LibJS: Allow advancing built-in iterators without result object creation

Expose a method on built-in iterators that allows retrieving the next
iteration result without allocating a JS::Object. This change is a
preparation for optimizing for..of and for..in loops.

Libraries/LibJS/Forward.h

@@ -158,6 +158,7 @@ class Agent;
 struct AsyncGeneratorRequest;
 class BigInt;
 class BoundFunction;
+class BuiltinIterator;
 struct CachedSourceRange;
 class Cell;
 class ClassExpression;

Libraries/LibJS/Runtime/ArrayIterator.cpp

@@ -4,8 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/ArrayIterator.h>
-#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/TypedArray.h>
 
 namespace JS {
 
@@ -35,4 +36,108 @@ void ArrayIterator::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_array);
 }
 
+ThrowCompletionOr<void> ArrayIterator::next(VM& vm, bool& done, Value& value)
+{
+    // 1. Let O be the this value.
+    // 2. If O is not an Object, throw a TypeError exception.
+    // 3. If O does not have all of the internal slots of an Array Iterator Instance (23.1.5.3), throw a TypeError exception.
+
+    // 4. Let array be O.[[IteratedArrayLike]].
+    auto target_array = m_array;
+
+    // 5. If array is undefined, return CreateIteratorResultObject(undefined, true).
+    if (target_array.is_undefined()) {
+        value = js_undefined();
+        done = true;
+        return {};
+    }
+
+    VERIFY(target_array.is_object());
+    auto& array = target_array.as_object();
+
+    // 6. Let index be O.[[ArrayLikeNextIndex]].
+    auto index = m_index;
+
+    // 7. Let kind be O.[[ArrayLikeIterationKind]].
+    auto kind = m_iteration_kind;
+
+    size_t length = 0;
+
+    // 8. If array has a [[TypedArrayName]] internal slot, then
+    if (array.is_typed_array()) {
+        auto& typed_array = static_cast<TypedArrayBase&>(array);
+
+        // a. Let taRecord be MakeTypedArrayWithBufferWitnessRecord(array, SEQ-CST).
+        auto typed_array_record = make_typed_array_with_buffer_witness_record(typed_array, ArrayBuffer::SeqCst);
+
+        // b. If IsTypedArrayOutOfBounds(taRecord) is true, throw a TypeError exception.
+        if (is_typed_array_out_of_bounds(typed_array_record))
+            return vm.throw_completion<TypeError>(ErrorType::BufferOutOfBounds, "TypedArray"sv);
+
+        // c. Let len be TypedArrayLength(taRecord).
+        length = typed_array_length(typed_array_record);
+    }
+    // 9. Else,
+    else {
+        // a. Let len be ? LengthOfArrayLike(array).
+        length = TRY(length_of_array_like(vm, array));
+    }
+
+    // 10. If index ≥ len, then
+    if (index >= length) {
+        // a. Set O.[[IteratedArrayLike]] to undefined.
+        m_array = js_undefined();
+
+        // b. Return CreateIteratorResultObject(undefined, true).
+        value = js_undefined();
+        done = true;
+        return {};
+    }
+
+    // 11. Set O.[[ArrayLikeNextIndex]] to index + 1.
+    m_index++;
+
+    // 12. Let indexNumber be 𝔽(index).
+
+    Value result;
+
+    // 13. If kind is KEY, then
+    if (kind == PropertyKind::Key) {
+        // a. Let result be indexNumber.
+        result = Value { static_cast<i32>(index) };
+    }
+    // 14. Else,
+    else {
+        // a. Let elementKey be ! ToString(indexNumber).
+        // b. Let elementValue be ? Get(array, elementKey).
+        auto element_value = TRY([&]() -> ThrowCompletionOr<Value> {
+            // OPTIMIZATION: For objects that don't interfere with indexed property access, we try looking directly at storage.
+            if (!array.may_interfere_with_indexed_property_access() && array.indexed_properties().has_index(index)) {
+                if (auto value = array.indexed_properties().get(index)->value; !value.is_accessor())
+                    return value;
+            }
+
+            return array.get(index);
+        }());
+
+        // c. If kind is VALUE, then
+        if (kind == PropertyKind::Value) {
+            // i. Let result be elementValue.
+            result = element_value;
+        }
+        // d. Else,
+        else {
+            // i. Assert: kind is KEY+VALUE.
+            VERIFY(kind == PropertyKind::KeyAndValue);
+
+            // ii. Let result be CreateArrayFromList(« indexNumber, elementValue »).
+            result = Array::create_from(*vm.current_realm(), { Value(static_cast<i32>(index)), element_value });
+        }
+    }
+
+    // 15. Return CreateIteratorResultObject(result, false).
+    value = result;
+    return {};
+}
+
 }

