LibJS: Implement generator functions (only in bytecode mode)

21 files changed, 407 insertions, 34 deletions

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 203bc880eb..f56a5c33bb 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -259,6 +259,12 @@ Value CallExpression::execute(Interpreter& interpreter, GlobalObject& global_obj
     return result;
 }
 
+Value YieldExpression::execute(Interpreter&, GlobalObject&) const
+{
+    // This should be transformed to a return.
+    VERIFY_NOT_REACHED();
+}
+
 Value ReturnStatement::execute(Interpreter& interpreter, GlobalObject& global_object) const
 {
     InterpreterNodeScope node_scope { interpreter, *this };
@@ -1151,7 +1157,7 @@ void BindingPattern::dump(int indent) const
 void FunctionNode::dump(int indent, const String& class_name) const
 {
     print_indent(indent);
-    outln("{} '{}'", class_name, name());
+    outln("{}{} '{}'", class_name, m_is_generator ? "*" : "", name());
     if (!m_parameters.is_empty()) {
         print_indent(indent + 1);
         outln("(Parameters)");
@@ -1193,6 +1199,13 @@ void FunctionExpression::dump(int indent) const
     FunctionNode::dump(indent, class_name());
 }
 
+void YieldExpression::dump(int indent) const
+{
+    ASTNode::dump(indent);
+    if (argument())
+        argument()->dump(indent + 1);
+}
+
 void ReturnStatement::dump(int indent) const
 {
     ASTNode::dump(indent);
diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h
index 946a60feaa..1f32544e71 100644
--- a/Userland/Libraries/LibJS/AST.h
+++ b/Userland/Libraries/LibJS/AST.h
@@ -227,14 +227,16 @@ public:
     const Vector<Parameter>& parameters() const { return m_parameters; };
     i32 function_length() const { return m_function_length; }
     bool is_strict_mode() const { return m_is_strict_mode; }
+    bool is_generator() const { return m_is_generator; }
 
 protected:
-    FunctionNode(const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_strict_mode)
+    FunctionNode(const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_generator, bool is_strict_mode)
         : m_name(name)
         , m_body(move(body))
         , m_parameters(move(parameters))
         , m_variables(move(variables))
         , m_function_length(function_length)
+        , m_is_generator(is_generator)
         , m_is_strict_mode(is_strict_mode)
     {
     }
@@ -256,6 +258,7 @@ private:
     const Vector<Parameter> m_parameters;
     NonnullRefPtrVector<VariableDeclaration> m_variables;
     const i32 m_function_length;
+    bool m_is_generator;
     bool m_is_strict_mode;
 };
 
@@ -265,9 +268,9 @@ class FunctionDeclaration final
 public:
     static bool must_have_name() { return true; }
 
-    FunctionDeclaration(SourceRange source_range, const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_strict_mode = false)
+    FunctionDeclaration(SourceRange source_range, const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_generator, bool is_strict_mode = false)
         : Declaration(move(source_range))
-        , FunctionNode(name, move(body), move(parameters), function_length, move(variables), is_strict_mode)
+        , FunctionNode(name, move(body), move(parameters), function_length, move(variables), is_generator, is_strict_mode)
     {
     }
 
@@ -282,9 +285,9 @@ class FunctionExpression final
 public:
     static bool must_have_name() { return false; }
 
-    FunctionExpression(SourceRange source_range, const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_strict_mode, bool is_arrow_function = false)
+    FunctionExpression(SourceRange source_range, const FlyString& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, NonnullRefPtrVector<VariableDeclaration> variables, bool is_generator, bool is_strict_mode, bool is_arrow_function = false)
         : Expression(source_range)
-        , FunctionNode(name, move(body), move(parameters), function_length, move(variables), is_strict_mode)
+        , FunctionNode(name, move(body), move(parameters), function_length, move(variables), is_generator, is_strict_mode)
         , m_is_arrow_function(is_arrow_function)
     {
     }
@@ -318,6 +321,24 @@ public:
     Value execute(Interpreter&, GlobalObject&) const override { return {}; }
 };
 
