Implement Generator parsing (#1575)

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

@@ -22,7 +22,7 @@ use serde::{Deserialize, Serialize};
 #[cfg_attr(feature = "deser", derive(Serialize, Deserialize))]
 #[derive(Clone, Debug, Trace, Finalize, PartialEq)]
 pub struct AsyncFunctionDecl {
-    name: Option<Box<str>>,
+    name: Box<str>,
     parameters: Box<[FormalParameter]>,
     body: StatementList,
 }
@@ -31,7 +31,7 @@ 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>>>,
+        N: Into<Box<str>>,
         P: Into<Box<[FormalParameter]>>,
         B: Into<StatementList>,
     {
@@ -43,8 +43,8 @@ impl AsyncFunctionDecl {
     }
 
     /// Gets the name of the async function declaration.
-    pub fn name(&self) -> Option<&str> {
+    pub fn name(&self) -> &str {
-        self.name.as_deref()
+        &self.name
     }
 
     /// Gets the list of parameters of the async function declaration.
@@ -63,10 +63,7 @@ impl AsyncFunctionDecl {
         f: &mut fmt::Formatter<'_>,
         indentation: usize,
     ) -> fmt::Result {
-        match &self.name {
+        write!(f, "async function {}(", self.name())?;
-            Some(name) => write!(f, "async function {}(", name)?,
-            None => write!(f, "async function (")?,
-        }
         join_nodes(f, &self.parameters)?;
         if self.body().is_empty() {
             f.write_str(") {}")

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

@@ -0,0 +1,96 @@
+use crate::{
+    exec::Executable,
+    gc::{Finalize, Trace},
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    BoaProfiler, Context, JsResult, JsValue,
+};
+use std::fmt;
+
+#[cfg(feature = "deser")]
+use serde::{Deserialize, Serialize};
+
+/// The `function*` declaration (`function` keyword followed by an asterisk) defines a generator function,
+/// which returns a `Generator` object.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-GeneratorDeclaration
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*
+#[cfg_attr(feature = "deser", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct GeneratorDecl {
+    name: Box<str>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl GeneratorDecl {
+    /// Creates a new generator declaration.
+    pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
+    where
+        N: Into<Box<str>>,
+        P: Into<Box<[FormalParameter]>>,
+        B: Into<StatementList>,
+    {
+        Self {
+            name: name.into(),
+            parameters: parameters.into(),
+            body: body.into(),
+        }
+    }
+
+    /// Gets the name of the generator declaration.
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    /// Gets the list of parameters of the generator declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the generator declaration.
+    pub fn body(&self) -> &[Node] {
+        self.body.items()
+    }
+
+    /// Implements the display formatting with indentation.
+    pub(in crate::syntax::ast::node) fn display(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        write!(f, "function* {}(", self.name)?;
+        join_nodes(f, &self.parameters)?;
+        if self.body().is_empty() {
+            f.write_str(") {}")
+        } else {
+            f.write_str(") {\n")?;
+            self.body.display(f, indentation + 1)?;
+            write!(f, "{}}}", "    ".repeat(indentation))
+        }
+    }
+}
+
+impl Executable for GeneratorDecl {
+    fn run(&self, _context: &mut Context) -> JsResult<JsValue> {
+        let _timer = BoaProfiler::global().start_event("GeneratorDecl", "exec");
+        // TODO: Implement GeneratorFunction
+        // https://tc39.es/ecma262/#sec-generatorfunction-objects
+        Ok(JsValue::undefined())
+    }
+}
+
+impl From<GeneratorDecl> for Node {
+    fn from(decl: GeneratorDecl) -> Self {
+        Self::GeneratorDecl(decl)
+    }
+}
+
+impl fmt::Display for GeneratorDecl {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}

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

@@ -0,0 +1,109 @@
+use crate::{
+    exec::Executable,
+    gc::{Finalize, Trace},
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    Context, JsResult, JsValue,
+};
+use std::fmt;
+
+#[cfg(feature = "deser")]
+use serde::{Deserialize, Serialize};
+
+/// The `function*` keyword can be used to define a generator function inside an expression.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-GeneratorExpression
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function*
+#[cfg_attr(feature = "deser", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct GeneratorExpr {
+    name: Option<Box<str>>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl GeneratorExpr {
+    /// Creates a new generator 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 generator declaration.
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_ref().map(Box::as_ref)
+    }
+
+    /// Gets the list of parameters of the generator declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the generator declaration.
+    pub fn body(&self) -> &StatementList {
+        &self.body
+    }
+
+    /// 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.display_block(f, indentation)
+    }
+
+    /// Displays the generator's body. This includes the curly braces at the start and end.
+    /// This will not indent the first brace, but will indent the last brace.
+    pub(in crate::syntax::ast::node) fn display_block(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        if self.body().items().is_empty() {
+            f.write_str("{}")
+        } else {
+            f.write_str("{\n")?;
+            self.body.display(f, indentation + 1)?;
+            write!(f, "{}}}", "    ".repeat(indentation))
+        }
+    }
+}
+
+impl Executable for GeneratorExpr {
+    fn run(&self, _context: &mut Context) -> JsResult<JsValue> {
+        // TODO: Implement GeneratorFunction
+        // https://tc39.es/ecma262/#sec-generatorfunction-objects
+        Ok(JsValue::undefined())
+    }
+}
+
+impl fmt::Display for GeneratorExpr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
+
+impl From<GeneratorExpr> for Node {
+    fn from(expr: GeneratorExpr) -> Self {
+        Self::GeneratorExpr(expr)
+    }
+}

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

@@ -17,6 +17,8 @@ pub mod async_function_decl;
 pub mod async_function_expr;
 pub mod function_decl;
 pub mod function_expr;
+pub mod generator_decl;
+pub mod generator_expr;
 
 pub use self::{
     arrow_function_decl::ArrowFunctionDecl, async_function_decl::AsyncFunctionDecl,

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

@@ -20,6 +20,7 @@ pub mod switch;
 pub mod template;
 pub mod throw;
 pub mod try_node;
+pub mod r#yield;
 
 pub use self::{
     array::ArrayDecl,
@@ -29,8 +30,9 @@ pub use self::{
     call::Call,
     conditional::{ConditionalOp, If},
     declaration::{
-        ArrowFunctionDecl, AsyncFunctionDecl, AsyncFunctionExpr, Declaration, DeclarationList,
+        generator_decl::GeneratorDecl, generator_expr::GeneratorExpr, ArrowFunctionDecl,
-        FunctionDecl, FunctionExpr,
+        AsyncFunctionDecl, AsyncFunctionExpr, Declaration, DeclarationList, FunctionDecl,
+        FunctionExpr,
     },
     field::{GetConstField, GetField},
     identifier::Identifier,
@@ -38,6 +40,7 @@ pub use self::{
     new::New,
     object::Object,
     operator::{Assign, BinOp, UnaryOp},
+    r#yield::Yield,
     return_smt::Return,
     spread::Spread,
     statement_list::{RcStatementList, StatementList},
@@ -209,6 +212,15 @@ pub enum Node {
     /// [spec]: https://tc39.es/ecma262/#prod-EmptyStatement
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/Empty
     Empty,
+
+    /// A `yield` node. [More information](./yield/struct.Yield.html).
+    Yield(Yield),
+
+    /// A generator function declaration node. [More information](./declaration/struct.GeneratorDecl.html).
+    GeneratorDecl(GeneratorDecl),
+
+    /// A generator function expression node. [More information](./declaration/struct.GeneratorExpr.html).
+    GeneratorExpr(GeneratorExpr),
 }
 
 impl Display for Node {
@@ -302,6 +314,9 @@ impl Node {
             Self::AsyncFunctionExpr(ref expr) => expr.display(f, indentation),
             Self::AwaitExpr(ref expr) => Display::fmt(expr, f),
             Self::Empty => write!(f, ";"),
+            Self::Yield(ref y) => Display::fmt(y, f),
+            Self::GeneratorDecl(ref decl) => Display::fmt(decl, f),
+            Self::GeneratorExpr(ref expr) => expr.display(f, indentation),
         }
     }
 }
@@ -363,6 +378,9 @@ impl Executable for Node {
             Node::Break(ref break_node) => break_node.run(context),
             Node::Continue(ref continue_node) => continue_node.run(context),
             Node::Empty => Ok(JsValue::undefined()),
+            Node::Yield(ref y) => y.run(context),
+            Node::GeneratorDecl(ref decl) => decl.run(context),
+            Node::GeneratorExpr(ref expr) => expr.run(context),
         }
     }
 }
@@ -597,7 +615,16 @@ pub enum MethodDefinitionKind {
     /// [spec]: https://tc39.es/ecma262/#prod-MethodDefinition
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions#Method_definition_syntax
     Ordinary,
-    // TODO: support other method definition kinds, like `Generator`.
+
+    /// Starting with ECMAScript 2015, you are able to define own methods in a shorter syntax, similar to the getters and setters.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#prod-MethodDefinition
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions#generator_methods
+    Generator,
 }
 
 unsafe impl Trace for MethodDefinitionKind {

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

@@ -72,7 +72,7 @@ impl Object {
                     match &kind {
                         MethodDefinitionKind::Get => write!(f, "get ")?,
                         MethodDefinitionKind::Set => write!(f, "set ")?,
-                        MethodDefinitionKind::Ordinary => (),
+                        MethodDefinitionKind::Ordinary | MethodDefinitionKind::Generator => (),
                     }
                     write!(f, "{}(", key)?;
                     join_nodes(f, node.parameters())?;
@@ -166,6 +166,19 @@ impl Executable for Object {
                                 context,
                             )?;
                         }
+                        &MethodDefinitionKind::Generator => {
+                            // TODO: Implement generator method definition execution.
+                            obj.__define_own_property__(
+                                name,
+                                PropertyDescriptor::builder()
+                                    .value(JsValue::undefined())
+                                    .writable(true)
+                                    .enumerable(true)
+                                    .configurable(true)
+                                    .build(),
+                                context,
+                            )?;
+                        }
                     }
                 }
                 // [spec]: https://tc39.es/ecma262/#sec-runtime-semantics-propertydefinitionevaluation

