LibJS: Implement Iterator.prototype.map

This uses a new Iterator type called IteratorHelper. This does not
implement IteratorHelper.prototype.return as that relies on generator
objects (i.e. the internal slots of JS::GeneratorObject), which are not
hooked up here.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -143,6 +143,8 @@ set(SOURCES
     Runtime/Intrinsics.cpp
     Runtime/Iterator.cpp
     Runtime/IteratorConstructor.cpp
+    Runtime/IteratorHelper.cpp
+    Runtime/IteratorHelperPrototype.cpp
     Runtime/IteratorOperations.cpp
     Runtime/IteratorPrototype.cpp
     Runtime/JSONObject.cpp

Userland/Libraries/LibJS/Forward.h

@@ -110,6 +110,7 @@
     __JS_ENUMERATE(ArrayIterator, array_iterator)                \
     __JS_ENUMERATE(AsyncIterator, async_iterator)                \
     __JS_ENUMERATE(Intl::SegmentIterator, intl_segment_iterator) \
+    __JS_ENUMERATE(IteratorHelper, iterator_helper)              \
     __JS_ENUMERATE(MapIterator, map_iterator)                    \
     __JS_ENUMERATE(RegExpStringIterator, regexp_string_iterator) \
     __JS_ENUMERATE(SetIterator, set_iterator)                    \

Userland/Libraries/LibJS/Runtime/Intrinsics.cpp

@@ -64,6 +64,7 @@
 #include <LibJS/Runtime/Intl/SegmentsPrototype.h>
 #include <LibJS/Runtime/Intrinsics.h>
 #include <LibJS/Runtime/IteratorConstructor.h>
+#include <LibJS/Runtime/IteratorHelperPrototype.h>
 #include <LibJS/Runtime/IteratorPrototype.h>
 #include <LibJS/Runtime/JSONObject.h>
 #include <LibJS/Runtime/MapConstructor.h>

Userland/Libraries/LibJS/Runtime/IteratorHelper.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorHelper.h>
+#include <LibJS/Runtime/IteratorOperations.h>
+#include <LibJS/Runtime/Realm.h>
+
+namespace JS {
+
+ThrowCompletionOr<NonnullGCPtr<IteratorHelper>> IteratorHelper::create(Realm& realm, IteratorRecord underlying_iterator, Closure closure)
+{
+    return TRY(realm.heap().allocate<IteratorHelper>(realm, realm.intrinsics().iterator_helper_prototype(), move(underlying_iterator), move(closure)));
+}
+
+IteratorHelper::IteratorHelper(Object& prototype, IteratorRecord underlying_iterator, Closure closure)
+    : Object(ConstructWithPrototypeTag::Tag, prototype)
+    , m_underlying_iterator(move(underlying_iterator))
+    , m_closure(move(closure))
+{
+}
+
+void IteratorHelper::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_underlying_iterator.iterator);
+}
+
+Value IteratorHelper::result(Value value)
+{
+    if (value.is_undefined())
+        m_done = true;
+    return value;
+}
+
+ThrowCompletionOr<Value> IteratorHelper::close_result(Completion completion)
+{
+    m_done = true;
+    return *TRY(iterator_close(vm(), underlying_iterator(), move(completion)));
+}
+
+}

Userland/Libraries/LibJS/Runtime/IteratorHelper.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Completion.h>
+#include <LibJS/Runtime/Iterator.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/SafeFunction.h>
+
+namespace JS {
+
+class IteratorHelper final : public Object {
+    JS_OBJECT(IteratorHelper, Object);
+
+public:
+    using Closure = JS::SafeFunction<ThrowCompletionOr<Value>(IteratorHelper&)>;
+
+    static ThrowCompletionOr<NonnullGCPtr<IteratorHelper>> create(Realm&, IteratorRecord, Closure);
+
+    IteratorRecord const& underlying_iterator() const { return m_underlying_iterator; }
+    Closure& closure() { return m_closure; }
+
+    size_t counter() const { return m_counter; }
+    void increment_counter() { ++m_counter; }
+
+    Value result(Value);
+    ThrowCompletionOr<Value> close_result(Completion);
+
+    bool done() const { return m_done; }
+
+private:
+    IteratorHelper(Object& prototype, IteratorRecord, Closure);
+
+    virtual void visit_edges(Visitor&) override;
+
+    IteratorRecord m_underlying_iterator; // [[UnderlyingIterator]]
+    Closure m_closure;
+
+    size_t m_counter { 0 };
+    bool m_done { false };
+};
+
+}