Libraries/LibJS/Runtime/ArrayIterator.h

@@ -6,11 +6,13 @@
 
 #pragma once
 
+#include <LibJS/Runtime/Iterator.h>
 #include <LibJS/Runtime/Object.h>
 
 namespace JS {
 
-class ArrayIterator final : public Object {
+class ArrayIterator final : public Object
+    , public BuiltinIterator {
     JS_OBJECT(ArrayIterator, Object);
     GC_DECLARE_ALLOCATOR(ArrayIterator);
 
@@ -19,13 +21,8 @@ public:
 
     virtual ~ArrayIterator() override = default;
 
-    Value array() const { return m_array; }
+    BuiltinIterator* as_builtin_iterator() override { return this; }
-    void set_array(Value array) { m_array = array; }
+    ThrowCompletionOr<void> next(VM&, bool& done, Value& value) override;
-
-    Object::PropertyKind iteration_kind() const { return m_iteration_kind; }
-
-    size_t index() const { return m_index; }
-    void set_index(size_t index) { m_index = index; }
 
 private:
     ArrayIterator(Value array, Object::PropertyKind iteration_kind, Object& prototype);

Libraries/LibJS/Runtime/ArrayIteratorPrototype.cpp

@@ -35,103 +35,13 @@ void ArrayIteratorPrototype::initialize(Realm& realm)
 // 23.1.5.2.1 %ArrayIteratorPrototype%.next ( ), https://tc39.es/ecma262/#sec-%arrayiteratorprototype%.next
 JS_DEFINE_NATIVE_FUNCTION(ArrayIteratorPrototype::next)
 {
-    auto& realm = *vm.current_realm();
-
-    // 1. Let O be the this value.
-    // 2. If O is not an Object, throw a TypeError exception.
-    // 3. If O does not have all of the internal slots of an Array Iterator Instance (23.1.5.3), throw a TypeError exception.
     auto iterator = TRY(typed_this_value(vm));
 
-    // 4. Let array be O.[[IteratedArrayLike]].
+    Value value;
-    auto target_array = iterator->array();
+    bool done = false;
+    TRY(iterator->next(vm, done, value));
 
