LibJS: Start implementing iterable framework, add ArrayIterator

With the addition of symbol keys, work can now be done on starting to
implement the well-known symbol functionality. The most important of
these well-known symbols is by far Symbol.iterator.

This patch adds IteratorPrototype, as well as ArrayIterator and
ArrayIteratorPrototype. In the future, sometime after StringIterator has
also been added, this will allow us to use Symbol.iterator directly in
for..of loops, enabling the use of custom iterator objects. Also makes
adding iterator support to native objects much easier (as will have to
be done for Map and Set, when they get added).

18 files changed, 657 insertions, 1 deletions

diff --git a/Libraries/LibJS/CMakeLists.txt b/Libraries/LibJS/CMakeLists.txt
index af35aa7a84..4b7f4d2af0 100644
--- a/Libraries/LibJS/CMakeLists.txt
+++ b/Libraries/LibJS/CMakeLists.txt
@@ -10,6 +10,8 @@ set(SOURCES
     Parser.cpp
     Runtime/Array.cpp
     Runtime/ArrayConstructor.cpp
+    Runtime/ArrayIterator.cpp
+    Runtime/ArrayIteratorPrototype.cpp
     Runtime/ArrayPrototype.cpp
     Runtime/BigInt.cpp
     Runtime/BigIntConstructor.cpp
@@ -34,6 +36,8 @@ set(SOURCES
     Runtime/FunctionPrototype.cpp
     Runtime/GlobalObject.cpp
     Runtime/IndexedProperties.cpp
+    Runtime/IteratorOperations.cpp
+    Runtime/IteratorPrototype.cpp
     Runtime/JSONObject.cpp
     Runtime/LexicalEnvironment.cpp
     Runtime/MarkedValueList.cpp
diff --git a/Libraries/LibJS/Forward.h b/Libraries/LibJS/Forward.h
index 833206e001..1883c67417 100644
--- a/Libraries/LibJS/Forward.h
+++ b/Libraries/LibJS/Forward.h
@@ -67,6 +67,10 @@
     __JS_ENUMERATE(TypeError, type_error, TypeErrorPrototype, TypeErrorConstructor)                     \
     __JS_ENUMERATE(URIError, uri_error, URIErrorPrototype, URIErrorConstructor)
 