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

@@ -0,0 +1,70 @@
+use crate::{
+    exec::Executable,
+    gc::{Finalize, Trace},
+    syntax::ast::node::Node,
+    Context, JsResult, JsValue,
+};
+use std::fmt;
+
+#[cfg(feature = "deser")]
+use serde::{Deserialize, Serialize};
+
+/// The `yield` keyword is used to pause and resume a generator function
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-YieldExpression
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield
+#[cfg_attr(feature = "deser", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct Yield {
+    expr: Option<Box<Node>>,
+    delegate: bool,
+}
+
+impl Yield {
+    pub fn expr(&self) -> Option<&Node> {
+        self.expr.as_ref().map(Box::as_ref)
+    }
+
+    pub fn delegate(&self) -> bool {
+        self.delegate
+    }
+
+    /// Creates a `Yield` AST node.
+    pub fn new<E, OE>(expr: OE, delegate: bool) -> Self
+    where
+        E: Into<Node>,
+        OE: Into<Option<E>>,
+    {
+        Self {
+            expr: expr.into().map(E::into).map(Box::new),
+            delegate,
+        }
+    }
+}
+
+impl Executable for Yield {
+    fn run(&self, _context: &mut Context) -> JsResult<JsValue> {
+        // TODO: Implement Generator execution
+        Ok(JsValue::undefined())
+    }
+}
+
+impl From<Yield> for Node {
+    fn from(r#yield: Yield) -> Node {
+        Node::Yield(r#yield)
+    }
+}
+
+impl fmt::Display for Yield {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let y = if self.delegate { "yield*" } else { "yield" };
+        match self.expr() {
+            Some(ex) => write!(f, "{} {}", y, ex),
+            None => write!(f, "{}", y),
+        }
+    }
+}

boa/src/syntax/parser/error.rs

@@ -92,6 +92,14 @@ impl ParseError {
         Self::General { message, position }
     }
 
+    /// Creates a "general" parsing error with the specific error message for a wrong function declaration in non-strict mode.
+    pub(super) fn wrong_function_declaration_non_strict(position: Position) -> Self {
+        Self::General {
+            message: "In non-strict mode code, functions can only be declared at top level, inside a block, or as the body of an if statement.",
+            position
+        }
+    }
+
     /// Creates a parsing error from a lexing error.
     pub(super) fn lex(e: LexError) -> Self {
         Self::Lex { err: e }

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

@@ -17,7 +17,7 @@ use crate::{
         lexer::{Error as LexError, Position, TokenKind},
         parser::{
             error::{ErrorContext, ParseError, ParseResult},
-            function::{FormalParameters, FunctionBody},
+            function::{FormalParameterList, FormalParameters, FunctionBody},
             statement::BindingIdentifier,
             AllowAwait, AllowIn, AllowYield, Cursor, TokenParser,
         },
@@ -72,18 +72,31 @@ where
         let _timer = BoaProfiler::global().start_event("ArrowFunction", "Parsing");
         let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
-        let params = if let TokenKind::Punctuator(Punctuator::OpenParen) = &next_token.kind() {
+        let (params, params_start_position) = if let TokenKind::Punctuator(Punctuator::OpenParen) =
+            &next_token.kind()
+        {
             // CoverParenthesizedExpressionAndArrowParameterList
-            cursor.expect(Punctuator::OpenParen, "arrow function")?;
+            let params_start_position = cursor
+                .expect(Punctuator::OpenParen, "arrow function")?
+                .span()
+                .end();
 
             let params = FormalParameters::new(self.allow_yield, self.allow_await).parse(cursor)?;
             cursor.expect(Punctuator::CloseParen, "arrow function")?;
-            params
+            (params, params_start_position)
         } else {
+            let params_start_position = next_token.span().start();
             let param = BindingIdentifier::new(self.allow_yield, self.allow_await)
                 .parse(cursor)
                 .context("arrow function")?;
-            Box::new([FormalParameter::new(param, None, false)])
+            (
+                FormalParameterList {
+                    parameters: Box::new([FormalParameter::new(param, None, false)]),
+                    is_simple: true,
+                    has_duplicates: false,
+                },
+                params_start_position,
+            )
         };
 
         cursor.peek_expect_no_lineterminator(0, "arrow function")?;
@@ -91,12 +104,29 @@ where
         cursor.expect(TokenKind::Punctuator(Punctuator::Arrow), "arrow function")?;
         let body = ConciseBody::new(self.allow_in).parse(cursor)?;
 
+        // Early Error: ArrowFormalParameters are UniqueFormalParameters.
+        if params.has_duplicates {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Duplicate parameter name not allowed in this context".into(),
+                params_start_position,
+            )));
+        }
+
+        // Early Error: It is a Syntax Error if ConciseBodyContainsUseStrict of ConciseBody is true
+        // and IsSimpleParameterList of ArrowParameters is false.
+        if body.strict() && !params.is_simple {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Illegal 'use strict' directive in function with non-simple parameter list".into(),
+                params_start_position,
+            )));
+        }
+
         // It is a Syntax Error if any element of the BoundNames of ArrowParameters
         // also occurs in the LexicallyDeclaredNames of ConciseBody.
         // https://tc39.es/ecma262/#sec-arrow-function-definitions-static-semantics-early-errors
         {
             let lexically_declared_names = body.lexically_declared_names();
-            for param in params.as_ref() {
+            for param in params.parameters.as_ref() {
                 if lexically_declared_names.contains(param.name()) {
                     return Err(ParseError::lex(LexError::Syntax(
                         format!("Redeclaration of formal parameter `{}`", param.name()).into(),
@@ -109,7 +139,7 @@ where
             }
         }
 
-        Ok(ArrowFunctionDecl::new(params, body))
+        Ok(ArrowFunctionDecl::new(params.parameters, body))
     }
 }
 

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

@@ -10,7 +10,9 @@
 mod arrow_function;
 mod conditional;
 mod exponentiation;
+mod r#yield;
 
+use self::r#yield::YieldExpression;
 use self::{arrow_function::ArrowFunction, conditional::ConditionalExpression};
 use crate::syntax::lexer::{Error as LexError, InputElement, TokenKind};
 use crate::{
@@ -82,9 +84,12 @@ where
         let _timer = BoaProfiler::global().start_event("AssignmentExpression", "Parsing");
         cursor.set_goal(InputElement::Div);
 
-        // Arrow function
         match cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind() {
-            // a=>{}
+            // [+Yield]YieldExpression[?In, ?Await]
+            TokenKind::Keyword(Keyword::Yield) if self.allow_yield.0 => {
+                return YieldExpression::new(self.allow_in, self.allow_await).parse(cursor)
+            }
+            // ArrowFunction[?In, ?Yield, ?Await] -> ArrowParameters[?Yield, ?Await] -> BindingIdentifier[?Yield, ?Await]
             TokenKind::Identifier(_)
             | TokenKind::Keyword(Keyword::Yield)
             | TokenKind::Keyword(Keyword::Await) => {
@@ -100,8 +105,7 @@ where
                     }
                 }
             }
-
+            // ArrowFunction[?In, ?Yield, ?Await] -> ArrowParameters[?Yield, ?Await] -> CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
-            // (a,b)=>{} or (a,b) or (Expression)
             TokenKind::Punctuator(Punctuator::OpenParen) => {
                 if let Some(next_token) = cursor.peek(1)? {
                     match *next_token.kind() {

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

@@ -0,0 +1,93 @@
+//! YieldExpression parsing.
+//!
+//! More information:
+//!  - [MDN documentation][mdn]
+//!  - [ECMAScript specification][spec]
+//!
+//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield
+//! [spec]: https://tc39.es/ecma262/#prod-YieldExpression
+
+use crate::{
+    syntax::{
+        ast::{
+            node::{Node, Yield},
+            Keyword, Punctuator,
+        },
+        lexer::TokenKind,
+        parser::{cursor::SemicolonResult, AllowAwait, AllowIn, Cursor, ParseResult, TokenParser},
+    },
+    BoaProfiler,
+};
+
+use std::io::Read;
+
+use super::AssignmentExpression;
+
+/// YieldExpression parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield
+/// [spec]: https://tc39.es/ecma262/#prod-YieldExpression
+#[derive(Debug, Clone, Copy)]
+pub(in crate::syntax::parser) struct YieldExpression {
+    allow_in: AllowIn,
+    allow_await: AllowAwait,
+}
+
+impl YieldExpression {
+    /// Creates a new `YieldExpression` parser.
+    pub(in crate::syntax::parser) fn new<I, A>(allow_in: I, allow_await: A) -> Self
+    where
+        I: Into<AllowIn>,
+        A: Into<AllowAwait>,
+    {
+        Self {
+            allow_in: allow_in.into(),
+            allow_await: allow_await.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for YieldExpression
+where
+    R: Read,
+{
+    type Output = Node;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> ParseResult {
+        let _timer = BoaProfiler::global().start_event("YieldExpression", "Parsing");
+
+        cursor.expect(TokenKind::Keyword(Keyword::Yield), "yield expression")?;
+
+        let mut expr = None;
+        let mut delegate = false;
+
+        if let SemicolonResult::Found(_) = cursor.peek_semicolon()? {
+            cursor.expect(
+                TokenKind::Punctuator(Punctuator::Semicolon),
+                "token disappeared",
+            )?;
+        } else if let Ok(next_token) = cursor.peek_expect_no_lineterminator(0, "yield expression") {
+            if let TokenKind::Punctuator(Punctuator::Mul) = next_token.kind() {
+                cursor.expect(TokenKind::Punctuator(Punctuator::Mul), "token disappeared")?;
+                delegate = true;
+                expr = Some(
+                    AssignmentExpression::new(self.allow_in, true, self.allow_await)
+                        .parse(cursor)?,
+                );
+            } else {
+                expr = Some(
+                    AssignmentExpression::new(self.allow_in, true, self.allow_await)
+                        .parse(cursor)?,
+                );
+            }
+        }
+
+        Ok(Node::Yield(Yield::new::<Node, Option<Node>>(
+            expr, delegate,
+        )))
+    }
+}

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

@@ -63,7 +63,24 @@ where
             return Err(ParseError::AbruptEnd);
         };
 
-        cursor.expect(Punctuator::OpenParen, "async function expression")?;
+        // Early Error: If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code,
+        // it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
+        if let Some(name) = &name {
+            if cursor.strict_mode() && ["eval", "arguments"].contains(&name.as_ref()) {
+                return Err(ParseError::lex(LexError::Syntax(
+                    "Unexpected eval or arguments in strict mode".into(),
+                    match cursor.peek(0)? {
+                        Some(token) => token.span().end(),
+                        None => Position::new(1, 1),
+                    },
+                )));
+            }
+        }
+
+        let params_start_position = cursor
+            .expect(Punctuator::OpenParen, "async function expression")?
+            .span()
+            .end();
 
         let params = FormalParameters::new(false, true).parse(cursor)?;
 
@@ -74,12 +91,30 @@ where
 
         cursor.expect(Punctuator::CloseBlock, "async function expression")?;
 
