Async/Await parse (#836)

* Add async keyword

* Created AsyncFunctionDecl/Expr nodes

* AsyncFunctionDecl/Expr parser created (not impl)

* Add async to HoistableDeclaration

* Added expect_no_lineterminator, updated some cursor docs

* AsyncFunctionDecl parsing

* AsyncFunctionExpr parsing

* Await expression parsing

* Added some async function expr parsing tests

* Reposition declaration parsing ready to add tests

* Moved function decl tests

* Added some failing async func decl tests

* mdn/spec links@

* Function expression parsing into own folder

* Added function expression test

* Fixed async function decl parse

* Async/await run returns undefined rather than panic

* Clippy :( -> :)

* Missing docs

* Update boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs

Co-authored-by: Halid Odat <halidodat@gmail.com>

* Update boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs

Co-authored-by: Halid Odat <halidodat@gmail.com>

* Update boa/src/syntax/parser/statement/mod.rs

Co-authored-by: Halid Odat <halidodat@gmail.com>

* Update boa/src/syntax/parser/statement/declaration/mod.rs

Co-authored-by: Halid Odat <halidodat@gmail.com>

* Update boa/src/syntax/ast/node/await_expr/mod.rs

Co-authored-by: Halid Odat <halidodat@gmail.com>

* Revert "Update boa/src/syntax/parser/statement/declaration/mod.rs"

This reverts commit 46b2917530.

* Revert "Update boa/src/syntax/parser/statement/mod.rs"

This reverts commit 4c0c1238ec.

* Update boa/src/syntax/parser/expression/primary/async_function_expression/mod.rs

Co-authored-by: Iban Eguia <razican@protonmail.ch>

* Update boa/src/syntax/parser/statement/declaration/hoistable/async_function_decl/mod.rs

Co-authored-by: Iban Eguia <razican@protonmail.ch>

* Addressed review comments

* Correct allow_yield parameters for async func

* Correct other usages of allow_yield

Co-authored-by: Halid Odat <halidodat@gmail.com>
Co-authored-by: Iban Eguia <razican@protonmail.ch>

boa/src/syntax/ast/keyword.rs

@@ -36,6 +36,16 @@ pub enum Keyword {
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
     Await,
 
+    /// The `async` keyword.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#prod-AsyncMethod
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
+    Async,
+
     /// The `break` keyword.
     ///
     /// More information:
@@ -454,6 +464,7 @@ impl Keyword {
     pub fn as_str(self) -> &'static str {
         match self {
             Self::Await => "await",
+            Self::Async => "async",
             Self::Break => "break",
             Self::Case => "case",
             Self::Catch => "catch",
@@ -526,6 +537,7 @@ impl FromStr for Keyword {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "await" => Ok(Self::Await),
+            "async" => Ok(Self::Async),
             "break" => Ok(Self::Break),
             "case" => Ok(Self::Case),
             "catch" => Ok(Self::Catch),

boa/src/syntax/ast/node/await_expr/mod.rs

@@ -0,0 +1,61 @@
+//! Await expression node.
+
+use super::Node;
+use crate::{exec::Executable, BoaProfiler, Context, Result, Value};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// An await expression is used within an async function to pause execution and wait for a
+/// promise to resolve.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-AwaitExpression
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct AwaitExpr {
+    expr: Box<Node>,
+}
+
+impl Executable for AwaitExpr {
+    fn run(&self, _: &mut Context) -> Result<Value> {
+        let _timer = BoaProfiler::global().start_event("AwaitExpression", "exec");
+        // TODO: Implement AwaitExpr
+        Ok(Value::Undefined)
+    }
+}
+
+impl AwaitExpr {
+    /// Implements the display formatting with indentation.
+    pub(super) fn display(&self, f: &mut fmt::Formatter<'_>, indentation: usize) -> fmt::Result {
+        writeln!(f, "await ")?;
+        self.expr.display(f, indentation)
+    }
+}
+
+impl<T> From<T> for AwaitExpr
+where
+    T: Into<Box<Node>>,
+{
+    fn from(e: T) -> Self {
+        Self { expr: e.into() }
+    }
+}
+
+impl fmt::Display for AwaitExpr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
+
+impl From<AwaitExpr> for Node {
+    fn from(awaitexpr: AwaitExpr) -> Self {
+        Self::AwaitExpr(awaitexpr)
+    }
+}

boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs

@@ -0,0 +1,101 @@
+//! Async Function Declaration.
+
+use crate::{
+    exec::Executable,
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    BoaProfiler, Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// An async function is used to specify an action (or series of actions) to perform asynchronously.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-async-function-prototype-properties
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct AsyncFunctionDecl {
+    name: Option<Box<str>>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl AsyncFunctionDecl {
+    /// Creates a new async function declaration.
+    pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
+    where
+        N: Into<Option<Box<str>>>,
+        P: Into<Box<[FormalParameter]>>,
+        B: Into<StatementList>,
+    {
+        Self {
+            name: name.into(),
+            parameters: parameters.into(),
+            body: body.into(),
+        }
+    }
+
+    /// Gets the name of the async function declaration.
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_deref()
+    }
+
+    /// Gets the list of parameters of the async function declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the async function declaration.
+    pub fn body(&self) -> &[Node] {
+        self.body.statements()
+    }
+
+    /// Implements the display formatting with indentation.
+    pub(in crate::syntax::ast::node) fn display(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        match &self.name {
+            Some(name) => {
+                write!(f, "async function {}(", name)?;
+            }
+            None => {
+                write!(f, "async function (")?;
+            }
+        }
+        join_nodes(f, &self.parameters)?;
+        f.write_str(") {{")?;
+
+        self.body.display(f, indentation + 1)?;
+
+        writeln!(f, "}}")
+    }
+}
+
+impl Executable for AsyncFunctionDecl {
+    fn run(&self, _: &mut Context) -> Result<Value> {
+        let _timer = BoaProfiler::global().start_event("AsyncFunctionDecl", "exec");
+        // TODO: Implement AsyncFunctionDecl
+        Ok(Value::undefined())
+    }
+}
+
+impl From<AsyncFunctionDecl> for Node {
+    fn from(decl: AsyncFunctionDecl) -> Self {
+        Self::AsyncFunctionDecl(decl)
+    }
+}
+
+impl fmt::Display for AsyncFunctionDecl {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}

boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs

@@ -0,0 +1,98 @@
+//! Async Function Expression.
+
+use crate::{
+    exec::Executable,
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// An async function expression is very similar to an async function declaration except used within
+/// a wider expression (for example during an assignment).
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-AsyncFunctionExpression
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct AsyncFunctionExpr {
+    name: Option<Box<str>>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl AsyncFunctionExpr {
+    /// Creates a new function expression
+    pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
+    where
+        N: Into<Option<Box<str>>>,
+        P: Into<Box<[FormalParameter]>>,
+        B: Into<StatementList>,
+    {
+        Self {
+            name: name.into(),
+            parameters: parameters.into(),
+            body: body.into(),
+        }
+    }
+
+    /// Gets the name of the function declaration.
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_ref().map(Box::as_ref)
+    }
+
+    /// Gets the list of parameters of the function declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the function declaration.
+    pub fn body(&self) -> &[Node] {
+        self.body.statements()
+    }
+
+    /// Implements the display formatting with indentation.
+    pub(in crate::syntax::ast::node) fn display(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        f.write_str("function")?;
+        if let Some(ref name) = self.name {
+            write!(f, " {}", name)?;
+        }
+        f.write_str("(")?;
+        join_nodes(f, &self.parameters)?;
+        f.write_str(") {{")?;
+
+        self.body.display(f, indentation + 1)?;
+
+        writeln!(f, "}}")
+    }
+}
+
+impl Executable for AsyncFunctionExpr {
+    fn run(&self, _: &mut Context) -> Result<Value> {
+        // TODO: Implement AsyncFunctionExpr
+        Ok(Value::Undefined)
+    }
+}
+
+impl fmt::Display for AsyncFunctionExpr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
+
+impl From<AsyncFunctionExpr> for Node {
+    fn from(expr: AsyncFunctionExpr) -> Self {
+        Self::AsyncFunctionExpr(expr)
+    }
+}

boa/src/syntax/ast/node/declaration/mod.rs

@@ -1,6 +1,8 @@
 //! Declaration nodes
 
 pub mod arrow_function_decl;
+pub mod async_function_decl;
+pub mod async_function_expr;
 pub mod const_decl_list;
 pub mod function_decl;
 pub mod function_expr;
@@ -9,6 +11,8 @@ pub mod var_decl_list;
 
 pub use self::{
     arrow_function_decl::ArrowFunctionDecl,
+    async_function_decl::AsyncFunctionDecl,
+    async_function_expr::AsyncFunctionExpr,
     const_decl_list::{ConstDecl, ConstDeclList},
     function_decl::FunctionDecl,
     function_expr::FunctionExpr,

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

@@ -1,6 +1,7 @@
 //! This module implements the `Node` structure, which composes the AST.
 