+#define JS_ENUMERATE_ITERATOR_PROTOTYPES            \
+    __JS_ENUMERATE(Iterator, iterator)              \
+    __JS_ENUMERATE(ArrayIterator, array_iterator)
+
 #define JS_ENUMERATE_BUILTIN_TYPES \
     JS_ENUMERATE_NATIVE_OBJECTS    \
     JS_ENUMERATE_ERROR_SUBCLASSES
diff --git a/Libraries/LibJS/Runtime/ArrayIterator.cpp b/Libraries/LibJS/Runtime/ArrayIterator.cpp
new file mode 100644
index 0000000000..6c01af00ab
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ArrayIterator.cpp
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/ArrayIterator.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS {
+
+ArrayIterator* ArrayIterator::create(GlobalObject& global_object, Value array, Object::PropertyKind iteration_kind)
+{
+    return global_object.heap().allocate<ArrayIterator>(global_object, *global_object.array_iterator_prototype(), array, iteration_kind);
+}
+
+ArrayIterator::ArrayIterator(Object& prototype, Value array, Object::PropertyKind iteration_kind)
+    : Object(prototype)
+    , m_array(array)
+    , m_iteration_kind(iteration_kind)
+{
+}
+
+ArrayIterator::~ArrayIterator()
+{
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/ArrayIterator.h b/Libraries/LibJS/Runtime/ArrayIterator.h
new file mode 100644
index 0000000000..408349f188
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ArrayIterator.h
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class ArrayIterator final : public Object {
+    JS_OBJECT(ArrayIterator, Object);
+
+public:
+    static ArrayIterator* create(GlobalObject&, Value array, Object::PropertyKind iteration_kind);
+
+    explicit ArrayIterator(Object& prototype, Value array, Object::PropertyKind iteration_kind);
+    virtual ~ArrayIterator() override;
+
+    Value array() const { return m_array; }
+    Object::PropertyKind iteration_kind() const { return m_iteration_kind; }
+    size_t index() const { return m_index; }
+
+private:
+    friend class ArrayIteratorPrototype;
+
+    virtual bool is_array_iterator_object() const override { return true; }
+
+    Value m_array;
+    Object::PropertyKind m_iteration_kind;
+    size_t m_index { 0 };
+};
+
+}
diff --git a/Libraries/LibJS/Runtime/ArrayIteratorPrototype.cpp b/Libraries/LibJS/Runtime/ArrayIteratorPrototype.cpp
new file mode 100644
index 0000000000..e5e381d34d
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ArrayIteratorPrototype.cpp
@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Runtime/Array.h>
+#include <LibJS/Runtime/ArrayIterator.h>
+#include <LibJS/Runtime/ArrayIteratorPrototype.h>
+#include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorOperations.h>
+
+namespace JS {
+
+ArrayIteratorPrototype::ArrayIteratorPrototype(GlobalObject& global_object)
+    : Object(*global_object.iterator_prototype())
+{
+}
+
+void ArrayIteratorPrototype::initialize(Interpreter& interpreter, GlobalObject& global_object)
+{
+    Object::initialize(interpreter, global_object);
+    define_native_function("next", next, 0, Attribute::Writable | Attribute::Configurable);
+}
+
+ArrayIteratorPrototype::~ArrayIteratorPrototype()
+{
+}
+
+JS_DEFINE_NATIVE_FUNCTION(ArrayIteratorPrototype::next)
+{
+    auto this_value = interpreter.this_value(global_object);
+    if (!this_value.is_object() || !this_value.as_object().is_array_iterator_object())
+        return interpreter.throw_exception<TypeError>(ErrorType::NotAn, "Array Iterator");
+
+    auto& this_object = this_value.as_object();
+
+    auto& iterator = static_cast<ArrayIterator&>(this_object);
+    auto target_array = iterator.array();
+    if (target_array.is_undefined())
+        return create_iterator_result_object(interpreter, global_object, js_undefined(), true);
+    ASSERT(target_array.is_object());
+    auto& array = target_array.as_object();
+
+    auto index = iterator.index();
+    auto iteration_kind = iterator.iteration_kind();
+    // FIXME: Typed array check
+    auto length = array.indexed_properties().array_like_size();
+
+    if (index >= length) {
+        iterator.m_array = js_undefined();
+        return create_iterator_result_object(interpreter, global_object, js_undefined(), true);
+    }
+
+    iterator.m_index++;
+    if (iteration_kind == Object::PropertyKind::Key)
+        return create_iterator_result_object(interpreter, global_object, Value(static_cast<i32>(index)), false);
+
+    auto value = array.get(index);
+    if (interpreter.exception())
+        return {};
+    if (iteration_kind == Object::PropertyKind::Value)
+        return create_iterator_result_object(interpreter, global_object, value, false);
+
+    auto* entry_array = Array::create(global_object);
+    entry_array->define_property(0, Value(static_cast<i32>(index)));
+    entry_array->define_property(1, value);
+    return create_iterator_result_object(interpreter, global_object, entry_array, false);
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/ArrayIteratorPrototype.h b/Libraries/LibJS/Runtime/ArrayIteratorPrototype.h
new file mode 100644
index 0000000000..a04c9aae08
--- /dev/null
+++ b/Libraries/LibJS/Runtime/ArrayIteratorPrototype.h
@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class ArrayIteratorPrototype final : public Object {
+    JS_OBJECT(ArrayIteratorPrototype, Object)
+
+public:
+    ArrayIteratorPrototype(GlobalObject&);
+    virtual void initialize(Interpreter&, GlobalObject&) override;
+    virtual ~ArrayIteratorPrototype() override;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(next);
+};
+
+}
diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index 7c376b56fa..2a3200690d 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -28,9 +28,9 @@
 
 #include <AK/Function.h>
 #include <AK/StringBuilder.h>
-#include <LibJS/Heap/Heap.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/Array.h>
+#include <LibJS/Runtime/ArrayIterator.h>
 #include <LibJS/Runtime/ArrayPrototype.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/Function.h>
@@ -75,7 +75,13 @@ void ArrayPrototype::initialize(Interpreter& interpreter, GlobalObject& global_o
     define_native_function("every", every, 1, attr);
     define_native_function("splice", splice, 2, attr);
     define_native_function("fill", fill, 1, attr);
+    define_native_function("values", values, 0, attr);
     define_property("length", Value(0), Attribute::Configurable);
+
+    // Use define_property here instead of define_native_function so that
+    // Object.is(Array.prototype[Symbol.iterator], Array.prototype.values)
+    // evaluates to true
+    define_property(interpreter.get_well_known_symbol("iterator"), get("values"), attr);
 }
 