Userland/Libraries/LibJS/Runtime/IteratorHelperPrototype.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/IteratorHelperPrototype.h>
+#include <LibJS/Runtime/IteratorOperations.h>
+#include <LibJS/Runtime/Realm.h>
+
+namespace JS {
+
+IteratorHelperPrototype::IteratorHelperPrototype(Realm& realm)
+    : PrototypeObject(realm.intrinsics().iterator_prototype())
+{
+}
+
+ThrowCompletionOr<void> IteratorHelperPrototype::initialize(Realm& realm)
+{
+    auto& vm = this->vm();
+    MUST_OR_THROW_OOM(Base::initialize(realm));
+
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(realm, vm.names.next, next, 0, attr);
+    define_native_function(realm, vm.names.return_, return_, 0, attr);
+
+    // 3.1.2.1.3 %IteratorHelperPrototype% [ @@toStringTag ], https://tc39.es/proposal-iterator-helpers/#sec-%iteratorhelperprototype%-@@tostringtag
+    define_direct_property(vm.well_known_symbol_to_string_tag(), MUST_OR_THROW_OOM(PrimitiveString::create(vm, "Iterator Helper"sv)), Attribute::Configurable);
+
+    return {};
+}
+
+// 3.1.2.1.1 %IteratorHelperPrototype%.next ( ), https://tc39.es/proposal-iterator-helpers/#sec-%iteratorhelperprototype%.next
+JS_DEFINE_NATIVE_FUNCTION(IteratorHelperPrototype::next)
+{
+    auto iterator = TRY(typed_this_object(vm));
+    if (iterator->done())
+        return create_iterator_result_object(vm, js_undefined(), true);
+
+    // 1. Return ? GeneratorResume(this value, undefined, "Iterator Helper").
+    auto result = TRY(iterator->closure()(*iterator));
+    return create_iterator_result_object(vm, result, iterator->done());
+}
+
+// 3.1.2.1.2 %IteratorHelperPrototype%.return ( ), https://tc39.es/proposal-iterator-helpers/#sec-%iteratorhelperprototype%.return
+JS_DEFINE_NATIVE_FUNCTION(IteratorHelperPrototype::return_)
+{
+    // 1. Let O be this value.
+    // 2. Perform ? RequireInternalSlot(O, [[UnderlyingIterator]]).
+    // 3. Assert: O has a [[GeneratorState]] slot.
+    // 4. If O.[[GeneratorState]] is suspendedStart, then
+    //     a. Set O.[[GeneratorState]] to completed.
+    //     b. NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with O can be discarded at this point.
+    //     c. Perform ? IteratorClose(O.[[UnderlyingIterator]], NormalCompletion(unused)).
+    //     d. Return CreateIterResultObject(undefined, true).
+    // 5. Let C be Completion { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.
+    // 6. Return ? GeneratorResumeAbrupt(O, C, "Iterator Helper").
+
+    return vm.throw_completion<InternalError>(ErrorType::NotImplemented, "IteratorHelper.prototype.return"sv);
+}
+
+}

Userland/Libraries/LibJS/Runtime/IteratorHelperPrototype.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/IteratorHelper.h>
+#include <LibJS/Runtime/PrototypeObject.h>
+
+namespace JS {
+
+class IteratorHelperPrototype final : public PrototypeObject<IteratorHelperPrototype, IteratorHelper> {
+    JS_PROTOTYPE_OBJECT(IteratorHelperPrototype, IteratorHelper, IteratorHelper);
+
+public:
+    virtual ThrowCompletionOr<void> initialize(Realm&) override;
+
+private:
+    explicit IteratorHelperPrototype(Realm&);
+
+    JS_DECLARE_NATIVE_FUNCTION(next);
+    JS_DECLARE_NATIVE_FUNCTION(return_);
+};
+
+}

Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp

@@ -5,14 +5,18 @@
  */
 
 #include <AK/Function.h>
+#include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/FunctionObject.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorHelper.h>
+#include <LibJS/Runtime/IteratorOperations.h>
 #include <LibJS/Runtime/IteratorPrototype.h>
 
 namespace JS {
 
 // 27.1.2 The %IteratorPrototype% Object, https://tc39.es/ecma262/#sec-%iteratorprototype%-object
 IteratorPrototype::IteratorPrototype(Realm& realm)
-    : Object(ConstructWithPrototypeTag::Tag, realm.intrinsics().object_prototype())
+    : PrototypeObject(realm.intrinsics().object_prototype())
 {
 }
 
@@ -26,6 +30,7 @@ ThrowCompletionOr<void> IteratorPrototype::initialize(Realm& realm)
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(realm, vm.well_known_symbol_iterator(), symbol_iterator, 0, attr);
+    define_native_function(realm, vm.names.map, map, 1, attr);
 
     return {};
 }
@@ -37,4 +42,65 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::symbol_iterator)
     return vm.this_value();
 }
 