 pub mod array;
+pub mod await_expr;
 pub mod block;
 pub mod break_node;
 pub mod call;
@@ -21,13 +22,14 @@ pub mod try_node;
 
 pub use self::{
     array::ArrayDecl,
+    await_expr::AwaitExpr,
     block::Block,
     break_node::Break,
     call::Call,
     conditional::{ConditionalOp, If},
     declaration::{
-        ArrowFunctionDecl, ConstDecl, ConstDeclList, FunctionDecl, FunctionExpr, LetDecl,
-        LetDeclList, VarDecl, VarDeclList,
+        ArrowFunctionDecl, AsyncFunctionDecl, AsyncFunctionExpr, ConstDecl, ConstDeclList,
+        FunctionDecl, FunctionExpr, LetDecl, LetDeclList, VarDecl, VarDeclList,
     },
     field::{GetConstField, GetField},
     identifier::Identifier,
@@ -65,6 +67,15 @@ pub enum Node {
     /// An assignment operator node. [More information](./operator/struct.Assign.html).
     Assign(Assign),
 
+    /// An async function declaration node. [More information](./declaration/struct.AsyncFunctionDecl.html).
+    AsyncFunctionDecl(AsyncFunctionDecl),
+
+    /// An async function expression node. [More information](./declaration/struct.AsyncFunctionExpr.html).
+    AsyncFunctionExpr(AsyncFunctionExpr),
+
+    /// An await expression node. [More information](./await_expr/struct.AwaitExpression.html).
+    AwaitExpr(AwaitExpr),
+
     /// A binary operator node. [More information](./operator/struct.BinOp.html).
     BinOp(BinOp),
 
@@ -104,7 +115,7 @@ pub enum Node {
     /// A function declaration node. [More information](./declaration/struct.FunctionDecl.html).
     FunctionDecl(FunctionDecl),
 
-    /// A function expressino node. [More information](./declaration/struct.FunctionExpr.html).
+    /// A function expression node. [More information](./declaration/struct.FunctionExpr.html).
     FunctionExpr(FunctionExpr),
 
     /// Provides access to an object types' constant properties. [More information](./declaration/struct.GetConstField.html).
@@ -243,6 +254,9 @@ impl Node {
             Self::Assign(ref op) => Display::fmt(op, f),
             Self::LetDeclList(ref decl) => Display::fmt(decl, f),
             Self::ConstDeclList(ref decl) => Display::fmt(decl, f),
+            Self::AsyncFunctionDecl(ref decl) => decl.display(f, indentation),
+            Self::AsyncFunctionExpr(ref expr) => expr.display(f, indentation),
+            Self::AwaitExpr(ref expr) => expr.display(f, indentation),
         }
     }
 }
@@ -251,6 +265,9 @@ impl Executable for Node {
     fn run(&self, interpreter: &mut Context) -> Result<Value> {
         let _timer = BoaProfiler::global().start_event("Executable", "exec");
         match *self {
+            Node::AsyncFunctionDecl(ref decl) => decl.run(interpreter),
+            Node::AsyncFunctionExpr(ref function_expr) => function_expr.run(interpreter),
+            Node::AwaitExpr(ref expr) => expr.run(interpreter),
             Node::Call(ref call) => call.run(interpreter),
             Node::Const(Const::Null) => Ok(Value::null()),
             Node::Const(Const::Num(num)) => Ok(Value::rational(num)),

boa/src/syntax/parser/cursor/mod.rs

@@ -67,10 +67,6 @@ where
     }
 
     /// Returns an error if the next token is not of kind `kind`.
-    ///
-    /// Note: it will consume the next token only if the next token is the expected type.
-    ///
-    /// If skip_line_terminators is true then line terminators will be discarded.
     #[inline]
     pub(super) fn expect<K>(&mut self, kind: K, context: &'static str) -> Result<Token, ParseError>
     where
@@ -134,15 +130,16 @@ where
     ///
     /// It expects that the token stream does not end here.
     ///
-    /// This is just syntatic sugar for a .peek(skip_n, false) call followed by a check that the result is not a line terminator or None.
+    /// This is just syntatic sugar for a .peek(skip_n) call followed by a check that the result is not a line terminator or None.
     #[inline]
     pub(super) fn peek_expect_no_lineterminator(
         &mut self,
         skip_n: usize,
+        context: &'static str,
     ) -> Result<&Token, ParseError> {
         if let Some(t) = self.buffered_lexer.peek(skip_n, false)? {
             if t.kind() == &TokenKind::LineTerminator {
-                Err(ParseError::unexpected(t.clone(), None))
+                Err(ParseError::unexpected(t.clone(), context))
             } else {
                 Ok(t)
             }
@@ -156,8 +153,6 @@ where
     /// When the next token is a `kind` token, get the token, otherwise return `None`.
     ///
     /// No next token also returns None.
-    ///
-    /// If skip_line_terminators is true then line terminators will be discarded.
     #[inline]
     pub(super) fn next_if<K>(&mut self, kind: K) -> Result<Option<Token>, ParseError>
     where

boa/src/syntax/parser/expression/assignment/arrow_function.rs

@@ -86,7 +86,7 @@ where
             Box::new([FormalParameter::new(param, None, false)])
         };
 