+class YieldExpression final : public Expression {
+public:
+    explicit YieldExpression(SourceRange source_range, RefPtr<Expression> argument)
+        : Expression(move(source_range))
+        , m_argument(move(argument))
+    {
+    }
+
+    const Expression* argument() const { return m_argument; }
+
+    virtual Value execute(Interpreter&, GlobalObject&) const override;
+    virtual void dump(int indent) const override;
+    virtual void generate_bytecode(Bytecode::Generator&) const override;
+
+private:
+    RefPtr<Expression> m_argument;
+};
+
 class ReturnStatement final : public Statement {
 public:
     explicit ReturnStatement(SourceRange source_range, RefPtr<Expression> argument)
diff --git a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
index 6397ca2e8c..e5e3704bbe 100644
--- a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
@@ -668,7 +668,23 @@ void ReturnStatement::generate_bytecode(Bytecode::Generator& generator) const
 {
     if (m_argument)
         m_argument->generate_bytecode(generator);
-    generator.emit<Bytecode::Op::Return>();
+
+    if (generator.is_in_generator_function())
+        generator.emit<Bytecode::Op::Yield>(nullptr);
+    else
+        generator.emit<Bytecode::Op::Return>();
+}
+
+void YieldExpression::generate_bytecode(Bytecode::Generator& generator) const
+{
+    VERIFY(generator.is_in_generator_function());
+
+    if (m_argument)
+        m_argument->generate_bytecode(generator);
+
+    auto& continuation_block = generator.make_block();
+    generator.emit<Bytecode::Op::Yield>(Bytecode::Label { continuation_block });
+    generator.switch_to_basic_block(continuation_block);
 }
 
 void IfStatement::generate_bytecode(Bytecode::Generator& generator) const
diff --git a/Userland/Libraries/LibJS/Bytecode/Generator.cpp b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
index c7e394ef2f..7d356588f3 100644
--- a/Userland/Libraries/LibJS/Bytecode/Generator.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Generator.cpp
@@ -23,11 +23,28 @@ Generator::~Generator()
 {
 }
 
-Executable Generator::generate(ASTNode const& node)
+Executable Generator::generate(ASTNode const& node, bool is_in_generator_function)
 {
     Generator generator;
     generator.switch_to_basic_block(generator.make_block());
+    if (is_in_generator_function) {
+        generator.enter_generator_context();
+        // Immediately yield with no value.
+        auto& start_block = generator.make_block();
+        generator.emit<Bytecode::Op::Yield>(Label { start_block });
+        generator.switch_to_basic_block(start_block);
+    }
     node.generate_bytecode(generator);
+    if (is_in_generator_function) {
+        // Terminate all unterminated blocks with yield return
+        for (auto& block : generator.m_root_basic_blocks) {
+            if (block.is_terminated())
+                continue;
+            generator.switch_to_basic_block(block);
+            generator.emit<Bytecode::Op::LoadImmediate>(js_undefined());
+            generator.emit<Bytecode::Op::Yield>(nullptr);
+        }
+    }
     return { move(generator.m_root_basic_blocks), move(generator.m_string_table), generator.m_next_register };
 }
 
diff --git a/Userland/Libraries/LibJS/Bytecode/Generator.h b/Userland/Libraries/LibJS/Bytecode/Generator.h
index 1ae3e0ab94..49b34e138a 100644
--- a/Userland/Libraries/LibJS/Bytecode/Generator.h
+++ b/Userland/Libraries/LibJS/Bytecode/Generator.h
@@ -28,7 +28,7 @@ struct Executable {
 
 class Generator {
 public:
-    static Executable generate(ASTNode const&);
+    static Executable generate(ASTNode const&, bool is_in_generator_function = false);
 
     Register allocate_register();
 
@@ -109,6 +109,10 @@ public:
         return m_string_table->insert(string);
     }
 
+    bool is_in_generator_function() const { return m_is_in_generator_function; }
+    void enter_generator_context() { m_is_in_generator_function = true; }
+    void leave_generator_context() { m_is_in_generator_function = false; }
+
 private:
     Generator();
     ~Generator();
@@ -122,6 +126,7 @@ private:
 
     u32 m_next_register { 2 };
     u32 m_next_block { 1 };
+    bool m_is_in_generator_function { false };
     Vector<Label> m_continuable_scopes;
     Vector<Label> m_breakable_scopes;
 };
diff --git a/Userland/Libraries/LibJS/Bytecode/Instruction.h b/Userland/Libraries/LibJS/Bytecode/Instruction.h
index 901b291139..b3a622898f 100644
--- a/Userland/Libraries/LibJS/Bytecode/Instruction.h
+++ b/Userland/Libraries/LibJS/Bytecode/Instruction.h
@@ -61,7 +61,8 @@
     O(PushLexicalEnvironment)     \
     O(EnterUnwindContext)         \
     O(LeaveUnwindContext)         \
-    O(ContinuePendingUnwind)
+    O(ContinuePendingUnwind)      \
+    O(Yield)
 
 namespace JS::Bytecode {
 
diff --git a/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp b/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
index fb2a493d83..94c3e1e512 100644
--- a/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Interpreter.cpp
@@ -35,7 +35,7 @@ Interpreter::~Interpreter()
     s_current = nullptr;
 }
 
-Value Interpreter::run(Executable const& executable)
+Value Interpreter::run(Executable const& executable, BasicBlock const* entry_point)
 {
     dbgln_if(JS_BYTECODE_DEBUG, "Bytecode::Interpreter will run unit {:p}", &executable);
 
@@ -56,10 +56,14 @@ Value Interpreter::run(Executable const& executable)
         VERIFY(!vm().exception());
     }
 
