LibJS: Implement Iterator.prototype.filter

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/deeee65e37
Pull-request: https://github.com/SerenityOS/serenity/pull/19642

Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp

@@ -31,6 +31,7 @@ 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);
+    define_native_function(realm, vm.names.filter, filter, 1, attr);
 
     return {};
 }
@@ -103,4 +104,70 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::map)
     return result;
 }
 
+// 3.1.3.3 Iterator.prototype.filter ( predicate ), https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.filter
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::filter)
+{
+    auto& realm = *vm.current_realm();
+
+    auto predicate = 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(predicate) is false, throw a TypeError exception.
+    if (!predicate.is_function())
+        return vm.throw_completion<TypeError>(ErrorType::NotAFunction, "predicate"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 predicate and performs the following steps when called:
+    IteratorHelper::Closure closure = [predicate = NonnullGCPtr { predicate.as_function() }](auto& iterator) -> ThrowCompletionOr<Value> {
+        auto& vm = iterator.vm();
+
+        auto const& iterated = iterator.underlying_iterator();
+
+        // a. Let counter be 0.
+
+        // b. Repeat,
+        while (true) {
+            // 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 selected be Completion(Call(predicate, undefined, « value, 𝔽(counter) »)).
+            auto selected = call(vm, *predicate, js_undefined(), value, Value { iterator.counter() });
+
+            // v. IfAbruptCloseIterator(selected, iterated).
+            if (selected.is_error())
+                return iterator.close_result(selected.release_error());
+
+            // vii. Set counter to counter + 1.
+            // NOTE: We do this step early to ensure it occurs before returning.
+            iterator.increment_counter();
+
+            // vi. If ToBoolean(selected) is true, then
+            if (selected.value().to_boolean()) {
+                // 1. Let completion be Completion(Yield(value)).
+                // 2. IfAbruptCloseIterator(completion, iterated).
+                return iterator.result(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

@@ -23,6 +23,7 @@ private:
 
     JS_DECLARE_NATIVE_FUNCTION(symbol_iterator);
     JS_DECLARE_NATIVE_FUNCTION(map);
+    JS_DECLARE_NATIVE_FUNCTION(filter);
 };
 
 }

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

@@ -0,0 +1,156 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.filter(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "predicate 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().filter(() => true);
+            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().filter(() => true);
+            iterator.next();
+        }).toThrow(TestError);
+    });
+
+    test("predicate function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().filter(() => {
+                throw new TestError();
+            });
+            iterator.next();
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.filter).toHaveLength(1);
+    });
+
+    test("predicate function sees every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        const iterator = generator().filter((value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected predicate invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return true;
+        });
+
+        for (const i of iterator) {
+        }
+
+        expect(count).toBe(2);
+    });
+
+    test("predicate function can select values", () => {
+        function* generator() {
+            yield 1;
+            yield 0;
+            yield 2;
+            yield 0;
+        }
+
+        const iterator = generator().filter(value => value > 0);
+
+        let value = iterator.next();
+        expect(value.value).toBe(1);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBe(2);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+
+    test("predicates can be chained", () => {
+        function* generator() {
+            yield 1;
+            yield 0;
+            yield 2;
+            yield 0;
+        }
+
+        let firstFilterCount = 0;
+        let secondFilterCount = 0;
+
+        const iterator = generator()
+            .filter(value => {
+                ++firstFilterCount;
+                return value > 0;
+            })
+            .filter(value => {
+                ++secondFilterCount;
+                return value > 1;
+            });
+
+        let value = iterator.next();
+        expect(value.value).toBe(2);
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+
+        expect(firstFilterCount).toBe(4);
+        expect(secondFilterCount).toBe(2);
+    });
+});