-        cursor.peek_expect_no_lineterminator(0)?;
+        cursor.peek_expect_no_lineterminator(0, "arrow function")?;
 
         cursor.expect(TokenKind::Punctuator(Punctuator::Arrow), "arrow function")?;
         let body = ConciseBody::new(self.allow_in).parse(cursor)?;

boa/src/syntax/parser/expression/assignment/mod.rs

@@ -88,7 +88,7 @@ where
             TokenKind::Identifier(_)
             | TokenKind::Keyword(Keyword::Yield)
             | TokenKind::Keyword(Keyword::Await) => {
-                if let Ok(tok) = cursor.peek_expect_no_lineterminator(1) {
+                if let Ok(tok) = cursor.peek_expect_no_lineterminator(1, "assignment expression") {
                     if tok.kind() == &TokenKind::Punctuator(Punctuator::Arrow) {
                         return ArrowFunction::new(
                             self.allow_in,

boa/src/syntax/parser/expression/await_expr.rs

@@ -0,0 +1,58 @@
+//! Await expression parsing.
+//!
+//! More information:
+//!  - [MDN documentation][mdn]
+//!  - [ECMAScript specification][spec]
+//!
+//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
+//! [spec]: https://tc39.es/ecma262/#prod-AwaitExpression
+
+use super::unary::UnaryExpression;
+
+use crate::syntax::{
+    ast::{node::AwaitExpr, Keyword},
+    lexer::TokenKind,
+    parser::{AllowYield, Cursor, ParseError, TokenParser},
+};
+use std::io::Read;
+
+/// Parses an await expression.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
+/// [spec]: https://tc39.es/ecma262/#prod-AwaitExpression
+#[derive(Debug, Clone, Copy)]
+pub(in crate::syntax::parser) struct AwaitExpression {
+    allow_yield: AllowYield,
+}
+
+impl AwaitExpression {
+    /// Creates a new `AwaitExpression` parser.
+    pub(in crate::syntax::parser) fn new<Y>(allow_yield: Y) -> Self
+    where
+        Y: Into<AllowYield>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for AwaitExpression
+where
+    R: Read,
+{
+    type Output = AwaitExpr;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        cursor.expect(
+            TokenKind::Keyword(Keyword::Await),
+            "Await expression parsing",
+        )?;
+        let expr = UnaryExpression::new(self.allow_yield, true).parse(cursor)?;
+        Ok(expr.into())
+    }
+}

boa/src/syntax/parser/expression/mod.rs

@@ -15,6 +15,8 @@ mod tests;
 mod unary;
 mod update;
 
+pub(in crate::syntax::parser) mod await_expr;
+
 use self::assignment::ExponentiationExpression;
 pub(super) use self::{assignment::AssignmentExpression, primary::Initializer};
 use super::{AllowAwait, AllowIn, AllowYield, Cursor, ParseResult, TokenParser};

boa/src/syntax/parser/expression/primary/async_function_expression/mod.rs

@@ -0,0 +1,79 @@
+#[cfg(test)]
+mod tests;
+
+use crate::{
+    syntax::{
+        ast::{node::AsyncFunctionExpr, Keyword, Punctuator},
+        lexer::TokenKind,
+        parser::{
+            function::{FormalParameters, FunctionBody},
+            statement::BindingIdentifier,
+            AllowYield, Cursor, ParseError, TokenParser,
+        },
+    },
+    BoaProfiler,
+};
+
+use std::io::Read;
+
+/// Async Function expression parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function
+/// [spec]: https://tc39.es/ecma262/#prod-AsyncFunctionExpression
+#[derive(Debug, Clone, Copy)]
+pub(super) struct AsyncFunctionExpression {
+    allow_yield: AllowYield,
+}
+
+impl AsyncFunctionExpression {
+    /// Creates a new `AsyncFunctionExpression` parser.
+    pub(super) fn new<Y>(allow_yield: Y) -> Self
+    where
+        Y: Into<AllowYield>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for AsyncFunctionExpression
+where
+    R: Read,
+{
+    type Output = AsyncFunctionExpr;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        let _timer = BoaProfiler::global().start_event("AsyncFunctionExpression", "Parsing");
+        cursor.peek_expect_no_lineterminator(0, "async function expression")?;
+        cursor.expect(Keyword::Function, "async function expression")?;
+
+        let tok = cursor.peek(0)?;
+
+        let name = if let Some(token) = tok {
+            match token.kind() {
+                TokenKind::Punctuator(Punctuator::OpenParen) => None,
+                _ => Some(BindingIdentifier::new(self.allow_yield, true).parse(cursor)?),
+            }
+        } else {
+            return Err(ParseError::AbruptEnd);
+        };
+
+        cursor.expect(Punctuator::OpenParen, "async function expression")?;
+
+        let params = FormalParameters::new(false, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseParen, "async function expression")?;
+        cursor.expect(Punctuator::OpenBlock, "async function expression")?;
+
+        let body = FunctionBody::new(false, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseBlock, "async function expression")?;
+
+        Ok(AsyncFunctionExpr::new(name, params, body))
+    }
+}