+        // Early Error: If the source code matching FormalParameters is strict mode code,
+        // the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
+        if (cursor.strict_mode() || body.strict()) && params.has_duplicates {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Duplicate parameter name not allowed in this context".into(),
+                params_start_position,
+            )));
+        }
+
+        // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncFunctionBody is true
+        // and IsSimpleParameterList of FormalParameters is false.
+        if body.strict() && !params.is_simple {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Illegal 'use strict' directive in function with non-simple parameter list".into(),
+                params_start_position,
+            )));
+        }
+
         // It is a Syntax Error if any element of the BoundNames of FormalParameters
         // also occurs in the LexicallyDeclaredNames of FunctionBody.
         // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
         {
             let lexically_declared_names = body.lexically_declared_names();
-            for param in params.as_ref() {
+            for param in params.parameters.as_ref() {
                 if lexically_declared_names.contains(param.name()) {
                     return Err(ParseError::lex(LexError::Syntax(
                         format!("Redeclaration of formal parameter `{}`", param.name()).into(),
@@ -92,6 +127,6 @@ where
             }
         }
 
-        Ok(AsyncFunctionExpr::new(name, params, body))
+        Ok(AsyncFunctionExpr::new(name, params.parameters, body))
     }
 }

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

@@ -58,7 +58,24 @@ where
             None
         };
 
-        cursor.expect(Punctuator::OpenParen, "function expression")?;
+        // Early Error: If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code,
+        // it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
+        if let Some(name) = &name {
+            if cursor.strict_mode() && ["eval", "arguments"].contains(&name.as_ref()) {
+                return Err(ParseError::lex(LexError::Syntax(
+                    "Unexpected eval or arguments in strict mode".into(),
+                    match cursor.peek(0)? {
+                        Some(token) => token.span().end(),
+                        None => Position::new(1, 1),
+                    },
+                )));
+            }
+        }
+
+        let params_start_position = cursor
+            .expect(Punctuator::OpenParen, "function expression")?
+            .span()
+            .end();
 
         let params = FormalParameters::new(false, false).parse(cursor)?;
 
@@ -69,12 +86,30 @@ where
 
         cursor.expect(Punctuator::CloseBlock, "function expression")?;
 
+        // Early Error: If the source code matching FormalParameters is strict mode code,
+        // the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
+        if (cursor.strict_mode() || body.strict()) && params.has_duplicates {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Duplicate parameter name not allowed in this context".into(),
+                params_start_position,
+            )));
+        }
+
+        // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is true
+        // and IsSimpleParameterList of FormalParameters is false.
+        if body.strict() && !params.is_simple {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Illegal 'use strict' directive in function with non-simple parameter list".into(),
+                params_start_position,
+            )));
+        }
+
         // It is a Syntax Error if any element of the BoundNames of FormalParameters
         // also occurs in the LexicallyDeclaredNames of FunctionBody.
         // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
         {
             let lexically_declared_names = body.lexically_declared_names();
-            for param in params.as_ref() {
+            for param in params.parameters.as_ref() {
                 if lexically_declared_names.contains(param.name()) {
                     return Err(ParseError::lex(LexError::Syntax(
                         format!("Redeclaration of formal parameter `{}`", param.name()).into(),
@@ -87,6 +122,6 @@ where
             }
         }
 
-        Ok(FunctionExpr::new(name, params, body))
+        Ok(FunctionExpr::new(name, params.parameters, body))
     }
 }

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

@@ -0,0 +1,132 @@
+//! Generator expression parsing.
+//!
+//! More information:
+//!  - [MDN documentation][mdn]
+//!  - [ECMAScript specification][spec]
+//!
+//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function*
+//! [spec]: https://tc39.es/ecma262/#prod-GeneratorExpression
+
+#[cfg(test)]
+mod tests;
+
+use crate::{
+    syntax::{
+        ast::{node::GeneratorExpr, Keyword, Punctuator},
+        lexer::{Error as LexError, Position, TokenKind},
+        parser::{
+            function::{FormalParameters, FunctionBody},
+            statement::BindingIdentifier,
+            Cursor, ParseError, TokenParser,
+        },
+    },
+    BoaProfiler,
+};
+
+use std::io::Read;
+
+/// Generator expression parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function*
+/// [spec]: https://tc39.es/ecma262/#prod-GeneratorExpression
+#[derive(Debug, Clone, Copy)]
+pub(super) struct GeneratorExpression;
+
+impl<R> TokenParser<R> for GeneratorExpression
+where
+    R: Read,
+{
+    type Output = GeneratorExpr;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        let _timer = BoaProfiler::global().start_event("GeneratorExpression", "Parsing");
+
+        cursor.expect(
+            TokenKind::Punctuator(Punctuator::Mul),
+            "generator expression",
+        )?;
+
+        let name = if let Some(token) = cursor.peek(0)? {
+            match token.kind() {
+                TokenKind::Identifier(_)
+                | TokenKind::Keyword(Keyword::Yield)
+                | TokenKind::Keyword(Keyword::Await) => {
+                    Some(BindingIdentifier::new(true, false).parse(cursor)?)
+                }
+                _ => None,
+            }
+        } else {
+            None
+        };
+
+        // Early Error: If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code,
+        // it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
+        if let Some(name) = &name {
+            if cursor.strict_mode() && ["eval", "arguments"].contains(&name.as_ref()) {
+                return Err(ParseError::lex(LexError::Syntax(
+                    "Unexpected eval or arguments in strict mode".into(),
+                    match cursor.peek(0)? {
+                        Some(token) => token.span().end(),
+                        None => Position::new(1, 1),
+                    },
+                )));
+            }
+        }
+
+        let params_start_position = cursor
+            .expect(Punctuator::OpenParen, "generator expression")?
+            .span()
+            .end();
+
+        let params = FormalParameters::new(true, false).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseParen, "generator expression")?;
+        cursor.expect(Punctuator::OpenBlock, "generator expression")?;
+
+        let body = FunctionBody::new(true, false).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseBlock, "generator expression")?;
+
+        // Early Error: If the source code matching FormalParameters is strict mode code,
+        // the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
+        if (cursor.strict_mode() || body.strict()) && params.has_duplicates {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Duplicate parameter name not allowed in this context".into(),
+                params_start_position,
+            )));
+        }
+
+        // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is true
+        // and IsSimpleParameterList of FormalParameters is false.
+        if body.strict() && !params.is_simple {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Illegal 'use strict' directive in function with non-simple parameter list".into(),
+                params_start_position,
+            )));
+        }
+
+        // It is a Syntax Error if any element of the BoundNames of FormalParameters
+        // also occurs in the LexicallyDeclaredNames of FunctionBody.
+        // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
+        {
+            let lexically_declared_names = body.lexically_declared_names();
+            for param in params.parameters.as_ref() {
+                if lexically_declared_names.contains(param.name()) {
+                    return Err(ParseError::lex(LexError::Syntax(
+                        format!("Redeclaration of formal parameter `{}`", param.name()).into(),
+                        match cursor.peek(0)? {
+                            Some(token) => token.span().end(),
+                            None => Position::new(1, 1),
+                        },
+                    )));
+                }
+            }
+        }
+
+        Ok(GeneratorExpr::new(name, params.parameters, body))
+    }
+}

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

@@ -0,0 +1,57 @@
+use crate::syntax::{
+    ast::{
+        node::{Declaration, DeclarationList, GeneratorExpr, StatementList, Yield},
+        Const,
+    },
+    parser::tests::check_parser,
+};
+
+#[test]
+fn check_generator_function_expression() {
+    check_parser(
+        "const gen = function*() {
+            yield 1;
+        };
+        ",
+        vec![DeclarationList::Const(
+            vec![Declaration::new_with_identifier(
+                "gen",
+                Some(
+                    GeneratorExpr::new::<Option<Box<str>>, _, StatementList>(
+                        None,
+                        [],
+                        vec![Yield::new(Const::from(1), false).into()].into(),
+                    )
+                    .into(),
+                ),
+            )]
+            .into(),
+        )
+        .into()],
+    );
+}
+
+#[test]
+fn check_generator_function_delegate_yield_expression() {
+    check_parser(
+        "const gen = function*() {
+            yield* 1;
+        };
+        ",
+        vec![DeclarationList::Const(
+            vec![Declaration::new_with_identifier(
+                "gen",
+                Some(
+                    GeneratorExpr::new::<Option<Box<str>>, _, StatementList>(
+                        None,
+                        [],
+                        vec![Yield::new(Const::from(1), true).into()].into(),
+                    )
+                    .into(),
+                ),
+            )]
+            .into(),
+        )
+        .into()],
+    );
+}

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

@@ -10,6 +10,7 @@
 mod array_initializer;
 mod async_function_expression;
 mod function_expression;
+mod generator_expression;
 mod object_initializer;
 mod template;
 #[cfg(test)]
@@ -17,7 +18,8 @@ mod tests;
 
 use self::{
     array_initializer::ArrayLiteral, async_function_expression::AsyncFunctionExpression,
-    function_expression::FunctionExpression, object_initializer::ObjectLiteral,
+    function_expression::FunctionExpression, generator_expression::GeneratorExpression,
+    object_initializer::ObjectLiteral,
 };
 use super::Expression;
 use crate::{
@@ -80,8 +82,13 @@ where
         match tok.kind() {
             TokenKind::Keyword(Keyword::This) => Ok(Node::This),
             TokenKind::Keyword(Keyword::Function) => {
+                let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+                if next_token.kind() == &TokenKind::Punctuator(Punctuator::Mul) {
+                    GeneratorExpression.parse(cursor).map(Node::from)
+                } else {
                     FunctionExpression.parse(cursor).map(Node::from)
                 }
+            }
             TokenKind::Keyword(Keyword::Async) => AsyncFunctionExpression::new(self.allow_yield)
                 .parse(cursor)
                 .map(Node::from),
@@ -106,7 +113,39 @@ where
             }
             TokenKind::BooleanLiteral(boolean) => Ok(Const::from(*boolean).into()),
             TokenKind::NullLiteral => Ok(Const::Null.into()),
-            TokenKind::Identifier(ident) => Ok(Identifier::from(ident.as_ref()).into()), // TODO: IdentifierReference
+            TokenKind::Identifier(ident) => Ok(Identifier::from(ident.as_ref()).into()),
+            TokenKind::Keyword(Keyword::Yield) if self.allow_yield.0 => {
+                // Early Error: It is a Syntax Error if this production has a [Yield] parameter and StringValue of Identifier is "yield".
+                Err(ParseError::general(
+                    "Unexpected identifier",
+                    tok.span().start(),
+                ))
+            }
+            TokenKind::Keyword(Keyword::Yield) if !self.allow_yield.0 => {
+                if cursor.strict_mode() {
+                    return Err(ParseError::general(
+                        "Unexpected strict mode reserved word",
+                        tok.span().start(),
+                    ));
+                }
+                Ok(Identifier::from("yield").into())
+            }
+            TokenKind::Keyword(Keyword::Await) if self.allow_await.0 => {
+                // Early Error: It is a Syntax Error if this production has an [Await] parameter and StringValue of Identifier is "await".
+                Err(ParseError::general(
+                    "Unexpected identifier",
+                    tok.span().start(),
+                ))
+            }
+            TokenKind::Keyword(Keyword::Await) if !self.allow_await.0 => {
+                if cursor.strict_mode() {
+                    return Err(ParseError::general(
+                        "Unexpected strict mode reserved word",
+                        tok.span().start(),
+                    ));
+                }
+                Ok(Identifier::from("await").into())
+            }
             TokenKind::StringLiteral(s) => Ok(Const::from(s.as_ref()).into()),
             TokenKind::TemplateNoSubstitution(template_string) => {
                 Ok(Const::from(template_string.to_owned_cooked().map_err(ParseError::lex)?).into())