 ArrayPrototype::~ArrayPrototype()
@@ -850,4 +856,13 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::fill)
     return this_object;
 }
 
+JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::values)
+{
+    auto* this_object = interpreter.this_value(global_object).to_object(interpreter, global_object);
+    if (!this_object)
+        return {};
+
+    return ArrayIterator::create(global_object, this_object, Object::PropertyKind::Value);
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/ArrayPrototype.h b/Libraries/LibJS/Runtime/ArrayPrototype.h
index 989603dafa..a286725136 100644
--- a/Libraries/LibJS/Runtime/ArrayPrototype.h
+++ b/Libraries/LibJS/Runtime/ArrayPrototype.h
@@ -64,6 +64,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(every);
     JS_DECLARE_NATIVE_FUNCTION(splice);
     JS_DECLARE_NATIVE_FUNCTION(fill);
+    JS_DECLARE_NATIVE_FUNCTION(values);
 };
 
 }
diff --git a/Libraries/LibJS/Runtime/GlobalObject.cpp b/Libraries/LibJS/Runtime/GlobalObject.cpp
index 9dd2b4e9d7..6096eb1eb7 100644
--- a/Libraries/LibJS/Runtime/GlobalObject.cpp
+++ b/Libraries/LibJS/Runtime/GlobalObject.cpp
@@ -28,6 +28,7 @@
 #include <AK/LogStream.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/ArrayConstructor.h>
+#include <LibJS/Runtime/ArrayIteratorPrototype.h>
 #include <LibJS/Runtime/ArrayPrototype.h>
 #include <LibJS/Runtime/BigIntConstructor.h>
 #include <LibJS/Runtime/BigIntPrototype.h>
@@ -41,6 +42,7 @@
 #include <LibJS/Runtime/FunctionConstructor.h>
 #include <LibJS/Runtime/FunctionPrototype.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorPrototype.h>
 #include <LibJS/Runtime/JSONObject.h>
 #include <LibJS/Runtime/MathObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
@@ -84,6 +86,13 @@ void GlobalObject::initialize()
     JS_ENUMERATE_BUILTIN_TYPES
 #undef __JS_ENUMERATE
 
+#define __JS_ENUMERATE(ClassName, snake_name)                                    \
+    if (!m_##snake_name##_prototype)                                             \
+        m_##snake_name##_prototype = heap().allocate<ClassName##Prototype>(*this, *this);
+    JS_ENUMERATE_ITERATOR_PROTOTYPES
+#undef __JS_ENUMERATE
+
+
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function("gc", gc, 0, attr);
     define_native_function("isNaN", is_nan, 1, attr);
diff --git a/Libraries/LibJS/Runtime/GlobalObject.h b/Libraries/LibJS/Runtime/GlobalObject.h
index 862522a0f1..36472a7f2f 100644
--- a/Libraries/LibJS/Runtime/GlobalObject.h
+++ b/Libraries/LibJS/Runtime/GlobalObject.h
@@ -49,6 +49,11 @@ public:
     JS_ENUMERATE_BUILTIN_TYPES
 #undef __JS_ENUMERATE
 
+#define __JS_ENUMERATE(ClassName, snake_name) \
+    Object* snake_name##_prototype() { return m_##snake_name##_prototype; }
+    JS_ENUMERATE_ITERATOR_PROTOTYPES
+#undef __JS_ENUMERATE
+
 protected:
     virtual void visit_children(Visitor&) override;
 
@@ -68,6 +73,12 @@ private:
     Object* m_##snake_name##_prototype { nullptr };
     JS_ENUMERATE_BUILTIN_TYPES
 #undef __JS_ENUMERATE
+
+#define __JS_ENUMERATE(ClassName, snake_name) \
+    Object* m_##snake_name##_prototype { nullptr };
+    JS_ENUMERATE_ITERATOR_PROTOTYPES
+#undef __JS_ENUMERATE
+
 };
 