-    auto block = &executable.basic_blocks.first();
-    m_register_windows.append(make<RegisterWindow>());
-    registers().resize(executable.number_of_registers);
-    registers()[Register::global_object_index] = Value(&global_object());
+    auto block = entry_point ?: &executable.basic_blocks.first();
+    if (m_manually_entered_frames) {
+        VERIFY(registers().size() >= executable.number_of_registers);
+    } else {
+        m_register_windows.append(make<RegisterWindow>());
+        registers().resize(executable.number_of_registers);
+        registers()[Register::global_object_index] = Value(&global_object());
+    }
 
     for (;;) {
         Bytecode::InstructionStreamIterator pc(block->instruction_stream());
@@ -124,7 +128,8 @@ Value Interpreter::run(Executable const& executable)
 
     vm().set_last_value(Badge<Interpreter> {}, accumulator());
 
-    m_register_windows.take_last();
+    if (!m_manually_entered_frames)
+        m_register_windows.take_last();
 
     auto return_value = m_return_value.value_or(js_undefined());
     m_return_value = {};
diff --git a/Userland/Libraries/LibJS/Bytecode/Interpreter.h b/Userland/Libraries/LibJS/Bytecode/Interpreter.h
index 2e2c3f8109..26b2dc9078 100644
--- a/Userland/Libraries/LibJS/Bytecode/Interpreter.h
+++ b/Userland/Libraries/LibJS/Bytecode/Interpreter.h
@@ -30,10 +30,23 @@ public:
     GlobalObject& global_object() { return m_global_object; }
     VM& vm() { return m_vm; }
 
-    Value run(Bytecode::Executable const&);
+    Value run(Bytecode::Executable const&, Bytecode::BasicBlock const* entry_point = nullptr);
 
     ALWAYS_INLINE Value& accumulator() { return reg(Register::accumulator()); }
     Value& reg(Register const& r) { return registers()[r.index()]; }
+    [[nodiscard]] RegisterWindow snapshot_frame() const { return m_register_windows.last(); }
+
+    void enter_frame(RegisterWindow const& frame)
+    {
+        ++m_manually_entered_frames;
+        m_register_windows.append(make<RegisterWindow>(frame));
+    }
+    void leave_frame()
+    {
+        VERIFY(m_manually_entered_frames);
+        --m_manually_entered_frames;
+        m_register_windows.take_last();
+    }
 
     void jump(Label const& label)
     {
@@ -55,6 +68,7 @@ private:
     NonnullOwnPtrVector<RegisterWindow> m_register_windows;
     Optional<BasicBlock const*> m_pending_jump;
     Value m_return_value;
+    size_t m_manually_entered_frames { 0 };
     Executable const* m_current_executable { nullptr };
     Vector<UnwindInfo> m_unwind_contexts;
     Handle<Exception> m_saved_exception;
diff --git a/Userland/Libraries/LibJS/Bytecode/Op.cpp b/Userland/Libraries/LibJS/Bytecode/Op.cpp
index f97fd1b538..2d080627c1 100644
--- a/Userland/Libraries/LibJS/Bytecode/Op.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/Op.cpp
@@ -218,8 +218,7 @@ void Call::execute(Bytecode::Interpreter& interpreter) const
 void NewFunction::execute(Bytecode::Interpreter& interpreter) const
 {
     auto& vm = interpreter.vm();
-    auto& global_object = interpreter.global_object();
-    interpreter.accumulator() = ScriptFunction::create(global_object, m_function_node.name(), m_function_node.body(), m_function_node.parameters(), m_function_node.function_length(), vm.current_scope(), m_function_node.is_strict_mode());
+    interpreter.accumulator() = ScriptFunction::create(interpreter.global_object(), m_function_node.name(), m_function_node.body(), m_function_node.parameters(), m_function_node.function_length(), vm.current_scope(), m_function_node.is_generator(), m_function_node.is_strict_mode());
 }
 
 void Return::execute(Bytecode::Interpreter& interpreter) const
@@ -280,6 +279,18 @@ void PushLexicalEnvironment::execute(Bytecode::Interpreter& interpreter) const
     interpreter.vm().call_frame().scope = block_lexical_environment;
 }
 
+void Yield::execute(Bytecode::Interpreter& interpreter) const
+{
+    auto yielded_value = interpreter.accumulator().value_or(js_undefined());
+    auto object = JS::Object::create_empty(interpreter.global_object());
+    object->put("result", yielded_value);
+    if (m_continuation_label.has_value())
+        object->put("continuation", Value(static_cast<double>(reinterpret_cast<u64>(&m_continuation_label->block()))));
+    else
+        object->put("continuation", Value(0));
+    interpreter.do_return(object);
+}
+
 String Load::to_string(Bytecode::Executable const&) const
 {
     return String::formatted("Load {}", m_src);
@@ -445,4 +456,11 @@ String PushLexicalEnvironment::to_string(const Bytecode::Executable& executable)
     return builder.to_string();
 }
 