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

@@ -9,14 +9,13 @@
 
 #[cfg(test)]
 mod tests;
-use crate::syntax::ast::node::{Identifier, PropertyName};
-use crate::syntax::lexer::TokenKind;
 use crate::{
     syntax::{
         ast::{
-            node::{self, FunctionExpr, MethodDefinitionKind, Node, Object},
+            node::{self, FunctionExpr, Identifier, MethodDefinitionKind, Node, Object},
-            Punctuator,
+            Keyword, Punctuator,
         },
+        lexer::{Error as LexError, Position, TokenKind},
         parser::{
             expression::AssignmentExpression,
             function::{FormalParameters, FunctionBody},
@@ -129,226 +128,331 @@ where
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("PropertyDefinition", "Parsing");
 
-        if cursor.next_if(Punctuator::Spread)?.is_some() {
+        // IdentifierReference[?Yield, ?Await]
-            let node = AssignmentExpression::new(true, self.allow_yield, self.allow_await)
+        if let Some(next_token) = cursor.peek(1)? {
-                .parse(cursor)?;
-            return Ok(node::PropertyDefinition::SpreadObject(node));
-        }
-
-        // ComputedPropertyName
-        // https://tc39.es/ecma262/#prod-ComputedPropertyName
-        if cursor.next_if(Punctuator::OpenBracket)?.is_some() {
-            let node = AssignmentExpression::new(false, self.allow_yield, self.allow_await)
-                .parse(cursor)?;
-            cursor.expect(Punctuator::CloseBracket, "expected token ']'")?;
-            let next_token = cursor.next()?.ok_or(ParseError::AbruptEnd)?;
             match next_token.kind() {
-                TokenKind::Punctuator(Punctuator::Colon) => {
+                TokenKind::Punctuator(Punctuator::CloseBlock)
-                    let val = AssignmentExpression::new(false, self.allow_yield, self.allow_await)
+                | TokenKind::Punctuator(Punctuator::Comma) => {
-                        .parse(cursor)?;
+                    let token = cursor.next()?.ok_or(ParseError::AbruptEnd)?;
-                    return Ok(node::PropertyDefinition::property(node, val));
+                    let ident = match token.kind() {
+                        TokenKind::Identifier(ident) => Identifier::from(ident.as_ref()),
+                        TokenKind::Keyword(Keyword::Yield) if self.allow_yield.0 => {
+                            // Early Error: It is a Syntax Error if this production has a [Yield] parameter and StringValue of Identifier is "yield".
+                            return Err(ParseError::general(
+                                "Unexpected identifier",
+                                token.span().start(),
+                            ));
+                        }
+                        TokenKind::Keyword(Keyword::Yield) if !self.allow_yield.0 => {
+                            if cursor.strict_mode() {
+                                // Early Error: It is a Syntax Error if the code matched by this production is contained in strict mode code.
+                                return Err(ParseError::general(
+                                    "Unexpected strict mode reserved word",
+                                    token.span().start(),
+                                ));
+                            }
+                            Identifier::from("yield")
+                        }
+                        TokenKind::Keyword(Keyword::Await) if self.allow_await.0 => {
+                            // Early Error: It is a Syntax Error if this production has an [Await] parameter and StringValue of Identifier is "await".
+                            return Err(ParseError::general(
+                                "Unexpected identifier",
+                                token.span().start(),
+                            ));
+                        }
+                        TokenKind::Keyword(Keyword::Await) if !self.allow_await.0 => {
+                            if cursor.strict_mode() {
+                                // Early Error: It is a Syntax Error if the code matched by this production is contained in strict mode code.
+                                return Err(ParseError::general(
+                                    "Unexpected strict mode reserved word",
+                                    token.span().start(),
+                                ));
                             }
-                TokenKind::Punctuator(Punctuator::OpenParen) => {
+                            Identifier::from("yield")
-                    return MethodDefinition::new(self.allow_yield, self.allow_await, node)
-                        .parse(cursor);
                         }
                         _ => {
                             return Err(ParseError::unexpected(
-                        next_token,
+                                token.clone(),
-                        "expected AssignmentExpression or MethodDefinition",
+                                "expected IdentifierReference",
-                    ))
+                            ));
+                        }
+                    };
+                    return Ok(node::PropertyDefinition::property(
+                        ident.clone().as_ref(),
+                        ident,
+                    ));
+                }
+                _ => {}
             }
         }
+
+        //  ... AssignmentExpression[+In, ?Yield, ?Await]
+        if cursor.next_if(Punctuator::Spread)?.is_some() {
+            let node = AssignmentExpression::new(true, self.allow_yield, self.allow_await)
+                .parse(cursor)?;
+            return Ok(node::PropertyDefinition::SpreadObject(node));
         }
 
-        // Peek for '}' or ',' to indicate shorthand property name
+        // MethodDefinition[?Yield, ?Await] -> GeneratorMethod[?Yield, ?Await]
-        if let Some(next_token) = cursor.peek(1)? {
+        if cursor.next_if(Punctuator::Mul)?.is_some() {
-            match next_token.kind() {
+            let property_name =
-                TokenKind::Punctuator(Punctuator::CloseBlock)
+                PropertyName::new(self.allow_yield, self.allow_await).parse(cursor)?;
-                | TokenKind::Punctuator(Punctuator::Comma) => {
+
-                    let token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+            let params_start_position = cursor
-                    if let TokenKind::Identifier(ident) = token.kind() {
+                .expect(Punctuator::OpenParen, "generator method definition")?
-                        // ident is both the name and value in a shorthand property
+                .span()
-                        let name = ident.to_string();
+                .start();
-                        let value = Identifier::from(ident.to_owned());
+            let params = FormalParameters::new(false, false).parse(cursor)?;
-                        cursor.next()?.expect("token vanished"); // Consume the token.
+            cursor.expect(Punctuator::CloseParen, "generator method definition")?;
-                        return Ok(node::PropertyDefinition::property(name, value));
+
-                    } else {
+            // Early Error: UniqueFormalParameters : FormalParameters
-                        // Anything besides an identifier is a syntax error
+            if params.has_duplicates {
-                        return Err(ParseError::unexpected(token.clone(), "object literal"));
+                return Err(ParseError::lex(LexError::Syntax(
+                    "Duplicate parameter name not allowed in this context".into(),
+                    params_start_position,
+                )));
+            }
+
+            cursor.expect(
+                TokenKind::Punctuator(Punctuator::OpenBlock),
+                "generator method definition",
+            )?;
+            let body = FunctionBody::new(true, false).parse(cursor)?;
+            cursor.expect(
+                TokenKind::Punctuator(Punctuator::CloseBlock),
+                "generator method definition",
+            )?;
+
+            // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
+            // and IsSimpleParameterList of UniqueFormalParameters is false.
+            if body.strict() && !params.is_simple {
+                return Err(ParseError::lex(LexError::Syntax(
+                    "Illegal 'use strict' directive in function with non-simple parameter list"
+                        .into(),
+                    params_start_position,
+                )));
+            }
+
+            // Early Error: It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also
+            // occurs in the LexicallyDeclaredNames of GeneratorBody.
+            {
+                let lexically_declared_names = body.lexically_declared_names();
+                for param in params.parameters.as_ref() {
+                    if lexically_declared_names.contains(param.name()) {
+                        return Err(ParseError::lex(LexError::Syntax(
+                            format!("Redeclaration of formal parameter `{}`", param.name()).into(),
+                            match cursor.peek(0)? {
+                                Some(token) => token.span().end(),
+                                None => Position::new(1, 1),
+                            },
+                        )));
                     }
                 }
-                _ => {}
             }
+
+            return Ok(node::PropertyDefinition::method_definition(
+                MethodDefinitionKind::Generator,
+                property_name,
+                FunctionExpr::new(None, params.parameters, body),
+            ));
         }
 
-        let prop_name = cursor.next()?.ok_or(ParseError::AbruptEnd)?.to_string();
+        let mut property_name =
+            PropertyName::new(self.allow_yield, self.allow_await).parse(cursor)?;
+
+        //  PropertyName[?Yield, ?Await] : AssignmentExpression[+In, ?Yield, ?Await]
         if cursor.next_if(Punctuator::Colon)?.is_some() {
-            let val = AssignmentExpression::new(true, self.allow_yield, self.allow_await)
+            let value = AssignmentExpression::new(true, self.allow_yield, self.allow_await)
                 .parse(cursor)?;
-            return Ok(node::PropertyDefinition::property(prop_name, val));
+            return Ok(node::PropertyDefinition::property(property_name, value));
         }
 
-        // TODO GeneratorMethod
+        let ordinary_method = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind()
-        // https://tc39.es/ecma262/#prod-GeneratorMethod
+            == &TokenKind::Punctuator(Punctuator::OpenParen);
 
-        if prop_name.as_str() == "async" {
+        match property_name {
-            // TODO - AsyncMethod.
+            // MethodDefinition[?Yield, ?Await] -> get ClassElementName[?Yield, ?Await] ( ) { FunctionBody[~Yield, ~Await] }
-            // https://tc39.es/ecma262/#prod-AsyncMethod
+            node::PropertyName::Literal(str) if str.as_ref() == "get" && !ordinary_method => {
+                property_name =
+                    PropertyName::new(self.allow_yield, self.allow_await).parse(cursor)?;
 