-    // 5. If array is undefined, return CreateIteratorResultObject(undefined, true).
+    return create_iterator_result_object(vm, value, done);
-    if (target_array.is_undefined())
-        return create_iterator_result_object(vm, js_undefined(), true);
-
-    VERIFY(target_array.is_object());
-    auto& array = target_array.as_object();
-
-    // 6. Let index be O.[[ArrayLikeNextIndex]].
-    auto index = iterator->index();
-
-    // 7. Let kind be O.[[ArrayLikeIterationKind]].
-    auto kind = iterator->iteration_kind();
-
-    size_t length = 0;
-
-    // 8. If array has a [[TypedArrayName]] internal slot, then
-    if (array.is_typed_array()) {
-        auto& typed_array = static_cast<TypedArrayBase&>(array);
-
-        // a. Let taRecord be MakeTypedArrayWithBufferWitnessRecord(array, SEQ-CST).
-        auto typed_array_record = make_typed_array_with_buffer_witness_record(typed_array, ArrayBuffer::SeqCst);
-
-        // b. If IsTypedArrayOutOfBounds(taRecord) is true, throw a TypeError exception.
-        if (is_typed_array_out_of_bounds(typed_array_record))
-            return vm.throw_completion<TypeError>(ErrorType::BufferOutOfBounds, "TypedArray"sv);
-
-        // c. Let len be TypedArrayLength(taRecord).
-        length = typed_array_length(typed_array_record);
-    }
-    // 9. Else,
-    else {
-        // a. Let len be ? LengthOfArrayLike(array).
-        length = TRY(length_of_array_like(vm, array));
-    }
-
-    // 10. If index ≥ len, then
-    if (index >= length) {
-        // a. Set O.[[IteratedArrayLike]] to undefined.
-        iterator->set_array(js_undefined());
-
-        // b. Return CreateIteratorResultObject(undefined, true).
-        return create_iterator_result_object(vm, js_undefined(), true);
-    }
-
-    // 11. Set O.[[ArrayLikeNextIndex]] to index + 1.
-    iterator->set_index(index + 1);
-
-    // 12. Let indexNumber be 𝔽(index).
-
-    Value result;
-
-    // 13. If kind is KEY, then
-    if (kind == PropertyKind::Key) {
-        // a. Let result be indexNumber.
-        result = Value { static_cast<i32>(index) };
-    }
-    // 14. Else,
-    else {
-        // a. Let elementKey be ! ToString(indexNumber).
-        // b. Let elementValue be ? Get(array, elementKey).
-        auto element_value = TRY([&]() -> ThrowCompletionOr<Value> {
-            // OPTIMIZATION: For objects that don't interfere with indexed property access, we try looking directly at storage.
-            if (!array.may_interfere_with_indexed_property_access() && array.indexed_properties().has_index(index)) {
-                if (auto value = array.indexed_properties().get(index)->value; !value.is_accessor())
-                    return value;
-            }
-
-            return array.get(index);
-        }());
-
-        // c. If kind is VALUE, then
-        if (kind == PropertyKind::Value) {
-            // i. Let result be elementValue.
-            result = element_value;
-        }
-        // d. Else,
-        else {
-            // i. Assert: kind is KEY+VALUE.
-            VERIFY(kind == PropertyKind::KeyAndValue);
-
-            // ii. Let result be CreateArrayFromList(« indexNumber, elementValue »).
-            result = Array::create_from(realm, { Value(static_cast<i32>(index)), element_value });
-        }
-    }
-
-    // 15. Return CreateIteratorResultObject(result, false).
-    return create_iterator_result_object(vm, result, false);
 }
 
 }

Libraries/LibJS/Runtime/Iterator.h

@@ -67,6 +67,12 @@ enum class PrimitiveHandling {
     RejectPrimitives,
 };
 
+class BuiltinIterator {
+public:
+    virtual ~BuiltinIterator() = default;
+    virtual ThrowCompletionOr<void> next(VM&, bool& done, Value& value) = 0;
+};
+
 // 7.4.12 IfAbruptCloseIterator ( value, iteratorRecord ), https://tc39.es/ecma262/#sec-ifabruptcloseiterator
 #define TRY_OR_CLOSE_ITERATOR(vm, iterator_record, expression)                                                    \
     ({                                                                                                            \

Libraries/LibJS/Runtime/MapIterator.cpp

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/MapIterator.h>
 
 namespace JS {
@@ -30,4 +30,34 @@ void MapIterator::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_map);
 }
 
+ThrowCompletionOr<void> MapIterator::next(VM& vm, bool& done, Value& value)
+{
+    if (m_done) {
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    if (m_iterator.is_end()) {
+        m_done = true;
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    auto entry = *m_iterator;
+    ++m_iterator;
+    if (m_iteration_kind == Object::PropertyKind::Key) {
+        value = entry.key;
+        return {};
+    }
+    if (m_iteration_kind == Object::PropertyKind::Value) {
+        value = entry.value;
+        return {};
+    }
+
+    value = Array::create_from(*vm.current_realm(), { entry.key, entry.value });
+    return {};
+}
+
 }

Libraries/LibJS/Runtime/MapIterator.h

@@ -6,12 +6,14 @@
 
 #pragma once
 
+#include <LibJS/Runtime/Iterator.h>
 #include <LibJS/Runtime/Map.h>
 #include <LibJS/Runtime/Object.h>
 
 namespace JS {
 
-class MapIterator final : public Object {
+class MapIterator final : public Object
+    , public BuiltinIterator {
     JS_OBJECT(MapIterator, Object);
     GC_DECLARE_ALLOCATOR(MapIterator);
 
@@ -20,9 +22,8 @@ public:
 
     virtual ~MapIterator() override = default;
 