+String Yield::to_string(Bytecode::Executable const&) const
+{
+    if (m_continuation_label.has_value())
+        return String::formatted("Yield continuation:@{}", m_continuation_label->block().name());
+    return String::formatted("Yield return");
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Bytecode/Op.h b/Userland/Libraries/LibJS/Bytecode/Op.h
index ca0497807e..f5961b87e0 100644
--- a/Userland/Libraries/LibJS/Bytecode/Op.h
+++ b/Userland/Libraries/LibJS/Bytecode/Op.h
@@ -184,8 +184,6 @@ public:
     void execute(Bytecode::Interpreter&) const;
     String to_string(Bytecode::Executable const&) const;
 
-    size_t length() const { return sizeof(*this) + sizeof(Register) * m_element_count; }
-
 private:
     size_t m_element_count { 0 };
     Register m_elements[];
@@ -462,6 +460,28 @@ private:
     Label m_resume_target;
 };
 
+class Yield final : public Instruction {
+public:
+    constexpr static bool IsTerminator = true;
+
+    explicit Yield(Label continuation_label)
+        : Instruction(Type::Yield)
+        , m_continuation_label(continuation_label)
+    {
+    }
+
+    explicit Yield(std::nullptr_t)
+        : Instruction(Type::Yield)
+    {
+    }
+
+    void execute(Bytecode::Interpreter&) const;
+    String to_string(Bytecode::Executable const&) const;
+
+private:
+    Optional<Label> m_continuation_label;
+};
+
 class PushLexicalEnvironment final : public Instruction {
 public:
     PushLexicalEnvironment(HashMap<u32, Variable> variables)
@@ -472,6 +492,7 @@ public:
     void execute(Bytecode::Interpreter&) const;
     String to_string(Bytecode::Executable const&) const;
 
+private:
     HashMap<u32, Variable> m_variables;
 };
 }
diff --git a/Userland/Libraries/LibJS/CMakeLists.txt b/Userland/Libraries/LibJS/CMakeLists.txt
index ffcba7b6a1..104538c481 100644
--- a/Userland/Libraries/LibJS/CMakeLists.txt
+++ b/Userland/Libraries/LibJS/CMakeLists.txt
@@ -45,6 +45,7 @@ set(SOURCES
     Runtime/FunctionConstructor.cpp
     Runtime/Function.cpp
     Runtime/FunctionPrototype.cpp
+    Runtime/GeneratorObject.cpp
     Runtime/GlobalObject.cpp
     Runtime/IndexedProperties.cpp
     Runtime/IteratorOperations.cpp
diff --git a/Userland/Libraries/LibJS/Interpreter.cpp b/Userland/Libraries/LibJS/Interpreter.cpp
index db06157740..2b383987a3 100644
--- a/Userland/Libraries/LibJS/Interpreter.cpp
+++ b/Userland/Libraries/LibJS/Interpreter.cpp
@@ -79,7 +79,7 @@ void Interpreter::enter_scope(const ScopeNode& scope_node, ScopeType scope_type,
 {
     ScopeGuard guard([&] {
         for (auto& declaration : scope_node.functions()) {
-            auto* function = ScriptFunction::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), current_scope(), declaration.is_strict_mode());
+            auto* function = ScriptFunction::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), current_scope(), declaration.is_generator(), declaration.is_strict_mode());
             vm().set_variable(declaration.name(), function, global_object);
         }
     });
diff --git a/Userland/Libraries/LibJS/Parser.cpp b/Userland/Libraries/LibJS/Parser.cpp
index c1a8051f94..bca6539426 100644
--- a/Userland/Libraries/LibJS/Parser.cpp
+++ b/Userland/Libraries/LibJS/Parser.cpp
@@ -590,9 +590,9 @@ NonnullRefPtr<ClassExpression> Parser::parse_class_expression(bool expect_class_
             constructor_body->append(create_ast_node<ExpressionStatement>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, move(super_call)));
             constructor_body->add_variables(m_parser_state.m_var_scopes.last());
 
-            constructor = create_ast_node<FunctionExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body), Vector { FunctionNode::Parameter { FlyString { "args" }, nullptr, true } }, 0, NonnullRefPtrVector<VariableDeclaration>(), true);
+            constructor = create_ast_node<FunctionExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body), Vector { FunctionNode::Parameter { FlyString { "args" }, nullptr, true } }, 0, NonnullRefPtrVector<VariableDeclaration>(), false, true);
         } else {
-            constructor = create_ast_node<FunctionExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body), Vector<FunctionNode::Parameter> {}, 0, NonnullRefPtrVector<VariableDeclaration>(), true);
+            constructor = create_ast_node<FunctionExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body), Vector<FunctionNode::Parameter> {}, 0, NonnullRefPtrVector<VariableDeclaration>(), false, true);
         }
     }
 