-            // TODO - AsyncGeneratorMethod
+                cursor.expect(
-            // https://tc39.es/ecma262/#prod-AsyncGeneratorMethod
+                    TokenKind::Punctuator(Punctuator::OpenParen),
+                    "get method definition",
+                )?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseParen),
+                    "get method definition",
+                )?;
+
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::OpenBlock),
+                    "get method definition",
+                )?;
+                let body = FunctionBody::new(false, false).parse(cursor)?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseBlock),
+                    "get method definition",
+                )?;
+
+                Ok(node::PropertyDefinition::method_definition(
+                    MethodDefinitionKind::Get,
+                    property_name,
+                    FunctionExpr::new(None, [], body),
+                ))
             }
+            // MethodDefinition[?Yield, ?Await] -> set ClassElementName[?Yield, ?Await] ( PropertySetParameterList ) { FunctionBody[~Yield, ~Await] }
+            node::PropertyName::Literal(str) if str.as_ref() == "set" && !ordinary_method => {
+                property_name =
+                    PropertyName::new(self.allow_yield, self.allow_await).parse(cursor)?;
 
-        if cursor
+                let params_start_position = cursor
-            .next_if(TokenKind::Punctuator(Punctuator::OpenParen))?
+                    .expect(
-            .is_some()
+                        TokenKind::Punctuator(Punctuator::OpenParen),
-            || ["get", "set"].contains(&prop_name.as_str())
+                        "set method definition",
-        {
+                    )?
-            return MethodDefinition::new(self.allow_yield, self.allow_await, prop_name)
+                    .span()
-                .parse(cursor);
+                    .end();
+                let params = FormalParameters::new(false, false).parse(cursor)?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseParen),
+                    "set method definition",
+                )?;
+                if params.parameters.len() != 1 {
+                    return Err(ParseError::general(
+                        "set method definition must have one parameter",
+                        params_start_position,
+                    ));
+                }
+
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::OpenBlock),
+                    "set method definition",
+                )?;
+                let body = FunctionBody::new(false, false).parse(cursor)?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseBlock),
+                    "set method definition",
+                )?;
+
+                // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
+                // and IsSimpleParameterList of PropertySetParameterList is false.
+                if body.strict() && !params.is_simple {
+                    return Err(ParseError::lex(LexError::Syntax(
+                        "Illegal 'use strict' directive in function with non-simple parameter list"
+                            .into(),
+                        params_start_position,
+                    )));
+                }
+
+                Ok(node::PropertyDefinition::method_definition(
+                    MethodDefinitionKind::Set,
+                    property_name,
+                    FunctionExpr::new(None, params.parameters, body),
+                ))
+            }
+            // MethodDefinition[?Yield, ?Await] -> ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] }
+            _ => {
+                let params_start_position = cursor
+                    .expect(
+                        TokenKind::Punctuator(Punctuator::OpenParen),
+                        "method definition",
+                    )?
+                    .span()
+                    .end();
+                let params = FormalParameters::new(false, false).parse(cursor)?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseParen),
+                    "method definition",
+                )?;
+
+                // Early Error: UniqueFormalParameters : FormalParameters
+                if params.has_duplicates {
+                    return Err(ParseError::lex(LexError::Syntax(
+                        "Duplicate parameter name not allowed in this context".into(),
+                        params_start_position,
+                    )));
+                }
+
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::OpenBlock),
+                    "method definition",
+                )?;
+                let body = FunctionBody::new(false, false).parse(cursor)?;
+                cursor.expect(
+                    TokenKind::Punctuator(Punctuator::CloseBlock),
+                    "method definition",
+                )?;
+
+                // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
+                // and IsSimpleParameterList of UniqueFormalParameters is false.
+                if body.strict() && !params.is_simple {
+                    return Err(ParseError::lex(LexError::Syntax(
+                        "Illegal 'use strict' directive in function with non-simple parameter list"
+                            .into(),
+                        params_start_position,
+                    )));
                 }
 
-        let pos = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.span().start();
+                Ok(node::PropertyDefinition::method_definition(
-        Err(ParseError::general("expected property definition", pos))
+                    MethodDefinitionKind::Ordinary,
+                    property_name,
+                    FunctionExpr::new(None, params.parameters, body),
+                ))
+            }
+        }
     }
 }
 
-/// Parses a method definition.
+/// Parses a property name.
 ///
 /// More information:
 ///  - [ECMAScript specification][spec]
 ///
-/// [spec]: https://tc39.es/ecma262/#prod-MethodDefinition
+/// [spec]: https://tc39.es/ecma262/#prod-PropertyName
 #[derive(Debug, Clone)]
-struct MethodDefinition {
+struct PropertyName {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
-    identifier: PropertyName,
 }
 
-impl MethodDefinition {
+impl PropertyName {
-    /// Creates a new `MethodDefinition` parser.
+    /// Creates a new `PropertyName` parser.
-    fn new<Y, A, I>(allow_yield: Y, allow_await: A, identifier: I) -> Self
+    fn new<Y, A>(allow_yield: Y, allow_await: A) -> Self
     where
         Y: Into<AllowYield>,
         A: Into<AllowAwait>,
-        I: Into<PropertyName>,
     {
         Self {
             allow_yield: allow_yield.into(),
             allow_await: allow_await.into(),
-            identifier: identifier.into(),
         }
     }
 }
 
-impl<R> TokenParser<R> for MethodDefinition
+impl<R> TokenParser<R> for PropertyName
 where
     R: Read,
 {
-    type Output = node::PropertyDefinition;
+    type Output = node::PropertyName;
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
-        let _timer = BoaProfiler::global().start_event("MethodDefinition", "Parsing");
+        let _timer = BoaProfiler::global().start_event("PropertyName", "Parsing");
-
+
-        let (method_kind, prop_name, params) = match self.identifier {
+        // ComputedPropertyName[?Yield, ?Await] -> [ AssignmentExpression[+In, ?Yield, ?Await] ]
-            PropertyName::Literal(ident)
+        if cursor.next_if(Punctuator::OpenBracket)?.is_some() {
-                if ["get", "set"].contains(&ident.as_ref())
+            let node = AssignmentExpression::new(false, self.allow_yield, self.allow_await)
-                    && matches!(
-                        cursor.peek(0)?.map(|t| t.kind()),
-                        Some(&TokenKind::Identifier(_))
-                            | Some(&TokenKind::Keyword(_))
-                            | Some(&TokenKind::BooleanLiteral(_))
-                            | Some(&TokenKind::NullLiteral)
-                            | Some(&TokenKind::NumericLiteral(_))
-                    ) =>
-            {
-                let prop_name = cursor.next()?.ok_or(ParseError::AbruptEnd)?.to_string();
-                cursor.expect(
-                    TokenKind::Punctuator(Punctuator::OpenParen),
-                    "property method definition",
-                )?;
-                let first_param = cursor.peek(0)?.expect("current token disappeared").clone();
-                let params = FormalParameters::new(false, false).parse(cursor)?;
-                cursor.expect(Punctuator::CloseParen, "method definition")?;
-                if ident.as_ref() == "get" {
-                    if !params.is_empty() {
-                        return Err(ParseError::unexpected(
-                            first_param,
-                            "getter functions must have no arguments",
-                        ));
-                    }
-                    (MethodDefinitionKind::Get, prop_name.into(), params)
-                } else {
-                    if params.len() != 1 {
-                        return Err(ParseError::unexpected(
-                            first_param,
-                            "setter functions must have one argument",
-                        ));
-                    }
-                    (MethodDefinitionKind::Set, prop_name.into(), params)
-                }
-            }
-            PropertyName::Literal(ident)
-                if ["get", "set"].contains(&ident.as_ref())
-                    && matches!(
-                        cursor.peek(0)?.map(|t| t.kind()),
-                        Some(&TokenKind::Punctuator(Punctuator::OpenBracket))
-                    ) =>
-            {
-                cursor.expect(Punctuator::OpenBracket, "token vanished")?;
-                let prop_name =
-                    AssignmentExpression::new(false, self.allow_yield, self.allow_await)
                 .parse(cursor)?;
             cursor.expect(Punctuator::CloseBracket, "expected token ']'")?;
-                cursor.expect(
+            return Ok(node.into());
-                    TokenKind::Punctuator(Punctuator::OpenParen),
-                    "property method definition",
-                )?;
-                let first_param = cursor.peek(0)?.expect("current token disappeared").clone();
-                let params = FormalParameters::new(false, false).parse(cursor)?;
-                cursor.expect(Punctuator::CloseParen, "method definition")?;
-                if ident.as_ref() == "get" {
-                    if !params.is_empty() {
-                        return Err(ParseError::unexpected(
-                            first_param,
-                            "getter functions must have no arguments",
-                        ));
-                    }
-                    (MethodDefinitionKind::Get, prop_name.into(), params)
-                } else {
-                    if params.len() != 1 {
-                        return Err(ParseError::unexpected(
-                            first_param,
-                            "setter functions must have one argument",
-                        ));
         }
-                    (MethodDefinitionKind::Set, prop_name.into(), params)
-                }
-            }
-            prop_name => {
-                let params = FormalParameters::new(false, false).parse(cursor)?;
-                cursor.expect(Punctuator::CloseParen, "method definition")?;
-                (MethodDefinitionKind::Ordinary, prop_name, params)
-            }
-        };
 
-        cursor.expect(
+        // LiteralPropertyName
-            TokenKind::Punctuator(Punctuator::OpenBlock),
+        Ok(cursor
-            "property method definition",
+            .next()?
-        )?;
+            .ok_or(ParseError::AbruptEnd)?
-        let body = FunctionBody::new(false, false).parse(cursor)?;
+            .to_string()
-        cursor.expect(
+            .into())
-            TokenKind::Punctuator(Punctuator::CloseBlock),
-            "property method definition",
-        )?;
-
-        Ok(node::PropertyDefinition::method_definition(
-            method_kind,
-            prop_name,
-            FunctionExpr::new(None, params, body),
-        ))
     }
 }
 

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

@@ -13,7 +13,7 @@ mod tests;
 use crate::{
     syntax::{
         ast::{node, Punctuator},
-        lexer::{InputElement, TokenKind},
+        lexer::{Error as LexError, InputElement, TokenKind},
         parser::{
             expression::Initializer,
             statement::{BindingIdentifier, StatementList},
@@ -22,7 +22,15 @@ use crate::{
     },
     BoaProfiler,
 };
-use std::{collections::HashSet, io::Read};
+use rustc_hash::FxHashSet;
+use std::io::Read;
+
+/// Intermediate type for a list of FormalParameters with some meta information.
+pub(in crate::syntax::parser) struct FormalParameterList {
+    pub(in crate::syntax::parser) parameters: Box<[node::FormalParameter]>,
+    pub(in crate::syntax::parser) is_simple: bool,
+    pub(in crate::syntax::parser) has_duplicates: bool,
+}
 