boa/src/syntax/parser/expression/primary/async_function_expression/tests.rs

@@ -0,0 +1,65 @@
+use crate::syntax::{
+    ast::{
+        node::{AsyncFunctionExpr, ConstDecl, ConstDeclList, Return, StatementList},
+        Const,
+    },
+    parser::tests::check_parser,
+};
+
+/// Checks async expression parsing.
+#[test]
+fn check_async_expression() {
+    check_parser(
+        "const add = async function() {
+            return 1; 
+        };
+        ",
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "add",
+            Some(
+                AsyncFunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                    None,
+                    [],
+                    vec![Return::new::<_, _, Option<Box<str>>>(Const::from(1), None).into()].into(),
+                ),
+            ),
+        )])
+        .into()],
+    );
+}
+
+#[test]
+fn check_nested_async_expression() {
+    check_parser(
+        "const a = async function() {
+            const b = async function() {
+                return 1; 
+            };
+        };
+        ",
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "a",
+            Some(
+                AsyncFunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                    None,
+                    [],
+                    vec![ConstDeclList::from(vec![ConstDecl::new(
+                        "b",
+                        Some(
+                            AsyncFunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                                None,
+                                [],
+                                vec![Return::new::<_, _, Option<Box<str>>>(Const::from(1), None)
+                                    .into()]
+                                .into(),
+                            ),
+                        ),
+                    )])
+                    .into()]
+                    .into(),
+                ),
+            ),
+        )])
+        .into()],
+    );
+}

boa/src/syntax/parser/expression/primary/function_expression/mod.rs

@@ -7,6 +7,9 @@
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function
 //! [spec]: https://tc39.es/ecma262/#prod-FunctionExpression
 
+#[cfg(test)]
+mod tests;
+
 use crate::{
     syntax::{
         ast::{node::FunctionExpr, Keyword, Punctuator},

boa/src/syntax/parser/expression/primary/function_expression/tests.rs

@@ -0,0 +1,58 @@
+use crate::syntax::{
+    ast::{
+        node::{ConstDecl, ConstDeclList, FunctionExpr, Return, StatementList},
+        Const,
+    },
+    parser::tests::check_parser,
+};
+
+/// Checks async expression parsing.
+#[test]
+fn check_function_expression() {
+    check_parser(
+        "const add = function() {
+            return 1;
+        };
+        ",
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "add",
+            Some(FunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                None,
+                [],
+                vec![Return::new::<_, _, Option<Box<str>>>(Const::from(1), None).into()].into(),
+            )),
+        )])
+        .into()],
+    );
+}
+
+#[test]
+fn check_nested_function_expression() {
+    check_parser(
+        "const a = function() {
+            const b = function() {
+                return 1; 
+            };
+        };
+        ",
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "a",
+            Some(FunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                None,
+                [],
+                vec![ConstDeclList::from(vec![ConstDecl::new(
+                    "b",
+                    Some(FunctionExpr::new::<Option<Box<str>>, _, StatementList>(
+                        None,
+                        [],
+                        vec![Return::new::<_, _, Option<Box<str>>>(Const::from(1), None).into()]
+                            .into(),
+                    )),
+                )])
+                .into()]
+                .into(),
+            )),
+        )])
+        .into()],
+    );
+}

boa/src/syntax/parser/expression/primary/mod.rs

@@ -8,14 +8,15 @@
 //! [spec]: https://tc39.es/ecma262/#prod-PrimaryExpression
 
 mod array_initializer;
+mod async_function_expression;
 mod function_expression;
 mod object_initializer;
 #[cfg(test)]
 mod tests;
 
 use self::{
-    array_initializer::ArrayLiteral, function_expression::FunctionExpression,
-    object_initializer::ObjectLiteral,
+    array_initializer::ArrayLiteral, async_function_expression::AsyncFunctionExpression,
+    function_expression::FunctionExpression, object_initializer::ObjectLiteral,
 };
 use super::Expression;
 use crate::{
@@ -77,6 +78,9 @@ where
             TokenKind::Keyword(Keyword::Function) => {
                 FunctionExpression.parse(cursor).map(Node::from)
             }
+            TokenKind::Keyword(Keyword::Async) => AsyncFunctionExpression::new(self.allow_yield)
+                .parse(cursor)
+                .map(Node::from),
             TokenKind::Punctuator(Punctuator::OpenParen) => {
                 cursor.set_goal(InputElement::RegExp);
                 let expr =

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

@@ -141,6 +141,17 @@ where
             return Ok(node::PropertyDefinition::property(prop_name, val));
         }
 