@@ -634,6 +634,7 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
             syntax_error("'super' keyword unexpected here");
         return create_ast_node<SuperExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() });
     case TokenType::Identifier: {
+    read_as_identifier:;
         if (!try_parse_arrow_function_expression_failed_at_position(position())) {
             auto arrow_function_result = try_parse_arrow_function_expression(false);
             if (!arrow_function_result.is_null())
@@ -674,6 +675,10 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
         }
         return parse_new_expression();
     }
+    case TokenType::Yield:
+        if (!m_parser_state.m_in_generator_function_context)
+            goto read_as_identifier;
+        return parse_yield_expression();
     default:
         expected("primary expression");
         consume();
@@ -1256,6 +1261,16 @@ NonnullRefPtr<NewExpression> Parser::parse_new_expression()
     return create_ast_node<NewExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, move(callee), move(arguments));
 }
 
+NonnullRefPtr<YieldExpression> Parser::parse_yield_expression()
+{
+    auto rule_start = push_start();
+    consume(TokenType::Yield);
+    RefPtr<Expression> argument;
+    if (match_expression())
+        argument = parse_expression(0);
+    return create_ast_node<YieldExpression>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, move(argument));
+}
+
 NonnullRefPtr<ReturnStatement> Parser::parse_return_statement()
 {
     auto rule_start = push_start();
@@ -1336,9 +1351,14 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
 
     ScopePusher scope(*this, ScopePusher::Var | ScopePusher::Function);
 
+    auto is_generator = false;
     String name;
     if (parse_options & FunctionNodeParseOptions::CheckForFunctionAndName) {
         consume(TokenType::Function);
+        is_generator = match(TokenType::Asterisk);
+        if (is_generator)
+            consume(TokenType::Asterisk);
+
         if (FunctionNodeType::must_have_name() || match(TokenType::Identifier))
             name = consume(TokenType::Identifier).value();
     }
@@ -1351,6 +1371,7 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
         function_length = parameters.size();
 
     TemporaryChange change(m_parser_state.m_in_function_context, true);
+    TemporaryChange generator_change(m_parser_state.m_in_generator_function_context, m_parser_state.m_in_generator_function_context || is_generator);
     auto old_labels_in_scope = move(m_parser_state.m_labels_in_scope);
     ScopeGuard guard([&]() {
         m_parser_state.m_labels_in_scope = move(old_labels_in_scope);
@@ -1360,7 +1381,7 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
     auto body = parse_block_statement(is_strict);
     body->add_variables(m_parser_state.m_var_scopes.last());
     body->add_functions(m_parser_state.m_function_scopes.last());
-    return create_ast_node<FunctionNodeType>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, name, move(body), move(parameters), function_length, NonnullRefPtrVector<VariableDeclaration>(), is_strict);
+    return create_ast_node<FunctionNodeType>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, name, move(body), move(parameters), function_length, NonnullRefPtrVector<VariableDeclaration>(), is_generator, is_strict);
 }
 
 Vector<FunctionNode::Parameter> Parser::parse_formal_parameters(int& function_length, u8 parse_options)
@@ -2009,6 +2030,7 @@ bool Parser::match_expression() const
         || type == TokenType::This
         || type == TokenType::Super
         || type == TokenType::RegexLiteral
+        || type == TokenType::Yield
         || match_unary_prefixed_expression();
 }
 
@@ -2085,6 +2107,7 @@ bool Parser::match_statement() const
     auto type = m_parser_state.m_current_token.type();
     return match_expression()
         || type == TokenType::Return
+        || type == TokenType::Yield
         || type == TokenType::Do
         || type == TokenType::If
         || type == TokenType::Throw
diff --git a/Userland/Libraries/LibJS/Parser.h b/Userland/Libraries/LibJS/Parser.h
index 85000c7a1e..83a6a01e5e 100644
--- a/Userland/Libraries/LibJS/Parser.h
+++ b/Userland/Libraries/LibJS/Parser.h
@@ -77,6 +77,7 @@ public:
     NonnullRefPtr<NewExpression> parse_new_expression();
     NonnullRefPtr<ClassDeclaration> parse_class_declaration();
     NonnullRefPtr<ClassExpression> parse_class_expression(bool expect_class_name);
+    NonnullRefPtr<YieldExpression> parse_yield_expression();
     NonnullRefPtr<Expression> parse_property_key();
     NonnullRefPtr<AssignmentExpression> parse_assignment_expression(AssignmentOp, NonnullRefPtr<Expression> lhs, int min_precedence, Associativity);
 
