From 134bb44ca05d2bc065b19cce9c2d38325042b0b8 Mon Sep 17 00:00:00 2001
From: Timothy Flynn <trflynn89@pm.me>
Date: Sun, 25 Jun 2023 13:58:45 -0400
Subject: [PATCH] LibJS: Implement Iterator.prototype.forEach

---
 .../LibJS/Runtime/IteratorPrototype.cpp       | 44 +++++++++
 .../LibJS/Runtime/IteratorPrototype.h         |  1 +
 .../Iterator/Iterator.prototype.forEach.js    | 94 +++++++++++++++++++
 3 files changed, 139 insertions(+)
 create mode 100644 Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.forEach.js

diff --git a/Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp b/Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp
index 9fca042f276..0a60414032e 100644
--- a/Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/IteratorPrototype.cpp
@@ -38,6 +38,7 @@ ThrowCompletionOr<void> IteratorPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.flatMap, flat_map, 1, attr);
     define_native_function(realm, vm.names.reduce, reduce, 1, attr);
     define_native_function(realm, vm.names.toArray, to_array, 0, attr);
+    define_native_function(realm, vm.names.forEach, for_each, 1, attr);
 
     return {};
 }
@@ -550,4 +551,47 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::to_array)
     }
 }
 
+// 3.1.3.9 Iterator.prototype.forEach ( fn ), https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.foreach
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::for_each)
+{
+    auto function = 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(fn) is false, throw a TypeError exception.
+    if (!function.is_function())
+        return vm.throw_completion<TypeError>(ErrorType::NotAFunction, "fn"sv);
+
+    // 4. Let iterated be ? GetIteratorDirect(O).
+    auto iterated = TRY(get_iterator_direct(vm, object));
+
+    // 5. Let counter be 0.
+    size_t counter = 0;
+
+    // 6. Repeat,
+    while (true) {
+        // a. Let next be ? IteratorStep(iterated).
+        auto next = TRY(iterator_step(vm, iterated));
+
+        // b. If next is false, return undefined.
+        if (!next)
+            return js_undefined();
+
+        // c. Let value be ? IteratorValue(next).
+        auto value = TRY(iterator_value(vm, *next));
+
+        // d. Let result be Completion(Call(fn, undefined, « value, 𝔽(counter) »)).
+        auto result = call(vm, function.as_function(), js_undefined(), value, Value { counter });
+
+        // e. IfAbruptCloseIterator(result, iterated).
+        if (result.is_error())
+            return *TRY(iterator_close(vm, iterated, result.release_error()));
+
+        // f. Set counter to counter + 1.
+        ++counter;
+    }
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/IteratorPrototype.h b/Userland/Libraries/LibJS/Runtime/IteratorPrototype.h
index d696bf47670..b94e1657024 100644
--- a/Userland/Libraries/LibJS/Runtime/IteratorPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/IteratorPrototype.h
@@ -29,6 +29,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(flat_map);
     JS_DECLARE_NATIVE_FUNCTION(reduce);
     JS_DECLARE_NATIVE_FUNCTION(to_array);
+    JS_DECLARE_NATIVE_FUNCTION(for_each);
 };
 
 }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.forEach.js b/Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.forEach.js
new file mode 100644
index 00000000000..c286ad060a5
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Iterator/Iterator.prototype.forEach.js
@@ -0,0 +1,94 @@
+describe("errors", () => {
+    test("called with non-callable object", () => {
+        expect(() => {
+            Iterator.prototype.forEach(Symbol.hasInstance);
+        }).toThrowWithMessage(TypeError, "fn is not a function");
+    });
+
+    test("iterator's next method throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                throw new TestError();
+            }
+        }
+
+        expect(() => {
+            new TestIterator().forEach(() => 0);
+        }).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(() => {
+            new TestIterator().forEach(() => 0);
+        }).toThrow(TestError);
+    });
+
+    test("for-each function throws", () => {
+        function TestError() {}
+
+        class TestIterator extends Iterator {
+            next() {
+                return {
+                    done: false,
+                    value: 1,
+                };
+            }
+        }
+
+        expect(() => {
+            new TestIterator().forEach(() => {
+                throw new TestError();
+            });
+        }).toThrow(TestError);
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Iterator.prototype.forEach).toHaveLength(1);
+    });
+
+    test("for-each function sees every value", () => {
+        function* generator() {
+            yield "a";
+            yield "b";
+        }
+
+        let count = 0;
+
+        generator().forEach((value, index) => {
+            ++count;
+
+            switch (index) {
+                case 0:
+                    expect(value).toBe("a");
+                    break;
+                case 1:
+                    expect(value).toBe("b");
+                    break;
+                default:
+                    expect().fail(`Unexpected reducer invocation: value=${value} index=${index}`);
+                    break;
+            }
+
+            return value;
+        }, "");
+
+        expect(count).toBe(2);
+    });
+});