+        // TODO GeneratorMethod
+        // https://tc39.es/ecma262/#prod-GeneratorMethod
+
+        if prop_name.as_str() == "async" {
+            // TODO - AsyncMethod.
+            // https://tc39.es/ecma262/#prod-AsyncMethod
+
+            // TODO - AsyncGeneratorMethod
+            // https://tc39.es/ecma262/#prod-AsyncGeneratorMethod
+        }
+
         if cursor
             .next_if(TokenKind::Punctuator(Punctuator::OpenParen))?
             .is_some()

boa/src/syntax/parser/expression/unary.rs

@@ -33,14 +33,14 @@ use std::io::Read;
 /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Unary
 /// [spec]: https://tc39.es/ecma262/#prod-UnaryExpression
 #[derive(Debug, Clone, Copy)]
-pub(super) struct UnaryExpression {
+pub(in crate::syntax::parser) struct UnaryExpression {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
 }
 
 impl UnaryExpression {
     /// Creates a new `UnaryExpression` parser.
-    pub(super) fn new<Y, A>(allow_yield: Y, allow_await: A) -> Self
+    pub(in crate::syntax::parser) fn new<Y, A>(allow_yield: Y, allow_await: A) -> Self
     where
         Y: Into<AllowYield>,
         A: Into<AllowAwait>,

boa/src/syntax/parser/statement/declaration/hoistable/async_function_decl/mod.rs

@@ -0,0 +1,89 @@
+#[cfg(test)]
+mod tests;
+
+use crate::syntax::{
+    ast::{node::AsyncFunctionDecl, Keyword, Punctuator},
+    lexer::TokenKind,
+    parser::{
+        function::FormalParameters, function::FunctionBody, statement::BindingIdentifier,
+        AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, TokenParser,
+    },
+};
+use std::io::Read;
+
+/// Async Function declaration parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
+/// [spec]: https://www.ecma-international.org/ecma-262/11.0/index.html#prod-AsyncFunctionDeclaration
+#[derive(Debug, Clone, Copy)]
+pub(super) struct AsyncFunctionDeclaration {
+    allow_yield: AllowYield,
+    allow_await: AllowAwait,
+    is_default: AllowDefault,
+}
+
+impl AsyncFunctionDeclaration {
+    /// Creates a new `FunctionDeclaration` parser.
+    pub(super) fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
+    where
+        Y: Into<AllowYield>,
+        A: Into<AllowAwait>,
+        D: Into<AllowDefault>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+            allow_await: allow_await.into(),
+            is_default: is_default.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for AsyncFunctionDeclaration
+where
+    R: Read,
+{
+    type Output = AsyncFunctionDecl;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        cursor.expect(Keyword::Async, "async function declaration")?;
+        cursor.peek_expect_no_lineterminator(0, "async function declaration")?;
+        cursor.expect(Keyword::Function, "async function declaration")?;
+        let tok = cursor.peek(0)?;
+
+        let name = if let Some(token) = tok {
+            match token.kind() {
+                TokenKind::Punctuator(Punctuator::OpenParen) => {
+                    if !self.is_default.0 {
+                        return Err(ParseError::unexpected(
+                            token.clone(),
+                            " in async function declaration",
+                        ));
+                    }
+                    None
+                }
+                _ => {
+                    Some(BindingIdentifier::new(self.allow_yield, self.allow_await).parse(cursor)?)
+                }
+            }
+        } else {
+            return Err(ParseError::AbruptEnd);
+        };
+
+        cursor.expect(Punctuator::OpenParen, "async function declaration")?;
+
+        let params = FormalParameters::new(false, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseParen, "async function declaration")?;
+        cursor.expect(Punctuator::OpenBlock, "async function declaration")?;
+
+        let body = FunctionBody::new(false, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseBlock, "async function declaration")?;
+
+        Ok(AsyncFunctionDecl::new(name, params, body))
+    }
+}

boa/src/syntax/parser/statement/declaration/hoistable/async_function_decl/tests.rs

@@ -0,0 +1,24 @@
+use crate::syntax::{ast::node::AsyncFunctionDecl, parser::tests::check_parser};
+
+/// Async function declaration parsing.
+#[test]
+fn async_function_declaration() {
+    check_parser(
+        "async function hello() {}",
+        vec![AsyncFunctionDecl::new(Box::from("hello"), vec![], vec![]).into()],
+    );
+}
+
+/// Async function declaration parsing with keywords.
+#[test]
+fn async_function_declaration_keywords() {
+    check_parser(
+        "async function yield() {}",
+        vec![AsyncFunctionDecl::new(Box::from("yield"), vec![], vec![]).into()],
+    );
+
+    check_parser(
+        "async function await() {}",
+        vec![AsyncFunctionDecl::new(Box::from("await"), vec![], vec![]).into()],
+    );
+}