 /// Formal parameters parsing.
 ///
@@ -56,25 +64,31 @@ impl<R> TokenParser<R> for FormalParameters
 where
     R: Read,
 {
-    type Output = Box<[node::FormalParameter]>;
+    type Output = FormalParameterList;
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("FormalParameters", "Parsing");
         cursor.set_goal(InputElement::RegExp);
 
         let mut params = Vec::new();
-        let mut param_names = HashSet::new();
+        let mut is_simple = true;
-
+        let mut has_duplicates = false;
-        if cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind()
+
-            == &TokenKind::Punctuator(Punctuator::CloseParen)
+        let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
-        {
+        if next_token.kind() == &TokenKind::Punctuator(Punctuator::CloseParen) {
-            return Ok(params.into_boxed_slice());
+            return Ok(FormalParameterList {
+                parameters: params.into_boxed_slice(),
+                is_simple,
+                has_duplicates,
+            });
         }
+        let start_position = next_token.span().start();
+
+        let mut parameter_names = FxHashSet::default();
 
         loop {
             let mut rest_param = false;
 
-            let position = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.span().start();
             let next_param = match cursor.peek(0)? {
                 Some(tok) if tok.kind() == &TokenKind::Punctuator(Punctuator::Spread) => {
                     rest_param = true;
@@ -82,11 +96,15 @@ where
                 }
                 _ => FormalParameter::new(self.allow_yield, self.allow_await).parse(cursor)?,
             };
-            if param_names.contains(next_param.name()) {
+
-                return Err(ParseError::general("duplicate parameter name", position));
+            if next_param.is_rest_param() || next_param.init().is_some() {
+                is_simple = false;
+            }
+            if parameter_names.contains(next_param.name()) {
+                has_duplicates = true;
             }
+            parameter_names.insert(Box::from(next_param.name()));
 
-            param_names.insert(Box::from(next_param.name()));
             params.push(next_param);
 
             if cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind()
@@ -105,7 +123,20 @@ where
             cursor.expect(Punctuator::Comma, "parameter list")?;
         }
 
-        Ok(params.into_boxed_slice())
+        // Early Error: It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false
+        // and BoundNames of FormalParameterList contains any duplicate elements.
+        if !is_simple && has_duplicates {
+            return Err(ParseError::lex(LexError::Syntax(
+                "Duplicate parameter name not allowed in this context".into(),
+                start_position,
+            )));
+        }
+
+        Ok(FormalParameterList {
+            parameters: params.into_boxed_slice(),
+            is_simple,
+            has_duplicates,
+        })
     }
 }
 
@@ -173,14 +204,14 @@ where
 /// [mdn]: https://developer.mozilla.org/en-US/docs/Glossary/Parameter
 /// [spec]: https://tc39.es/ecma262/#prod-FormalParameter
 #[derive(Debug, Clone, Copy)]
-struct FormalParameter {
+pub(in crate::syntax::parser) struct FormalParameter {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
 }
 
 impl FormalParameter {
     /// Creates a new `FormalParameter` parser.
-    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/function/tests.rs

@@ -21,10 +21,27 @@ fn check_basic() {
     );
 }
 
-/// Checks for duplicate parameter names.
+/// Checks if duplicate parameter names are allowed with strict mode off.
 #[test]
-fn check_duplicates() {
+fn check_duplicates_strict_off() {
-    let js = "function foo(a, a) {}";
+    check_parser(
+        "function foo(a, a) { return a; }",
+        vec![FunctionDecl::new(
+            Box::from("foo"),
+            vec![
+                FormalParameter::new("a", None, false),
+                FormalParameter::new("a", None, false),
+            ],
+            vec![Return::new(Identifier::from("a"), None).into()],
+        )
+        .into()],
+    );
+}
+
+/// Checks if duplicate parameter names are an error with strict mode on.
+#[test]
+fn check_duplicates_strict_on() {
+    let js = "'use strict'; function foo(a, a) {}";
 
     let res = Parser::new(js.as_bytes(), false).parse_all();
     dbg!(&res);

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

@@ -2,12 +2,9 @@
 mod tests;
 
 use crate::syntax::{
-    ast::{node::AsyncFunctionDecl, Keyword, Punctuator},
+    ast::{node::AsyncFunctionDecl, Keyword},
-    lexer::TokenKind,
     parser::{
-        function::FormalParameters,
+        statement::declaration::hoistable::{parse_callable_declaration, CallableDeclaration},
-        function::FunctionBody,
-        statement::{BindingIdentifier, LexError, Position},
         AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, TokenParser,
     },
 };
@@ -44,6 +41,33 @@ impl AsyncFunctionDeclaration {
     }
 }
 
+impl CallableDeclaration for AsyncFunctionDeclaration {
+    fn error_context(&self) -> &'static str {
+        "async function declaration"
+    }
+    fn is_default(&self) -> bool {
+        self.is_default.0
+    }
+    fn name_allow_yield(&self) -> bool {
+        self.allow_yield.0
+    }
+    fn name_allow_await(&self) -> bool {
+        self.allow_await.0
+    }
+    fn parameters_allow_yield(&self) -> bool {
+        false
+    }
+    fn parameters_allow_await(&self) -> bool {
+        true
+    }
+    fn body_allow_yield(&self) -> bool {
+        false
+    }
+    fn body_allow_await(&self) -> bool {
+        true
+    }
+}
+
 impl<R> TokenParser<R> for AsyncFunctionDeclaration
 where
     R: Read,
@@ -54,56 +78,9 @@ where
         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 {
+        let result = parse_callable_declaration(&self, cursor)?;
-            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")?;
-
-        // It is a Syntax Error if any element of the BoundNames of FormalParameters
-        // also occurs in the LexicallyDeclaredNames of FunctionBody.
-        // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
-        {
-            let lexically_declared_names = body.lexically_declared_names();
-            for param in params.as_ref() {
-                if lexically_declared_names.contains(param.name()) {
-                    return Err(ParseError::lex(LexError::Syntax(
-                        format!("Redeclaration of formal parameter `{}`", param.name()).into(),
-                        match cursor.peek(0)? {
-                            Some(token) => token.span().end(),
-                            None => Position::new(1, 1),
-                        },
-                    )));
-                }
-            }
-        }
 
-        Ok(AsyncFunctionDecl::new(name, params, body))
+        Ok(AsyncFunctionDecl::new(result.0, result.1, result.2))
     }
 }

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

@@ -2,11 +2,9 @@
 mod tests;
 
 use crate::syntax::{
-    ast::{node::FunctionDecl, Keyword, Punctuator},
+    ast::{node::FunctionDecl, Keyword},
     parser::{
-        function::FormalParameters,
+        statement::declaration::hoistable::{parse_callable_declaration, CallableDeclaration},
-        function::FunctionBody,
-        statement::{BindingIdentifier, LexError, Position},
         AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, TokenParser,
     },
 };
@@ -22,7 +20,7 @@ use std::io::Read;
 /// [spec]: https://tc39.es/ecma262/#prod-FunctionDeclaration
 
 #[derive(Debug, Clone, Copy)]
-pub(super) struct FunctionDeclaration {
+pub(in crate::syntax::parser) struct FunctionDeclaration {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
     is_default: AllowDefault,
@@ -30,7 +28,11 @@ pub(super) struct FunctionDeclaration {
 
 impl FunctionDeclaration {
     /// Creates a new `FunctionDeclaration` parser.
-    pub(super) fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
+    pub(in crate::syntax::parser) fn new<Y, A, D>(
+        allow_yield: Y,
+        allow_await: A,
+        is_default: D,
+    ) -> Self
     where
         Y: Into<AllowYield>,
         A: Into<AllowAwait>,
@@ -44,6 +46,33 @@ impl FunctionDeclaration {
     }
 }
 
+impl CallableDeclaration for FunctionDeclaration {
+    fn error_context(&self) -> &'static str {
+        "function declaration"
+    }
+    fn is_default(&self) -> bool {
+        self.is_default.0
+    }
+    fn name_allow_yield(&self) -> bool {
+        self.allow_yield.0
+    }
+    fn name_allow_await(&self) -> bool {
+        self.allow_await.0
+    }
+    fn parameters_allow_yield(&self) -> bool {
+        false
+    }
+    fn parameters_allow_await(&self) -> bool {
+        false
+    }
+    fn body_allow_yield(&self) -> bool {
+        self.allow_yield.0
+    }
+    fn body_allow_await(&self) -> bool {
+        self.allow_await.0
+    }
+}
+
 impl<R> TokenParser<R> for FunctionDeclaration
 where
     R: Read,
@@ -53,38 +82,8 @@ where
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         cursor.expect(Keyword::Function, "function declaration")?;
 
-        // TODO: If self.is_default, then this can be empty.
+        let result = parse_callable_declaration(&self, cursor)?;
-        let name = BindingIdentifier::new(self.allow_yield, self.allow_await).parse(cursor)?;
-
-        cursor.expect(Punctuator::OpenParen, "function declaration")?;
-
-        let params = FormalParameters::new(false, false).parse(cursor)?;
-
-        cursor.expect(Punctuator::CloseParen, "function declaration")?;
-        cursor.expect(Punctuator::OpenBlock, "function declaration")?;
-
-        let body = FunctionBody::new(self.allow_yield, self.allow_await).parse(cursor)?;
-
-        cursor.expect(Punctuator::CloseBlock, "function declaration")?;
-
-        // It is a Syntax Error if any element of the BoundNames of FormalParameters
-        // also occurs in the LexicallyDeclaredNames of FunctionBody.
-        // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
-        {
-            let lexically_declared_names = body.lexically_declared_names();
-            for param in params.as_ref() {
-                if lexically_declared_names.contains(param.name()) {
-                    return Err(ParseError::lex(LexError::Syntax(
-                        format!("Redeclaration of formal parameter `{}`", param.name()).into(),
-                        match cursor.peek(0)? {
-                            Some(token) => token.span().end(),
-                            None => Position::new(1, 1),
-                        },
-                    )));
-                }
-            }
-        }
 
-        Ok(FunctionDecl::new(name, params, body))
+        Ok(FunctionDecl::new(result.0, result.1, result.2))
     }
 }