+// 3.1.3.2 Iterator.prototype.map ( mapper ), https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.map
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::map)
+{
+    auto& realm = *vm.current_realm();
+
+    auto mapper = vm.argument(0);
+
+    // 1. Let O be the this value.
+    // 2. If O is not an Object, throw a TypeError exception.
+    auto object = TRY(this_object(vm));
+
+    // 3. If IsCallable(mapper) is false, throw a TypeError exception.
+    if (!mapper.is_function())
+        return vm.throw_completion<TypeError>(ErrorType::NotAFunction, "mapper"sv);
+
+    // 4. Let iterated be ? GetIteratorDirect(O).
+    auto iterated = TRY(get_iterator_direct(vm, object));
+
+    // 5. Let closure be a new Abstract Closure with no parameters that captures iterated and mapper and performs the following steps when called:
+    IteratorHelper::Closure closure = [mapper = NonnullGCPtr { mapper.as_function() }](auto& iterator) -> ThrowCompletionOr<Value> {
+        auto& vm = iterator.vm();
+
+        auto const& iterated = iterator.underlying_iterator();
+
+        // a. Let counter be 0.
+        // b. Repeat,
+
+        // i. Let next be ? IteratorStep(iterated).
+        auto next = TRY(iterator_step(vm, iterated));
+
+        // ii. If next is false, return undefined.
+        if (!next)
+            return iterator.result(js_undefined());
+
+        // iii. Let value be ? IteratorValue(next).
+        auto value = TRY(iterator_value(vm, *next));
+
+        // iv. Let mapped be Completion(Call(mapper, undefined, « value, 𝔽(counter) »)).
+        auto mapped = call(vm, *mapper, js_undefined(), value, Value { iterator.counter() });
+
+        // v. IfAbruptCloseIterator(mapped, iterated).
+        if (mapped.is_error())
+            return iterator.close_result(mapped.release_error());
+
+        // viii. Set counter to counter + 1.
+        // NOTE: We do this step early to ensure it occurs before returning.
+        iterator.increment_counter();
+
+        // vi. Let completion be Completion(Yield(mapped)).
+        // vii. IfAbruptCloseIterator(completion, iterated).
+        return iterator.result(mapped.release_value());
+    };
+
+    // 6. Let result be CreateIteratorFromClosure(closure, "Iterator Helper", %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
+    // 7. Set result.[[UnderlyingIterator]] to iterated.
+    auto result = TRY(IteratorHelper::create(realm, move(iterated), move(closure)));
+
+    // 8. Return result.
+    return result;
+}
+
 }

Userland/Libraries/LibJS/Runtime/IteratorPrototype.h

@@ -6,12 +6,13 @@
 
 #pragma once
 
-#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/Iterator.h>
+#include <LibJS/Runtime/PrototypeObject.h>
 
 namespace JS {
 
-class IteratorPrototype : public Object {
-    JS_OBJECT(IteratorPrototype, Object)
+class IteratorPrototype : public PrototypeObject<IteratorPrototype, Iterator> {
+    JS_PROTOTYPE_OBJECT(IteratorPrototype, Iterator, Iterator);
 
 public:
     virtual ThrowCompletionOr<void> initialize(Realm&) override;
@@ -21,6 +22,7 @@ private:
     IteratorPrototype(Realm&);
 
     JS_DECLARE_NATIVE_FUNCTION(symbol_iterator);
+    JS_DECLARE_NATIVE_FUNCTION(map);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.map.js

@@ -0,0 +1,144 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.map(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "mapper is not a function");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().map(() => 0);
+            iterator.next();
+        }).toThrow(TestError);
+    });
+
+    test("value returned by iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    get value() {
+                        throw new TestError();
+                    },
+                };
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().map(() => 0);
+            iterator.next();
+        }).toThrow(TestError);
+    });
+
+    test("mapper function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().map(() => {
+                throw new TestError();
+            });
+            iterator.next();
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.map).toHaveLength(1);
+    });
+
+    test("mapper function sees every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        const iterator = generator().map((value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected mapper invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return value;
+        });
+
+        for (const i of iterator) {
+        }
+
+        expect(count).toBe(2);
+    });
+
+    test("mapper function can modify values", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        const iterator = generator().map(value => value.toUpperCase());
+
+        let value = iterator.next();
+        expect(value.value).toBe("A");
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBe("B");
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+
+    test("mappers can be chained", () => {
+        function* generator() {
+            yield 1;
+            yield 2;
+        }
+
+        const iterator = generator()
+            .map(value => value * 2)
+            .map(value => value + 10);
+
+        let value = iterator.next();
+        expect(value.value).toBe(12);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBe(14);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+});