boa/src/syntax/parser/statement/declaration/hoistable/function_decl/mod.rs

@@ -1,67 +1,15 @@
-//! Hoistable declaration parsing.
-//!
-//! More information:
-//!  - [ECMAScript specification][spec]
-//!
-//! [spec]: https://tc39.es/ecma262/#prod-HoistableDeclaration
-
-use crate::{
-    syntax::{
-        ast::{node::FunctionDecl, Keyword, Node, Punctuator},
-        parser::{
-            function::FormalParameters, function::FunctionBody, statement::BindingIdentifier,
-            AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, ParseResult, TokenParser,
-        },
+#[cfg(test)]
+mod tests;
+
+use crate::syntax::{
+    ast::{node::FunctionDecl, Keyword, Punctuator},
+    parser::{
+        function::FormalParameters, function::FunctionBody, statement::BindingIdentifier,
+        AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, TokenParser,
     },
-    BoaProfiler,
 };
-
 use std::io::Read;
 
-/// Hoistable declaration parsing.
-///
-/// More information:
-///  - [ECMAScript specification][spec]
-///
-/// [spec]: https://tc39.es/ecma262/#prod-FunctionDeclaration
-#[derive(Debug, Clone, Copy)]
-pub(super) struct HoistableDeclaration {
-    allow_yield: AllowYield,
-    allow_await: AllowAwait,
-    is_default: AllowDefault,
-}
-
-impl HoistableDeclaration {
-    /// Creates a new `HoistableDeclaration` parser.
-    pub(super) fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
-    where
-        Y: Into<AllowYield>,
-        A: Into<AllowAwait>,
-        D: Into<AllowDefault>,
-    {
-        Self {
-            allow_yield: allow_yield.into(),
-            allow_await: allow_await.into(),
-            is_default: is_default.into(),
-        }
-    }
-}
-
-impl<R> TokenParser<R> for HoistableDeclaration
-where
-    R: Read,
-{
-    type Output = Node;
-
-    fn parse(self, cursor: &mut Cursor<R>) -> ParseResult {
-        let _timer = BoaProfiler::global().start_event("HoistableDeclaration", "Parsing");
-        // TODO: check for generators and async functions + generators
-        FunctionDeclaration::new(self.allow_yield, self.allow_await, self.is_default)
-            .parse(cursor)
-            .map(Node::from)
-    }
-}
-
 /// Function declaration parsing.
 ///
 /// More information:
@@ -70,8 +18,9 @@ where
 ///
 /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function
 /// [spec]: https://tc39.es/ecma262/#prod-FunctionDeclaration
+
 #[derive(Debug, Clone, Copy)]
-struct FunctionDeclaration {
+pub(super) struct FunctionDeclaration {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
     is_default: AllowDefault,
@@ -79,7 +28,7 @@ struct FunctionDeclaration {
 
 impl FunctionDeclaration {
     /// Creates a new `FunctionDeclaration` parser.
-    fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
+    pub(super) fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
     where
         Y: Into<AllowYield>,
         A: Into<AllowAwait>,

boa/src/syntax/parser/statement/declaration/hoistable/function_decl/tests.rs

@@ -0,0 +1,24 @@
+use crate::syntax::{ast::node::FunctionDecl, parser::tests::check_parser};
+
+/// Function declaration parsing.
+#[test]
+fn function_declaration() {
+    check_parser(
+        "function hello() {}",
+        vec![FunctionDecl::new(Box::from("hello"), vec![], vec![]).into()],
+    );
+}
+
+/// Function declaration parsing with keywords.
+#[test]
+fn function_declaration_keywords() {
+    check_parser(
+        "function yield() {}",
+        vec![FunctionDecl::new(Box::from("yield"), vec![], vec![]).into()],
+    );
+
+    check_parser(
+        "function await() {}",
+        vec![FunctionDecl::new(Box::from("await"), vec![], vec![]).into()],
+    );
+}