@@ -200,6 +201,7 @@ private:
         bool m_allow_super_property_lookup { false };
         bool m_allow_super_constructor_call { false };
         bool m_in_function_context { false };
+        bool m_in_generator_function_context { false };
         bool m_in_arrow_function_context { false };
         bool m_in_break_context { false };
         bool m_in_continue_context { false };
diff --git a/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h b/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
index 52dc0ef420..d0254ec4e9 100644
--- a/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
+++ b/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
@@ -293,6 +293,8 @@ struct CommonPropertyNames {
     FlyString catch_ { "catch" };
     FlyString delete_ { "delete" };
     FlyString for_ { "for" };
+    FlyString return_ { "return" };
+    FlyString throw_ { "throw" };
 #define __ENUMERATE(x) FlyString x { #x };
     ENUMERATE_STANDARD_PROPERTY_NAMES(__ENUMERATE)
 #undef __ENUMERATE
diff --git a/Userland/Libraries/LibJS/Runtime/GeneratorObject.cpp b/Userland/Libraries/LibJS/Runtime/GeneratorObject.cpp
new file mode 100644
index 0000000000..9422ed168c
--- /dev/null
+++ b/Userland/Libraries/LibJS/Runtime/GeneratorObject.cpp
@@ -0,0 +1,163 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/TemporaryChange.h>
+#include <LibJS/Bytecode/Generator.h>
+#include <LibJS/Bytecode/Interpreter.h>
+#include <LibJS/Runtime/GeneratorObject.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS {
+
+GeneratorObject* GeneratorObject::create(GlobalObject& global_object, Value initial_value, ScriptFunction* generating_function, ScopeObject* generating_scope, Bytecode::RegisterWindow frame)
+{
+    auto object = global_object.heap().allocate<GeneratorObject>(global_object, global_object);
+    object->m_generating_function = generating_function;
+    object->m_scope = generating_scope;
+    object->m_frame = move(frame);
+    object->m_previous_value = initial_value;
+    return object;
+}
+
+GeneratorObject::GeneratorObject(GlobalObject& global_object)
+    : Object(*global_object.object_prototype())
+{
+}
+
+void GeneratorObject::initialize(GlobalObject& global_object)
+{
+    auto& vm = this->vm();
+    Object::initialize(global_object);
+    define_native_function(vm.names.next, next);
+    define_native_function(vm.names.return_, return_);
+    define_native_function(vm.names.throw_, throw_);
+}
+
+GeneratorObject::~GeneratorObject()
+{
+}
+
+void GeneratorObject::visit_edges(Cell::Visitor& visitor)
+{
+    Object::visit_edges(visitor);
+    visitor.visit(m_scope);
+    visitor.visit(m_generating_function);
+    if (m_previous_value.is_object())
+        visitor.visit(&m_previous_value.as_object());
+}
+
+GeneratorObject* GeneratorObject::typed_this(VM& vm, GlobalObject& global_object)
+{
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return {};
+    if (!is<GeneratorObject>(this_object)) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::NotA, "Generator");
+        return nullptr;
+    }
+    return static_cast<GeneratorObject*>(this_object);
+}
+
+Value GeneratorObject::next_impl(VM& vm, GlobalObject& global_object, Optional<Value> value_to_throw)
+{
+    auto bytecode_interpreter = Bytecode::Interpreter::current();
+    VERIFY(bytecode_interpreter);
+
+    auto generated_value = [](Value value) {
+        if (value.is_object())
+            return value.as_object().get("result");
+        return value.is_empty() ? js_undefined() : value;
+    };
+
+    auto generated_continuation = [&](Value value) -> Bytecode::BasicBlock const* {
+        if (value.is_object())
+            return reinterpret_cast<Bytecode::BasicBlock const*>(static_cast<u64>(value.as_object().get("continuation").to_double(global_object)));
+        return nullptr;
+    };
+
+    Value previous_generated_value { generated_value(m_previous_value) };
+
+    if (vm.exception())
+        return {};
+
+    auto result = Object::create_empty(global_object);
+    result->put("value", previous_generated_value);
+
+    if (m_done) {
+        result->put("done", Value(true));
+        return result;
+    }
+
+    // Extract the continuation
+    auto next_block = generated_continuation(m_previous_value);
+    if (vm.exception())
+        return {};
+
+    if (!next_block) {
+        // The generator has terminated, now we can simply return done=true.
+        m_done = true;
+        result->put("done", Value(true));
+        return result;
+    }
+
+    // Make sure it's an actual block
+    VERIFY(!m_generating_function->bytecode_executable()->basic_blocks.find_if([next_block](auto& block) { return block == next_block; }).is_end());
+
+    // Restore the snapshot registers
+    bytecode_interpreter->enter_frame(m_frame);
+
+    // Pretend that 'yield' returned the passed value, or threw
+    if (value_to_throw.has_value()) {
+        vm.throw_exception(global_object, value_to_throw.release_value());
+        bytecode_interpreter->accumulator() = js_undefined();
+    } else {
+        bytecode_interpreter->accumulator() = vm.argument(0);
+    }
+
+    // Temporarily switch to the captured scope
+    TemporaryChange change { vm.call_frame().scope, m_scope };
+
+    m_previous_value = bytecode_interpreter->run(*m_generating_function->bytecode_executable(), next_block);
+
+    bytecode_interpreter->leave_frame();
+
+    m_done = generated_continuation(m_previous_value) == nullptr;
+
+    result->put("value", generated_value(m_previous_value));
+    result->put("done", Value(m_done));
+
+    if (vm.exception())
+        return {};
+
+    return result;
+}
+
+JS_DEFINE_NATIVE_FUNCTION(GeneratorObject::next)
+{
+    auto object = typed_this(vm, global_object);
+    if (!object)
+        return {};
+    return object->next_impl(vm, global_object, {});
+}
+
+JS_DEFINE_NATIVE_FUNCTION(GeneratorObject::return_)
+{
+    auto object = typed_this(vm, global_object);
+    if (!object)
+        return {};
+    object->m_done = true;
+    return object->next_impl(vm, global_object, {});
+}
+
+JS_DEFINE_NATIVE_FUNCTION(GeneratorObject::throw_)
+{
+    auto object = typed_this(vm, global_object);
+    if (!object)
+        return {};
+    return object->next_impl(vm, global_object, vm.argument(0));
+}
+
+}
diff --git a/Userland/Libraries/LibJS/Runtime/GeneratorObject.h b/Userland/Libraries/LibJS/Runtime/GeneratorObject.h
new file mode 100644
index 0000000000..e25d8e8198
--- /dev/null
+++ b/Userland/Libraries/LibJS/Runtime/GeneratorObject.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/ScriptFunction.h>
+
+namespace JS {
+
+class GeneratorObject final : public Object {
+    JS_OBJECT(GeneratorObject, Object);
+
+public:
+    static GeneratorObject* create(GlobalObject&, Value, ScriptFunction*, ScopeObject*, Bytecode::RegisterWindow);
+    explicit GeneratorObject(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~GeneratorObject() override;
+    void visit_edges(Cell::Visitor&) override;
+
+    static GeneratorObject* typed_this(VM&, GlobalObject&);
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(next);
+    JS_DECLARE_NATIVE_FUNCTION(return_);
+    JS_DECLARE_NATIVE_FUNCTION(throw_);
+
+    Value next_impl(VM&, GlobalObject&, Optional<Value> value_to_throw);
+
+    ScopeObject* m_scope { nullptr };
+    ScriptFunction* m_generating_function { nullptr };
+    Value m_previous_value;
+    Bytecode::RegisterWindow m_frame;
+    bool m_done { false };
+};
+
+}
diff --git a/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp b/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
index f57dad44eb..d91f65bc3a 100644
--- a/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ScriptFunction.cpp
@@ -13,7 +13,9 @@
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/GeneratorObject.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/ScriptFunction.h>
 #include <LibJS/Runtime/Value.h>
 