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

@@ -0,0 +1,85 @@
+#[cfg(test)]
+mod tests;
+
+use crate::syntax::{
+    ast::{node::declaration::generator_decl::GeneratorDecl, Keyword, Punctuator},
+    parser::{
+        statement::declaration::hoistable::{parse_callable_declaration, CallableDeclaration},
+        AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, TokenParser,
+    },
+};
+use std::io::Read;
+
+/// Generator declaration parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*
+/// [spec]: https://tc39.es/ecma262/#prod-GeneratorDeclaration
+#[derive(Debug, Clone, Copy)]
+pub(super) struct GeneratorDeclaration {
+    allow_yield: AllowYield,
+    allow_await: AllowAwait,
+    is_default: AllowDefault,
+}
+
+impl GeneratorDeclaration {
+    /// Creates a new `GeneratorDeclaration` 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 CallableDeclaration for GeneratorDeclaration {
+    fn error_context(&self) -> &'static str {
+        "generator declaration"
+    }
+    fn is_default(&self) -> bool {
+        self.is_default.0
+    }
+    fn name_allow_yield(&self) -> bool {
+        self.allow_yield.0
+    }
+    fn name_allow_await(&self) -> bool {
+        self.allow_await.0
+    }
+    fn parameters_allow_yield(&self) -> bool {
+        true
+    }
+    fn parameters_allow_await(&self) -> bool {
+        false
+    }
+    fn body_allow_yield(&self) -> bool {
+        true
+    }
+    fn body_allow_await(&self) -> bool {
+        false
+    }
+}
+
+impl<R> TokenParser<R> for GeneratorDeclaration
+where
+    R: Read,
+{
+    type Output = GeneratorDecl;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        cursor.expect(Keyword::Function, "generator declaration")?;
+        cursor.expect(Punctuator::Mul, "generator declaration")?;
+
+        let result = parse_callable_declaration(&self, cursor)?;
+
+        Ok(GeneratorDecl::new(result.0, result.1, result.2))
+    }
+}

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

@@ -0,0 +1,9 @@
+use crate::syntax::{ast::node::GeneratorDecl, parser::tests::check_parser};
+
+#[test]
+fn generator_function_declaration() {
+    check_parser(
+        "function* gen() {}",
+        vec![GeneratorDecl::new(Box::from("gen"), vec![], vec![]).into()],
+    );
+}

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

@@ -10,15 +10,20 @@ mod tests;
 
 mod async_function_decl;
 mod function_decl;
+mod generator_decl;
 
 use async_function_decl::AsyncFunctionDeclaration;
-use function_decl::FunctionDeclaration;
+pub(in crate::syntax::parser) use function_decl::FunctionDeclaration;
+use generator_decl::GeneratorDeclaration;
 
 use crate::{
     syntax::{
-        ast::{Keyword, Node},
+        ast::node::{FormalParameter, StatementList},
-        lexer::TokenKind,
+        ast::{Keyword, Node, Punctuator},
+        lexer::{Position, TokenKind},
         parser::{
+            function::{FormalParameters, FunctionBody},
+            statement::{BindingIdentifier, LexError},
             AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, ParseResult, TokenParser,
         },
     },
@@ -67,10 +72,17 @@ where
 
         match tok.kind() {
             TokenKind::Keyword(Keyword::Function) => {
+                let next_token = cursor.peek(1)?.ok_or(ParseError::AbruptEnd)?;
+                if let TokenKind::Punctuator(Punctuator::Mul) = next_token.kind() {
+                    GeneratorDeclaration::new(self.allow_yield, self.allow_await, self.is_default)
+                        .parse(cursor)
+                        .map(Node::from)
+                } else {
                     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)
@@ -80,3 +92,104 @@ where
         }
     }
 }
+
+trait CallableDeclaration {
+    fn error_context(&self) -> &'static str;
+    fn is_default(&self) -> bool;
+    fn name_allow_yield(&self) -> bool;
+    fn name_allow_await(&self) -> bool;
+    fn parameters_allow_yield(&self) -> bool;
+    fn parameters_allow_await(&self) -> bool;
+    fn body_allow_yield(&self) -> bool;
+    fn body_allow_await(&self) -> bool;
+}
+
+// This is a helper function to not duplicate code in the individual callable deceleration parsers.
+#[inline]
+#[allow(clippy::type_complexity)]
+fn parse_callable_declaration<R: Read, C: CallableDeclaration>(
+    c: &C,
+    cursor: &mut Cursor<R>,
+) -> Result<(Box<str>, Box<[FormalParameter]>, StatementList), ParseError> {
+    let next_token = cursor.peek(0)?;
+    let name = if let Some(token) = next_token {
+        match token.kind() {
+            TokenKind::Punctuator(Punctuator::OpenParen) => {
+                if !c.is_default() {
+                    return Err(ParseError::unexpected(token.clone(), c.error_context()));
+                }
+                "default".into()
+            }
+            _ => {
+                BindingIdentifier::new(c.name_allow_yield(), c.name_allow_await()).parse(cursor)?
+            }
+        }
+    } else {
+        return Err(ParseError::AbruptEnd);
+    };
+
+    // Early Error: If BindingIdentifier is present and the source code matching BindingIdentifier is strict mode code,
+    // it is a Syntax Error if the StringValue of BindingIdentifier is "eval" or "arguments".
+    if cursor.strict_mode() && ["eval", "arguments"].contains(&name.as_ref()) {
+        return Err(ParseError::lex(LexError::Syntax(
+            "Unexpected eval or arguments in strict mode".into(),
+            match cursor.peek(0)? {
+                Some(token) => token.span().end(),
+                None => Position::new(1, 1),
+            },
+        )));
+    }
+
+    let params_start_position = cursor
+        .expect(Punctuator::OpenParen, c.error_context())?
+        .span()
+        .end();
+
+    let params = FormalParameters::new(c.parameters_allow_yield(), c.parameters_allow_await())
+        .parse(cursor)?;
+
+    cursor.expect(Punctuator::CloseParen, c.error_context())?;
+    cursor.expect(Punctuator::OpenBlock, c.error_context())?;
+
+    let body = FunctionBody::new(c.body_allow_yield(), c.body_allow_await()).parse(cursor)?;
+
+    cursor.expect(Punctuator::CloseBlock, c.error_context())?;
+
+    // Early Error: If the source code matching FormalParameters is strict mode code,
+    // the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
+    if (cursor.strict_mode() || body.strict()) && params.has_duplicates {
+        return Err(ParseError::lex(LexError::Syntax(
+            "Duplicate parameter name not allowed in this context".into(),
+            params_start_position,
+        )));
+    }
+
+    // Early Error: It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true
+    // and IsSimpleParameterList of FormalParameters is false.
+    if body.strict() && !params.is_simple {
+        return Err(ParseError::lex(LexError::Syntax(
+            "Illegal 'use strict' directive in function with non-simple parameter list".into(),
+            params_start_position,
+        )));
+    }
+
+    // It is a Syntax Error if any element of the BoundNames of FormalParameters
+    // also occurs in the LexicallyDeclaredNames of FunctionBody.
+    // https://tc39.es/ecma262/#sec-function-definitions-static-semantics-early-errors
+    {
+        let lexically_declared_names = body.lexically_declared_names();
+        for param in params.parameters.as_ref() {
+            if lexically_declared_names.contains(param.name()) {
+                return Err(ParseError::lex(LexError::Syntax(
+                    format!("Redeclaration of formal parameter `{}`", param.name()).into(),
+                    match cursor.peek(0)? {
+                        Some(token) => token.span().end(),
+                        None => Position::new(1, 1),
+                    },
+                )));
+            }
+        }
+    }
+
+    Ok((name, params.parameters, body))
+}

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

@@ -7,7 +7,7 @@
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements#Declarations
 //! [spec]:https://tc39.es/ecma262/#sec-declarations-and-the-variable-statement
 
-mod hoistable;
+pub(in crate::syntax::parser) mod hoistable;
 mod lexical;
 #[cfg(test)]
 mod tests;

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

@@ -1,8 +1,9 @@
 use super::super::{expression::Expression, ParseResult};
 use crate::{
     syntax::{
-        ast::node::Node,
+        ast::{node::Node, Keyword, Punctuator},
-        parser::{AllowAwait, AllowYield, Cursor, TokenParser},
+        lexer::TokenKind,
+        parser::{AllowAwait, AllowYield, Cursor, ParseError, TokenParser},
     },
     BoaProfiler,
 };
@@ -42,7 +43,36 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> ParseResult {
         let _timer = BoaProfiler::global().start_event("ExpressionStatement", "Parsing");
-        // TODO: lookahead
+
+        let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+        match next_token.kind() {
+            TokenKind::Keyword(Keyword::Function) | TokenKind::Keyword(Keyword::Class) => {
+                return Err(ParseError::general(
+                    "expected statement",
+                    next_token.span().start(),
+                ));
+            }
+            TokenKind::Keyword(Keyword::Async) => {
+                let next_token = cursor.peek(1)?.ok_or(ParseError::AbruptEnd)?;
+                if next_token.kind() == &TokenKind::Keyword(Keyword::Function) {
+                    return Err(ParseError::general(
+                        "expected statement",
+                        next_token.span().start(),
+                    ));
+                }
+            }
+            TokenKind::Keyword(Keyword::Let) => {
+                let next_token = cursor.peek(1)?.ok_or(ParseError::AbruptEnd)?;
+                if next_token.kind() == &TokenKind::Punctuator(Punctuator::OpenBracket) {
+                    return Err(ParseError::general(
+                        "expected statement",
+                        next_token.span().start(),
+                    ));
+                }
+            }
+            _ => {}
+        }
+
         let expr = Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
 
         cursor.expect_semicolon("expression statement")?;

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

@@ -1,20 +1,18 @@
 #[cfg(test)]
 mod tests;
 
-use super::Statement;
-
-use crate::syntax::lexer::TokenKind;
 use crate::{
     syntax::{
         ast::{node::If, Keyword, Node, Punctuator},
+        lexer::TokenKind,
         parser::{
-            expression::Expression, AllowAwait, AllowReturn, AllowYield, Cursor, ParseError,
+            expression::Expression,
-            TokenParser,
+            statement::{declaration::hoistable::FunctionDeclaration, Statement},
+            AllowAwait, AllowReturn, AllowYield, Cursor, ParseError, TokenParser,
         },
     },
     BoaProfiler,
 };
-
 use std::io::Read;
 
 /// If statement parsing.
@@ -58,30 +56,67 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("IfStatement", "Parsing");
+
         cursor.expect(Keyword::If, "if statement")?;
         cursor.expect(Punctuator::OpenParen, "if statement")?;
 
-        let cond = Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
+        let condition = Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
+
+        let position = cursor
+            .expect(Punctuator::CloseParen, "if statement")?
+            .span()
+            .end();
 
-        cursor.expect(Punctuator::CloseParen, "if statement")?;
+        let then_node = if !cursor.strict_mode()
+            && cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind()
+                == &TokenKind::Keyword(Keyword::Function)
+        {
+            // FunctionDeclarations in IfStatement Statement Clauses
+            // https://tc39.es/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses
+            FunctionDeclaration::new(self.allow_yield, self.allow_await, false)
+                .parse(cursor)?
+                .into()
+        } else {
+            let node = Statement::new(self.allow_yield, self.allow_await, self.allow_return)
+                .parse(cursor)?;
 
-        let then_stm =
+            // Early Error: It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
-            Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
+            if let Node::FunctionDecl(_) = node {
+                return Err(ParseError::wrong_function_declaration_non_strict(position));
+            }
 