-    Map& map() const { return m_map; }
+    BuiltinIterator* as_builtin_iterator() override { return this; }
-    bool done() const { return m_done; }
+    ThrowCompletionOr<void> next(VM&, bool& done, Value& value) override;
-    Object::PropertyKind iteration_kind() const { return m_iteration_kind; }
 
 private:
     friend class MapIteratorPrototype;

Libraries/LibJS/Runtime/MapIteratorPrototype.cpp

@@ -6,8 +6,6 @@
 
 #include <AK/TypeCasts.h>
 #include <LibJS/Runtime/Array.h>
-#include <LibJS/Runtime/Error.h>
-#include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Iterator.h>
 #include <LibJS/Runtime/MapIteratorPrototype.h>
 
@@ -32,27 +30,13 @@ void MapIteratorPrototype::initialize(Realm& realm)
 // 24.1.5.2.1 %MapIteratorPrototype%.next ( ), https://tc39.es/ecma262/#sec-%mapiteratorprototype%.next
 JS_DEFINE_NATIVE_FUNCTION(MapIteratorPrototype::next)
 {
-    auto& realm = *vm.current_realm();
+    auto iterator = TRY(typed_this_value(vm));
 
-    auto map_iterator = TRY(typed_this_value(vm));
+    Value value;
-    if (map_iterator->done())
+    bool done = false;
-        return create_iterator_result_object(vm, js_undefined(), true);
+    TRY(iterator->next(vm, done, value));
 
-    if (map_iterator->m_iterator.is_end()) {
+    return create_iterator_result_object(vm, value, done);
-        map_iterator->m_done = true;
-        return create_iterator_result_object(vm, js_undefined(), true);
-    }
-
-    auto iteration_kind = map_iterator->iteration_kind();
-
-    auto entry = *map_iterator->m_iterator;
-    ++map_iterator->m_iterator;
-    if (iteration_kind == Object::PropertyKind::Key)
-        return create_iterator_result_object(vm, entry.key, false);
-    if (iteration_kind == Object::PropertyKind::Value)
-        return create_iterator_result_object(vm, entry.value, false);
-
-    return create_iterator_result_object(vm, Array::create_from(realm, { entry.key, entry.value }), false);
 }
 
 }

Libraries/LibJS/Runtime/Object.h

@@ -212,6 +212,8 @@ public:
     virtual bool is_array_iterator() const { return false; }
     virtual bool is_raw_json_object() const { return false; }
 
+    virtual BuiltinIterator* as_builtin_iterator() { return nullptr; }
+
     // B.3.7 The [[IsHTMLDDA]] Internal Slot, https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot
     virtual bool is_htmldda() const { return false; }
 

Libraries/LibJS/Runtime/SetIterator.cpp

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/SetIterator.h>
 
 namespace JS {
@@ -30,4 +30,31 @@ void SetIterator::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_set);
 }
 
+ThrowCompletionOr<void> SetIterator::next(VM& vm, bool& done, Value& value)
+{
+    if (m_done) {
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    if (m_iterator == m_set->end()) {
+        m_done = true;
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    VERIFY(m_iteration_kind != Object::PropertyKind::Key);
+
+    value = (*m_iterator).key;
+    ++m_iterator;
+    if (m_iteration_kind == Object::PropertyKind::Value) {
+        return {};
+    }
+
+    value = Array::create_from(*vm.current_realm(), { value, value });
+    return {};
+}
+
 }

Libraries/LibJS/Runtime/SetIterator.h

@@ -6,13 +6,14 @@
 
 #pragma once
 
-#include <AK/HashTable.h>
+#include <LibJS/Runtime/Iterator.h>
 #include <LibJS/Runtime/Object.h>
 #include <LibJS/Runtime/Set.h>
 
 namespace JS {
 
-class SetIterator final : public Object {
+class SetIterator final : public Object
+    , public BuiltinIterator {
     JS_OBJECT(SetIterator, Object);
     GC_DECLARE_ALLOCATOR(SetIterator);
 
@@ -21,9 +22,8 @@ public:
 
     virtual ~SetIterator() override = default;
 
-    Set& set() const { return m_set; }
+    BuiltinIterator* as_builtin_iterator() override { return this; }
-    bool done() const { return m_done; }
+    ThrowCompletionOr<void> next(VM&, bool& done, Value& value) override;
-    Object::PropertyKind iteration_kind() const { return m_iteration_kind; }
 
 private:
     friend class SetIteratorPrototype;