 template<typename ConstructorType>
diff --git a/Libraries/LibJS/Runtime/IteratorOperations.cpp b/Libraries/LibJS/Runtime/IteratorOperations.cpp
new file mode 100644
index 0000000000..3bee12548b
--- /dev/null
+++ b/Libraries/LibJS/Runtime/IteratorOperations.cpp
@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/IteratorOperations.h>
+
+namespace JS {
+
+Object* get_iterator(Object& obj, String hint, Value method)
+{
+    auto& interpreter = obj.interpreter();
+    ASSERT(hint == "sync" || hint == "async");
+    if (method.is_empty()) {
+        if (hint == "async")
+            TODO();
+        method = obj.get(obj.interpreter().get_well_known_symbol("iterator"));
+        if (interpreter.exception())
+            return {};
+    }
+    if (!method.is_function())
+        TODO();
+    auto iterator = interpreter.call(method.as_function(), &obj);
+    if (interpreter.exception())
+        return {};
+    if (!iterator.is_object())
+        TODO();
+    return &iterator.as_object();
+}
+
+Value iterator_next(Object& iterator, Value value)
+{
+    auto& interpreter = iterator.interpreter();
+    auto next_method = iterator.get("next");
+    if (interpreter.exception())
+        return {};
+
+    ASSERT(next_method.is_function());
+
+    Value result;
+    if (value.is_empty()) {
+        result = interpreter.call(next_method.as_function(), &iterator);
+    } else {
+        MarkedValueList arguments(iterator.heap());
+        arguments.append(value);
+        result = interpreter.call(next_method.as_function(), &iterator, move(arguments));
+    }
+
+    if (interpreter.exception())
+        return {};
+    if (!result.is_object())
+        TODO();
+
+    return result;
+}
+
+bool is_iterator_complete(Object& iterator_result)
+{
+    auto done = iterator_result.get("done");
+    if (iterator_result.interpreter().exception())
+        return false;
+    return done.to_boolean();
+}
+
+Value iterator_value(Object& iterator_result)
+{
+    return iterator_result.get("value");
+}
+
+Value iterator_step(Object& iterator)
+{
+    auto& interpreter = iterator.interpreter();
+    auto result = iterator_next(iterator);
+    if (interpreter.exception())
+        return {};
+    auto done = is_iterator_complete(result.as_object());
+    if (interpreter.exception())
+        return {};
+    if (done)
+        return Value(false);
+    return result;
+}
+
+void iterator_close(Object& iterator)
+{
+    (void)iterator;
+    TODO();
+}
+
+Value create_iterator_result_object(Interpreter& interpreter, GlobalObject& global_object, Value value, bool done)
+{
+    auto* object = Object::create_empty(interpreter, global_object);
+    object->put("value", value);
+    object->put("done", Value(done));
+    return object;
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/IteratorOperations.h b/Libraries/LibJS/Runtime/IteratorOperations.h
new file mode 100644
index 0000000000..9b9caae4e9
--- /dev/null
+++ b/Libraries/LibJS/Runtime/IteratorOperations.h
@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+// Common iterator operations defined in ECMA262 7.4
+// https://tc39.es/ecma262/#sec-operations-on-iterator-objects
+
+Object* get_iterator(Object& obj, String hint = "sync", Value method = {});
+bool is_iterator_complete(Object& iterator_result);
+Value create_iterator_result_object(Interpreter&, GlobalObject&, Value value, bool done);
+
+Value iterator_next(Object& iterator, Value value = {});
+Value iterator_value(Object& iterator_result);
+Value iterator_step(Object& iterator);
+void iterator_close(Object& iterator);
+
+}
diff --git a/Libraries/LibJS/Runtime/IteratorPrototype.cpp b/Libraries/LibJS/Runtime/IteratorPrototype.cpp
new file mode 100644
index 0000000000..e1fe68e125
--- /dev/null
+++ b/Libraries/LibJS/Runtime/IteratorPrototype.cpp
@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorPrototype.h>
+
+namespace JS {
+
+IteratorPrototype::IteratorPrototype(GlobalObject& global_object)
+    : Object(*global_object.object_prototype())
+{
+}
+
+void IteratorPrototype::initialize(Interpreter& interpreter, GlobalObject& global_object)
+{
+    Object::initialize(interpreter, global_object);
+    define_native_function(interpreter.get_well_known_symbol("iterator"), symbol_iterator, 0, Attribute::Writable | Attribute::Enumerable);
+}
+
+IteratorPrototype::~IteratorPrototype()
+{
+}
+
+JS_DEFINE_NATIVE_FUNCTION(IteratorPrototype::symbol_iterator)
+{
+    auto* this_object = interpreter.this_value(global_object).to_object(interpreter, global_object);
+    if (!this_object)
+        return {};
+    return this_object;
+}
+
+}
diff --git a/Libraries/LibJS/Runtime/IteratorPrototype.h b/Libraries/LibJS/Runtime/IteratorPrototype.h
new file mode 100644
index 0000000000..1cfd954632
--- /dev/null
+++ b/Libraries/LibJS/Runtime/IteratorPrototype.h
@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class IteratorPrototype : public Object {
+    JS_OBJECT(IteratorPrototype, Object)
+
+public:
+    IteratorPrototype(GlobalObject&);
+    virtual void initialize(Interpreter&, GlobalObject&) override;
+    virtual ~IteratorPrototype() override;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(symbol_iterator);
+};
+
+}
diff --git a/Libraries/LibJS/Runtime/Object.h b/Libraries/LibJS/Runtime/Object.h
index 507795ce1e..ff325b845b 100644
--- a/Libraries/LibJS/Runtime/Object.h
+++ b/Libraries/LibJS/Runtime/Object.h
@@ -123,6 +123,7 @@ public:
     virtual bool is_number_object() const { return false; }
     virtual bool is_symbol_object() const { return false; }
     virtual bool is_bigint_object() const { return false; }
+    virtual bool is_array_iterator_object() const { return false; }
 
