mut generator_object_mut = generator_object.borrow_mut();
+                        let generator = generator_object_mut
+                            .as_async_generator_mut()
+                            .expect("must be async generator");
+
+                        generator.state = AsyncGeneratorState::Completed;
+
+                        let next = generator
+                            .queue
+                            .pop_front()
+                            .expect("must have item in queue");
+                        drop(generator_object_mut);
+                        AsyncGenerator::complete_step(&next, Ok(result), true, self);
+                        AsyncGenerator::drain_queue(&generator_object, self);
+                        return Ok((JsValue::undefined(), ReturnType::Normal));
+                    }
 
                     return Ok((result, ReturnType::Normal));
                 }
@@ -2548,6 +2621,26 @@ impl Context {
 
                             return Ok((e, ReturnType::Normal));
                         }
+                        // Step 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
+                        else if let Some(generator_object) =
+                            self.vm.frame().async_generator.clone()
+                        {
+                            let mut generator_object_mut = generator_object.borrow_mut();
+                            let generator = generator_object_mut
+                                .as_async_generator_mut()
+                                .expect("must be async generator");
+
+                            generator.state = AsyncGeneratorState::Completed;
+
+                            let next = generator
+                                .queue
+                                .pop_front()
+                                .expect("must have item in queue");
+                            drop(generator_object_mut);
+                            AsyncGenerator::complete_step(&next, Err(e), true, self);
+                            AsyncGenerator::drain_queue(&generator_object, self);
+                            return Ok((JsValue::undefined(), ReturnType::Normal));
+                        }
 
                         return Err(e);
                     }
@@ -2586,6 +2679,23 @@ impl Context {
                     .call(&JsValue::undefined(), &[], self)
                     .expect("cannot fail per spec");
             }
+            // Step 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
+            else if let Some(generator_object) = self.vm.frame().async_generator.clone() {
+                let mut generator_object_mut = generator_object.borrow_mut();
+                let generator = generator_object_mut
+                    .as_async_generator_mut()
+                    .expect("must be async generator");
+
+                generator.state = AsyncGeneratorState::Completed;
+
+                let next = generator
+                    .queue
+                    .pop_front()
+                    .expect("must have item in queue");
+                drop(generator_object_mut);
+                AsyncGenerator::complete_step(&next, Ok(JsValue::undefined()), true, self);
+                AsyncGenerator::drain_queue(&generator_object, self);
+            }
 
             return Ok((JsValue::undefined(), ReturnType::Normal));
         }
@@ -2600,6 +2710,24 @@ impl Context {
                 .call(&JsValue::undefined(), &[result.clone()], self)
                 .expect("cannot fail per spec");
         }
+        // Step 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
+        else if let Some(generator_object) = self.vm.frame().async_generator.clone() {
+            let mut generator_object_mut = generator_object.borrow_mut();
+            let generator = generator_object_mut
+                .as_async_generator_mut()
+                .expect("must be async generator");
+
+            generator.state = AsyncGeneratorState::Completed;
+
+            let next = generator
+                .queue
+                .pop_front()
+                .expect("must have item in queue");
+            drop(generator_object_mut);
+            AsyncGenerator::complete_step(&next, Ok(result), true, self);
+            AsyncGenerator::drain_queue(&generator_object, self);
+            return Ok((JsValue::undefined(), ReturnType::Normal));
+        }
 
         Ok((result, ReturnType::Normal))
     }

boa_engine/src/vm/opcode.rs

@@ -927,6 +927,13 @@ pub enum Opcode {
     /// Stack: **=>** func
     GetGenerator,
 
+    /// Get async generator function from the pre-compiled inner functions.
+    ///
+    /// Operands: address: `u32`
+    ///
+    /// Stack: **=>** func
+    GetGeneratorAsync,
+
     /// Call a function named "eval".
     ///
     /// Operands: argument_count: `u32`
@@ -1125,6 +1132,13 @@ pub enum Opcode {
     /// Stack: received **=>**
     GeneratorNext,
 
+    /// Resumes the current generator function.
+    ///
+    /// Operands:
+    ///
+    /// Stack: received **=>** Option<value>, skip_0, skip_1
+    AsyncGeneratorNext,
+
     /// Delegates the current generator function another generator.
     ///
     /// Operands: done_address: `u32`
@@ -1285,6 +1299,7 @@ impl Opcode {
             Self::GetFunction => "GetFunction",
             Self::GetFunctionAsync => "GetFunctionAsync",
             Self::GetGenerator => "GetGenerator",
+            Self::GetGeneratorAsync => "GetGeneratorAsync",
             Self::CallEval => "CallEval",
             Self::CallEvalWithRest => "CallEvalWithRest",
             Self::Call => "Call",
@@ -1313,6 +1328,7 @@ impl Opcode {
             Self::PopOnReturnSub => "PopOnReturnSub",
             Self::Yield => "Yield",
             Self::GeneratorNext => "GeneratorNext",
+            Self::AsyncGeneratorNext => "AsyncGeneratorNext",
             Self::Await => "Await",
             Self::GeneratorNextDelegate => "GeneratorNextDelegate",
             Self::Nop => "Nop",
@@ -1425,6 +1441,7 @@ impl Opcode {
             Self::GetFunction => "INST - GetFunction",
             Self::GetFunctionAsync => "INST - GetFunctionAsync",
             Self::GetGenerator => "INST - GetGenerator",
+            Self::GetGeneratorAsync => "INST - GetGeneratorAsync",
             Self::CallEval => "INST - CallEval",
             Self::CallEvalWithRest => "INST - CallEvalWithRest",
             Self::Call => "INST - Call",
@@ -1453,6 +1470,7 @@ impl Opcode {
             Self::PopOnReturnSub => "INST - PopOnReturnSub",
             Self::Yield => "INST - Yield",
             Self::GeneratorNext => "INST - GeneratorNext",
+            Self::AsyncGeneratorNext => "INST - AsyncGeneratorNext",
             Self::Await => "INST - Await",
             Self::GeneratorNextDelegate => "INST - GeneratorNextDelegate",
             Self::Nop => "INST - Nop",