@@ -31,18 +33,19 @@ static ScriptFunction* typed_this(VM& vm, GlobalObject& global_object)
     return static_cast<ScriptFunction*>(this_object);
 }
 
-ScriptFunction* ScriptFunction::create(GlobalObject& global_object, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, bool is_strict, bool is_arrow_function)
+ScriptFunction* ScriptFunction::create(GlobalObject& global_object, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, bool is_generator, bool is_strict, bool is_arrow_function)
 {
-    return global_object.heap().allocate<ScriptFunction>(global_object, global_object, name, body, move(parameters), m_function_length, parent_scope, *global_object.function_prototype(), is_strict, is_arrow_function);
+    return global_object.heap().allocate<ScriptFunction>(global_object, global_object, name, body, move(parameters), m_function_length, parent_scope, *global_object.function_prototype(), is_generator, is_strict, is_arrow_function);
 }
 
-ScriptFunction::ScriptFunction(GlobalObject& global_object, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, Object& prototype, bool is_strict, bool is_arrow_function)
+ScriptFunction::ScriptFunction(GlobalObject& global_object, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, Object& prototype, bool is_generator, bool is_strict, bool is_arrow_function)
     : Function(prototype, is_arrow_function ? vm().this_value(global_object) : Value(), {})
     , m_name(name)
     , m_body(body)
     , m_parameters(move(parameters))
     , m_parent_scope(parent_scope)
     , m_function_length(m_function_length)
+    , m_is_generator(is_generator)
     , m_is_strict(is_strict)
     , m_is_arrow_function(is_arrow_function)
 {
@@ -152,15 +155,20 @@ Value ScriptFunction::execute_function_body()
     if (bytecode_interpreter) {
         prepare_arguments();
         if (!m_bytecode_executable.has_value()) {
-            m_bytecode_executable = Bytecode::Generator::generate(m_body);
+            m_bytecode_executable = Bytecode::Generator::generate(m_body, m_is_generator);
             if constexpr (JS_BYTECODE_DEBUG) {
                 dbgln("Compiled Bytecode::Block for function '{}':", m_name);
                 for (auto& block : m_bytecode_executable->basic_blocks)
                     block.dump(*m_bytecode_executable);
             }
         }
-        return bytecode_interpreter->run(*m_bytecode_executable);
+        auto result = bytecode_interpreter->run(*m_bytecode_executable);
+        if (!m_is_generator)
+            return result;
+
+        return GeneratorObject::create(global_object(), result, this, vm.call_frame().scope, bytecode_interpreter->snapshot_frame());
     } else {
+        VERIFY(!m_is_generator);
         OwnPtr<Interpreter> local_interpreter;
         ast_interpreter = vm.interpreter_if_exists();
 