-        let else_stm = if let Some(else_tok) = cursor.peek(0)? {
+            node
-            if else_tok.kind() == &TokenKind::Keyword(Keyword::Else) {
+        };
-                cursor.next()?.expect("else token vanished");
+
+        let else_node = if cursor.next_if(Keyword::Else)?.is_some() {
+            let position = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.span().start();
+
+            if !cursor.strict_mode()
+                && cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.kind()
+                    == &TokenKind::Keyword(Keyword::Function)
+            {
+                // FunctionDeclarations in IfStatement Statement Clauses
+                // https://tc39.es/ecma262/#sec-functiondeclarations-in-ifstatement-statement-clauses
                 Some(
-                    Statement::new(self.allow_yield, self.allow_await, self.allow_return)
+                    FunctionDeclaration::new(self.allow_yield, self.allow_await, false)
-                        .parse(cursor)?,
+                        .parse(cursor)?
+                        .into(),
                 )
             } else {
-                None
+                let node = Statement::new(self.allow_yield, self.allow_await, self.allow_return)
+                    .parse(cursor)?;
+
+                // Early Error: It is a Syntax Error if IsLabelledFunction(the second Statement) is true.
+                if let Node::FunctionDecl(_) = node {
+                    return Err(ParseError::wrong_function_declaration_non_strict(position));
+                }
+
+                Some(node)
             }
         } else {
             None
         };
 
-        Ok(If::new::<_, _, Node, _>(cond, then_stm, else_stm))
+        Ok(If::new::<_, _, Node, _>(condition, then_node, else_node))
     }
 }

boa/src/syntax/parser/statement/iteration/do_while_statement.rs

@@ -7,10 +7,10 @@
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/do...while
 //! [spec]: https://tc39.es/ecma262/#sec-do-while-statement
 
-use crate::syntax::lexer::TokenKind;
 use crate::{
     syntax::{
-        ast::{node::DoWhileLoop, Keyword, Punctuator},
+        ast::{node::DoWhileLoop, Keyword, Node, Punctuator},
+        lexer::TokenKind,
         parser::{
             expression::Expression, statement::Statement, AllowAwait, AllowReturn, AllowYield,
             Cursor, ParseError, TokenParser,
@@ -63,11 +63,20 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("DoWhileStatement", "Parsing");
-        cursor.expect(Keyword::Do, "do while statement")?;
+
+        let position = cursor
+            .expect(Keyword::Do, "do while statement")?
+            .span()
+            .end();
 
         let body =
             Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
 
+        // Early Error: It is a Syntax Error if IsLabelledFunction(Statement) is true.
+        if let Node::FunctionDecl(_) = body {
+            return Err(ParseError::wrong_function_declaration_non_strict(position));
+        }
+
         let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
         if next_token.kind() != &TokenKind::Keyword(Keyword::While) {
@@ -86,7 +95,7 @@ where
 
         cursor.expect(Punctuator::CloseParen, "do while statement")?;
 
-        // Here, we only care to read the next token if it's a smicolon. If it's not, we
+        // Here, we only care to read the next token if it's a semicolon. If it's not, we
         // automatically "enter" or assume a semicolon, since we have just read the `)` token:
         // https://tc39.es/ecma262/#sec-automatic-semicolon-insertion
         if let Some(tok) = cursor.peek(0)? {

boa/src/syntax/parser/statement/iteration/for_statement.rs

@@ -93,18 +93,40 @@ where
                 let _ = cursor.next();
                 let expr =
                     Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
-                cursor.expect(Punctuator::CloseParen, "for in statement")?;
+
+                let position = cursor
+                    .expect(Punctuator::CloseParen, "for in statement")?
+                    .span()
+                    .end();
+
                 let body = Statement::new(self.allow_yield, self.allow_await, self.allow_return)
                     .parse(cursor)?;
+
+                // Early Error: It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
+                if let Node::FunctionDecl(_) = body {
+                    return Err(ParseError::wrong_function_declaration_non_strict(position));
+                }
+
                 return Ok(ForInLoop::new(init.unwrap(), expr, body).into());
             }
             Some(tok) if tok.kind() == &TokenKind::Keyword(Keyword::Of) && init.is_some() => {
                 let _ = cursor.next();
                 let iterable =
                     Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
-                cursor.expect(Punctuator::CloseParen, "for of statement")?;
+
+                let position = cursor
+                    .expect(Punctuator::CloseParen, "for of statement")?
+                    .span()
+                    .end();
+
                 let body = Statement::new(self.allow_yield, self.allow_await, self.allow_return)
                     .parse(cursor)?;
+
+                // Early Error: It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
+                if let Node::FunctionDecl(_) = body {
+                    return Err(ParseError::wrong_function_declaration_non_strict(position));
+                }
+
                 return Ok(ForOfLoop::new(init.unwrap(), iterable, body).into());
             }
             _ => {}
@@ -131,9 +153,16 @@ where
             Some(step)
         };
 
+        let position = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?.span().start();
+
         let body =
             Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
 
+        // Early Error: It is a Syntax Error if IsLabelledFunction(the first Statement) is true.
+        if let Node::FunctionDecl(_) = body {
+            return Err(ParseError::wrong_function_declaration_non_strict(position));
+        }
+
         // TODO: do not encapsulate the `for` in a block just to have an inner scope.
         Ok(ForLoop::new(init, cond, step, body).into())
     }

boa/src/syntax/parser/statement/iteration/while_statement.rs

@@ -1,6 +1,6 @@
 use crate::{
     syntax::{
-        ast::{node::WhileLoop, Keyword, Punctuator},
+        ast::{node::WhileLoop, Keyword, Node, Punctuator},
         parser::{
             expression::Expression, statement::Statement, AllowAwait, AllowReturn, AllowYield,
             Cursor, ParseError, TokenParser,
@@ -60,11 +60,19 @@ where
 
         let cond = Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
 
-        cursor.expect(Punctuator::CloseParen, "while statement")?;
+        let position = cursor
+            .expect(Punctuator::CloseParen, "while statement")?
+            .span()
+            .end();
 
         let body =
             Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
 
+        // Early Error: It is a Syntax Error if IsLabelledFunction(Statement) is true.
+        if let Node::FunctionDecl(_) = body {
+            return Err(ParseError::wrong_function_declaration_non_strict(position));
+        }
+
         Ok(WhileLoop::new(cond, body))
     }
 }

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

@@ -1,12 +1,17 @@
 use std::io::Read;
 
-use super::{LabelIdentifier, Statement};
 use crate::{
-    syntax::ast::Node,
     syntax::{
-        ast::Punctuator,
+        ast::{Keyword, Node, Punctuator},
+        lexer::TokenKind,
         parser::{
-            cursor::Cursor, error::ParseError, AllowAwait, AllowReturn, AllowYield, TokenParser,
+            cursor::Cursor,
+            error::ParseError,
+            statement::{
+                declaration::hoistable::FunctionDeclaration, AllowAwait, AllowReturn,
+                LabelIdentifier, Statement,
+            },
+            AllowYield, TokenParser,
         },
     },
     BoaProfiler,
@@ -49,18 +54,38 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("Label", "Parsing");
+
         let name = LabelIdentifier::new(self.allow_yield, self.allow_await).parse(cursor)?;
+
         cursor.expect(Punctuator::Colon, "Labelled Statement")?;
-        let mut stmt =
-            Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
 
-        set_label_for_node(&mut stmt, name);
+        let strict = cursor.strict_mode();
-        Ok(stmt)
+        let next_token = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+        let mut node = match next_token.kind() {
+            // Early Error: It is a Syntax Error if any strict mode source code matches this rule.
+            // https://tc39.es/ecma262/#sec-labelled-statements-static-semantics-early-errors
+            // https://tc39.es/ecma262/#sec-labelled-function-declarations
+            TokenKind::Keyword(Keyword::Function) if strict => {
+                return Err(ParseError::general(
+                    "In strict mode code, functions can only be declared at top level or inside a block.",
+                    next_token.span().start()
+                ))
+            }
+            TokenKind::Keyword(Keyword::Function) => {
+                FunctionDeclaration::new(self.allow_yield, self.allow_await, false)
+                .parse(cursor)?
+                .into()
+            }
+            _ => Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?
+        };
+
+        set_label_for_node(&mut node, name);
+        Ok(node)
     }
 }
 
-fn set_label_for_node(stmt: &mut Node, name: Box<str>) {
+fn set_label_for_node(node: &mut Node, name: Box<str>) {
-    match stmt {
+    match node {
         Node::ForLoop(ref mut for_loop) => for_loop.set_label(name),
         Node::ForOfLoop(ref mut for_of_loop) => for_of_loop.set_label(name),
         Node::ForInLoop(ref mut for_in_loop) => for_in_loop.set_label(name),

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

@@ -520,24 +520,38 @@ where
 
         match next_token.kind() {
             TokenKind::Identifier(ref s) => Ok(s.clone()),
-            TokenKind::Keyword(k @ Keyword::Yield) if !self.allow_yield.0 => {
+            TokenKind::Keyword(Keyword::Yield) if self.allow_yield.0 => {
+                // Early Error: It is a Syntax Error if this production has a [Yield] parameter and StringValue of Identifier is "yield".
+                Err(ParseError::general(
+                    "Unexpected identifier",
+                    next_token.span().start(),
+                ))
+            }
+            TokenKind::Keyword(Keyword::Yield) if !self.allow_yield.0 => {
                 if cursor.strict_mode() {
-                    Err(ParseError::lex(LexError::Syntax(
+                    Err(ParseError::general(
-                        "yield keyword in binding identifier not allowed in strict mode".into(),
+                        "yield keyword in binding identifier not allowed in strict mode",
                         next_token.span().start(),
-                    )))
+                    ))
                 } else {
-                    Ok(k.as_str().into())
+                    Ok("yield".into())
                 }
             }
-            TokenKind::Keyword(k @ Keyword::Await) if !self.allow_await.0 => {
+            TokenKind::Keyword(Keyword::Await) if self.allow_await.0 => {
+                // Early Error: It is a Syntax Error if this production has an [Await] parameter and StringValue of Identifier is "await".
+                Err(ParseError::general(
+                    "Unexpected identifier",
+                    next_token.span().start(),
+                ))
+            }
+            TokenKind::Keyword(Keyword::Await) if !self.allow_await.0 => {
                 if cursor.strict_mode() {
-                    Err(ParseError::lex(LexError::Syntax(
+                    Err(ParseError::general(
-                        "await keyword in binding identifier not allowed in strict mode".into(),
+                        "await keyword in binding identifier not allowed in strict mode",
                         next_token.span().start(),
-                    )))
+                    ))
                 } else {
-                    Ok(k.as_str().into())
+                    Ok("await".into())
                 }
             }
             _ => Err(ParseError::expected(