boa/src/syntax/parser/statement/declaration/hoistable/mod.rs

@@ -0,0 +1,82 @@
+//! Hoistable declaration parsing.
+//!
+//! More information:
+//!  - [ECMAScript specification][spec]
+//!
+//! [spec]: https://tc39.es/ecma262/#prod-HoistableDeclaration
+
+#[cfg(test)]
+mod tests;
+
+mod async_function_decl;
+mod function_decl;
+
+use async_function_decl::AsyncFunctionDeclaration;
+use function_decl::FunctionDeclaration;
+
+use crate::{
+    syntax::{
+        ast::{Keyword, Node},
+        lexer::TokenKind,
+        parser::{
+            AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, ParseResult, TokenParser,
+        },
+    },
+    BoaProfiler,
+};
+use std::io::Read;
+
+/// Hoistable declaration parsing.
+///
+/// More information:
+///  - [ECMAScript specification][spec]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-FunctionDeclaration
+#[derive(Debug, Clone, Copy)]
+pub(super) struct HoistableDeclaration {
+    allow_yield: AllowYield,
+    allow_await: AllowAwait,
+    is_default: AllowDefault,
+}
+
+impl HoistableDeclaration {
+    /// Creates a new `HoistableDeclaration` parser.
+    pub(super) fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
+    where
+        Y: Into<AllowYield>,
+        A: Into<AllowAwait>,
+        D: Into<AllowDefault>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+            allow_await: allow_await.into(),
+            is_default: is_default.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for HoistableDeclaration
+where
+    R: Read,
+{
+    type Output = Node;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> ParseResult {
+        let _timer = BoaProfiler::global().start_event("HoistableDeclaration", "Parsing");
+        let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+
+        match tok.kind() {
+            TokenKind::Keyword(Keyword::Function) => {
+                FunctionDeclaration::new(self.allow_yield, self.allow_await, self.is_default)
+                    .parse(cursor)
+                    .map(Node::from)
+            }
+            TokenKind::Keyword(Keyword::Async) => {
+                AsyncFunctionDeclaration::new(self.allow_yield, self.allow_await, false)
+                    .parse(cursor)
+                    .map(Node::from)
+            }
+            _ => unreachable!("unknown token found: {:?}", tok),
+        }
+    }
+}

boa/src/syntax/parser/statement/declaration/hoistable/tests.rs

@@ -0,0 +1 @@
+

boa/src/syntax/parser/statement/declaration/mod.rs

@@ -63,7 +63,7 @@ where
         let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
         match tok.kind() {
-            TokenKind::Keyword(Keyword::Function) => {
+            TokenKind::Keyword(Keyword::Function) | TokenKind::Keyword(Keyword::Async) => {
                 HoistableDeclaration::new(self.allow_yield, self.allow_await, false).parse(cursor)
             }
             TokenKind::Keyword(Keyword::Const) | TokenKind::Keyword(Keyword::Let) => {

boa/src/syntax/parser/statement/declaration/tests.rs

@@ -1,9 +1,6 @@
 use crate::syntax::{
     ast::{
-        node::{
-            ConstDecl, ConstDeclList, FunctionDecl, LetDecl, LetDeclList, Node, VarDecl,
-            VarDeclList,
-        },
+        node::{ConstDecl, ConstDeclList, LetDecl, LetDeclList, Node, VarDecl, VarDeclList},
         Const,
     },
     parser::tests::{check_invalid, check_parser},
@@ -169,26 +166,3 @@ fn multiple_const_declaration() {
         .into()],
     );
 }
-
-/// Function declaration parsing.
-#[test]
-fn function_declaration() {
-    check_parser(
-        "function hello() {}",
-        vec![FunctionDecl::new(Box::from("hello"), vec![], vec![]).into()],
-    );
-}
-
-/// Function declaration parsing with keywords.
-#[test]
-fn function_declaration_keywords() {
-    check_parser(
-        "function yield() {}",
-        vec![FunctionDecl::new(Box::from("yield"), vec![], vec![]).into()],
-    );
-
-    check_parser(
-        "function await() {}",
-        vec![FunctionDecl::new(Box::from("await"), vec![], vec![]).into()],
-    );
-}

boa/src/syntax/parser/statement/mod.rs

@@ -42,6 +42,7 @@ use crate::{
     syntax::{
         ast::{node, Keyword, Node, Punctuator},
         lexer::{Error as LexError, InputElement, TokenKind},
+        parser::expression::await_expr::AwaitExpression,
     },
     BoaProfiler,
 };
@@ -110,6 +111,9 @@ where
         let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
         match tok.kind() {
+            TokenKind::Keyword(Keyword::Await) => AwaitExpression::new(self.allow_yield)
+                .parse(cursor)
+                .map(Node::from),
             TokenKind::Keyword(Keyword::If) => {
                 IfStatement::new(self.allow_yield, self.allow_await, self.allow_return)
                     .parse(cursor)
@@ -386,7 +390,7 @@ where
         let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
         match *tok.kind() {
-            TokenKind::Keyword(Keyword::Function) => {
+            TokenKind::Keyword(Keyword::Function) | TokenKind::Keyword(Keyword::Async) => {
                 if strict_mode && self.in_block {
                     return Err(ParseError::lex(LexError::Syntax(
                         "Function declaration in blocks not allowed in strict mode".into(),

boa/src/syntax/parser/statement/throw/mod.rs

@@ -50,7 +50,7 @@ where
         let _timer = BoaProfiler::global().start_event("ThrowStatement", "Parsing");
         cursor.expect(Keyword::Throw, "throw statement")?;
 
-        cursor.peek_expect_no_lineterminator(0)?;
+        cursor.peek_expect_no_lineterminator(0, "throw statement")?;
 
         let expr = Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
         if let Some(tok) = cursor.peek(0)? {