LibJS: Implement Iterator.prototype.drop

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/67028ee3a3
Pull-request: https://github.com/SerenityOS/serenity/pull/19642

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -157,6 +157,7 @@ namespace JS {
     P(disposed)                              \
     P(done)                                  \
     P(dotAll)                                \
+    P(drop)                                  \
     P(encodeURI)                             \
     P(encodeURIComponent)                    \
     P(endsWith)                              \

Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp

@@ -33,6 +33,7 @@ ThrowCompletionOr<void> IteratorPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.map, map, 1, attr);
     define_native_function(realm, vm.names.filter, filter, 1, attr);
     define_native_function(realm, vm.names.take, take, 1, attr);
+    define_native_function(realm, vm.names.drop, drop, 1, attr);
 
     return {};
 }
@@ -238,4 +239,75 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::take)
     return result;
 }
 
+// 3.1.3.5 Iterator.prototype.drop ( limit ), https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.drop
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::drop)
+{
+    auto& realm = *vm.current_realm();
+
+    auto limit = 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. Let numLimit be ? ToNumber(limit).
+    auto numeric_limit = TRY(limit.to_number(vm));
+
+    // 4. If numLimit is NaN, throw a RangeError exception.
+    if (numeric_limit.is_nan())
+        return vm.throw_completion<RangeError>(ErrorType::NumberIsNaN, "limit"sv);
+
+    // 5. Let integerLimit be ! ToIntegerOrInfinity(numLimit).
+    auto integer_limit = MUST(numeric_limit.to_integer_or_infinity(vm));
+
+    // 6. If integerLimit < 0, throw a RangeError exception.
+    if (integer_limit < 0)
+        return vm.throw_completion<RangeError>(ErrorType::NumberIsNegative, "limit"sv);
+
+    // 7. Let iterated be ? GetIteratorDirect(O).
+    auto iterated = TRY(get_iterator_direct(vm, object));
+
+    // 8. Let closure be a new Abstract Closure with no parameters that captures iterated and integerLimit and performs the following steps when called:
+    IteratorHelper::Closure closure = [integer_limit](auto& iterator) -> ThrowCompletionOr<Value> {
+        auto& vm = iterator.vm();
+
+        auto const& iterated = iterator.underlying_iterator();
+
+        // a. Let remaining be integerLimit.
+        // b. Repeat, while remaining > 0,
+        while (iterator.counter() < integer_limit) {
+            // i. If remaining is not +∞, then
+            //     1. Set remaining to remaining - 1.
+            iterator.increment_counter();
+
+            // ii. Let next be ? IteratorStep(iterated).
+            auto next = TRY(iterator_step(vm, iterated));
+
+            // iii. If next is false, return undefined.
+            if (!next)
+                return iterator.result(js_undefined());
+        }
+
+        // c. 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 completion be Completion(Yield(? IteratorValue(next))).
+        // iv. IfAbruptCloseIterator(completion, iterated).
+        return iterator.result(TRY(iterator_value(vm, *next)));
+    };
+
+    // 9. Let result be CreateIteratorFromClosure(closure, "Iterator Helper", %IteratorHelperPrototype%, « [[UnderlyingIterator]] »).
+    // 10. Set result.[[UnderlyingIterator]] to iterated.
+    auto result = TRY(IteratorHelper::create(realm, move(iterated), move(closure)));
+
+    // 11. Return result.
+    return result;
+}
+
 }

Userland/Libraries/LibJS/Runtime/IteratorPrototype.h

@@ -25,6 +25,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(map);
     JS_DECLARE_NATIVE_FUNCTION(filter);
     JS_DECLARE_NATIVE_FUNCTION(take);
+    JS_DECLARE_NATIVE_FUNCTION(drop);
 };
 
 }

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

@@ -0,0 +1,118 @@
+describe("errors", () => {
+    test("called with non-numeric object", () => {
+        expect(() => {
+            Iterator.prototype.drop(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "Cannot convert symbol to number");
+    });
+
+    test("called with invalid numbers", () => {
+        expect(() => {
+            Iterator.prototype.drop(NaN);
+        }).toThrowWithMessage(RangeError, "limit must not be NaN");
+
+        expect(() => {
+            Iterator.prototype.drop(-1);
+        }).toThrowWithMessage(RangeError, "limit must not be negative");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            const iterator = new TestIterator().drop(1);
+            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().drop(1);
+            iterator.next();
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.drop).toHaveLength(1);
+    });
+
+    test("drop zero values", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        const iterator = generator().drop(0);
+
+        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("drop fewer than the number of values", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+            yield "c";
+        }
+
+        const iterator = generator().drop(1);
+
+        let value = iterator.next();
+        expect(value.value).toBe("b");
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBe("c");
+        expect(value.done).toBeFalse();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+
+    test("drop more than the number of values", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        const iterator = generator().drop(Infinity);
+
+        let value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+
+        value = iterator.next();
+        expect(value.value).toBeUndefined();
+        expect(value.done).toBeTrue();
+    });
+});