diff --git a/Userland/Libraries/LibJS/Runtime/ScriptFunction.h b/Userland/Libraries/LibJS/Runtime/ScriptFunction.h
index edc748130b..fa93085e5d 100644
--- a/Userland/Libraries/LibJS/Runtime/ScriptFunction.h
+++ b/Userland/Libraries/LibJS/Runtime/ScriptFunction.h
@@ -16,9 +16,9 @@ class ScriptFunction final : public Function {
     JS_OBJECT(ScriptFunction, Function);
 
 public:
-    static ScriptFunction* create(GlobalObject&, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, bool is_strict, bool is_arrow_function = false);
+    static ScriptFunction* create(GlobalObject&, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, bool is_generator, bool is_strict, bool is_arrow_function = false);
 
-    ScriptFunction(GlobalObject&, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, Object& prototype, bool is_strict, bool is_arrow_function = false);
+    ScriptFunction(GlobalObject&, const FlyString& name, const Statement& body, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, ScopeObject* parent_scope, Object& prototype, bool is_generator, bool is_strict, bool is_arrow_function = false);
     virtual void initialize(GlobalObject&) override;
     virtual ~ScriptFunction();
 
@@ -33,6 +33,8 @@ public:
 
     void set_is_class_constructor() { m_is_class_constructor = true; };
 
+    auto& bytecode_executable() const { return m_bytecode_executable; }
+
 protected:
     virtual bool is_strict_mode() const final { return m_is_strict; }
 
@@ -51,6 +53,7 @@ private:
     Optional<Bytecode::Executable> m_bytecode_executable;
     ScopeObject* m_parent_scope { nullptr };
     i32 m_function_length { 0 };
+    bool m_is_generator { false };
     bool m_is_strict { false };
     bool m_is_arrow_function { false };
     bool m_is_class_constructor { false };
diff --git a/Userland/Libraries/LibWeb/HTML/GlobalEventHandlers.cpp b/Userland/Libraries/LibWeb/HTML/GlobalEventHandlers.cpp
index f6f49a72d8..320090d4e2 100644
--- a/Userland/Libraries/LibWeb/HTML/GlobalEventHandlers.cpp
+++ b/Userland/Libraries/LibWeb/HTML/GlobalEventHandlers.cpp
@@ -49,7 +49,7 @@ void GlobalEventHandlers::set_event_handler_attribute(const FlyString& name, HTM
             dbgln("Failed to parse script in event handler attribute '{}'", name);
             return;
         }
-        auto* function = JS::ScriptFunction::create(self.script_execution_context()->interpreter().global_object(), name, program->body(), program->parameters(), program->function_length(), nullptr, false, false);
+        auto* function = JS::ScriptFunction::create(self.script_execution_context()->interpreter().global_object(), name, program->body(), program->parameters(), program->function_length(), nullptr, false, false, false);
         VERIFY(function);
         listener = adopt_ref(*new DOM::EventListener(JS::make_handle(static_cast<JS::Function*>(function))));
     }
diff --git a/Userland/Libraries/LibWeb/HTML/WebSocket.cpp b/Userland/Libraries/LibWeb/HTML/WebSocket.cpp
index 6bc9fdf50f..3c5b524367 100644
--- a/Userland/Libraries/LibWeb/HTML/WebSocket.cpp
+++ b/Userland/Libraries/LibWeb/HTML/WebSocket.cpp
@@ -246,7 +246,7 @@ void WebSocket::set_event_handler_attribute(const FlyString& name, HTML::EventHa
             dbgln("Failed to parse script in event handler attribute '{}'", name);
             return;
         }
-        auto* function = JS::ScriptFunction::create(script_execution_context()->interpreter().global_object(), name, program->body(), program->parameters(), program->function_length(), nullptr, false, false);
+        auto* function = JS::ScriptFunction::create(script_execution_context()->interpreter().global_object(), name, program->body(), program->parameters(), program->function_length(), nullptr, false, false, false);
         VERIFY(function);
         listener = adopt_ref(*new DOM::EventListener(JS::make_handle(static_cast<JS::Function*>(function))));
     }