Libraries/LibJS/Runtime/SetIteratorPrototype.cpp

@@ -34,27 +34,13 @@ void SetIteratorPrototype::initialize(Realm& realm)
 // 24.2.5.2.1 %SetIteratorPrototype%.next ( ), https://tc39.es/ecma262/#sec-%setiteratorprototype%.next
 JS_DEFINE_NATIVE_FUNCTION(SetIteratorPrototype::next)
 {
-    auto& realm = *vm.current_realm();
+    auto iterator = TRY(typed_this_value(vm));
 
-    auto set_iterator = TRY(typed_this_value(vm));
+    Value value;
-    if (set_iterator->done())
+    bool done = false;
-        return create_iterator_result_object(vm, js_undefined(), true);
+    TRY(iterator->next(vm, done, value));
 
-    auto& set = set_iterator->set();
+    return create_iterator_result_object(vm, value, done);
-    if (set_iterator->m_iterator == set.end()) {
-        set_iterator->m_done = true;
-        return create_iterator_result_object(vm, js_undefined(), true);
-    }
-
-    auto iteration_kind = set_iterator->iteration_kind();
-    VERIFY(iteration_kind != Object::PropertyKind::Key);
-
-    auto value = (*set_iterator->m_iterator).key;
-    ++set_iterator->m_iterator;
-    if (iteration_kind == Object::PropertyKind::Value)
-        return create_iterator_result_object(vm, value, false);
-
-    return create_iterator_result_object(vm, Array::create_from(realm, { value, value }), false);
 }
 
 }

Libraries/LibJS/Runtime/StringIterator.cpp

@@ -24,4 +24,26 @@ StringIterator::StringIterator(String string, Object& prototype)
 {
 }
 
+ThrowCompletionOr<void> StringIterator::next(VM& vm, bool& done, Value& value)
+{
+    if (m_done) {
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    if (m_iterator.done()) {
+        m_done = true;
+        done = true;
+        value = js_undefined();
+        return {};
+    }
+
+    auto code_point = String::from_code_point(*m_iterator);
+    ++m_iterator;
+
+    value = PrimitiveString::create(vm, move(code_point));
+    return {};
+}
+
 }

Libraries/LibJS/Runtime/StringIterator.h

@@ -8,11 +8,13 @@
 
 #include <AK/String.h>
 #include <AK/Utf8View.h>
+#include <LibJS/Runtime/Iterator.h>
 #include <LibJS/Runtime/Object.h>
 
 namespace JS {
 
-class StringIterator final : public Object {
+class StringIterator final : public Object
+    , public BuiltinIterator {
     JS_OBJECT(StringIterator, Object);
     GC_DECLARE_ALLOCATOR(StringIterator);
 
@@ -21,8 +23,8 @@ public:
 
     virtual ~StringIterator() override = default;
 
-    Utf8CodePointIterator& iterator() { return m_iterator; }
+    BuiltinIterator* as_builtin_iterator() override { return this; }
-    bool done() const { return m_done; }
+    ThrowCompletionOr<void> next(VM&, bool& done, Value& value) override;
 
 private:
     explicit StringIterator(String string, Object& prototype);

Libraries/LibJS/Runtime/StringIteratorPrototype.cpp

@@ -33,20 +33,12 @@ void StringIteratorPrototype::initialize(Realm& realm)
 JS_DEFINE_NATIVE_FUNCTION(StringIteratorPrototype::next)
 {
     auto iterator = TRY(typed_this_value(vm));
-    if (iterator->done())
-        return create_iterator_result_object(vm, js_undefined(), true);
 
-    auto& utf8_iterator = iterator->iterator();
+    Value value;
+    bool done = false;
+    TRY(iterator->next(vm, done, value));
 
-    if (utf8_iterator.done()) {
+    return create_iterator_result_object(vm, value, done);
-        iterator->m_done = true;
-        return create_iterator_result_object(vm, js_undefined(), true);
-    }
-
-    auto code_point = String::from_code_point(*utf8_iterator);
-    ++utf8_iterator;
-
-    return create_iterator_result_object(vm, PrimitiveString::create(vm, move(code_point)), false);
 }
 
 }