     virtual const char* class_name() const override { return "Object"; }
     virtual void visit_children(Cell::Visitor&) override;
diff --git a/Libraries/LibJS/Tests/builtins/Array/Array.prototype.values.js b/Libraries/LibJS/Tests/builtins/Array/Array.prototype.values.js
new file mode 100644
index 0000000000..c382fe837d
--- /dev/null
+++ b/Libraries/LibJS/Tests/builtins/Array/Array.prototype.values.js
@@ -0,0 +1,44 @@
+test("length", () => {
+    expect(Array.prototype.values.length).toBe(0);
+});
+
+test("basic functionality", () => {
+    const a = [1, 2, 3];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("works when applied to non-object", () => {
+    [true, false, 9, 2n, Symbol()].forEach(primitive => {
+        const it = [].values.call(primitive);
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+    });
+});
+
+test("item added to array before exhaustion is accessible", () => {
+    const a = [1, 2];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    a.push(3);
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("item added to array after exhaustion is inaccesible", () => {
+    const a = [1, 2];
+    const it = a.values();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    a.push(3);
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
diff --git a/Libraries/LibJS/Tests/iterators/%IteratorPrototype%.js b/Libraries/LibJS/Tests/iterators/%IteratorPrototype%.js
new file mode 100644
index 0000000000..efdb32e51a
--- /dev/null
+++ b/Libraries/LibJS/Tests/iterators/%IteratorPrototype%.js
@@ -0,0 +1,12 @@
+const getIteratorPrototype = () =>
+    Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()));
+
+test("prototype of %IteratorPrototype% is %ObjectPrototype%", () => {
+    let itProto = getIteratorPrototype();
+    expect(Object.getPrototypeOf(itProto)).toBe(Object.getPrototypeOf({}));
+});
+
+test("@@iterator of %IteratorPrototype% is itself", () => {
+    let itProto = getIteratorPrototype();
+    expect(itProto[Symbol.iterator]()).toBe(itProto);
+});
diff --git a/Libraries/LibJS/Tests/iterators/array-iterator.js b/Libraries/LibJS/Tests/iterators/array-iterator.js
new file mode 100644
index 0000000000..117030ebdf
--- /dev/null
+++ b/Libraries/LibJS/Tests/iterators/array-iterator.js
@@ -0,0 +1,48 @@
+test("length", () => {
+    expect(Array.prototype[Symbol.iterator].length).toBe(0);
+});
+
+test("same function as Array.prototype.values", () => {
+    expect(Array.prototype[Symbol.iterator]).toBe(Array.prototype.values);
+});
+
+test("basic functionality", () => {
+    const a = [1, 2, 3];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("works when applied to non-object", () => {
+    [true, false, 9, 2n, Symbol()].forEach(primitive => {
+        const it = [][Symbol.iterator].call(primitive);
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+        expect(it.next()).toEqual({ value: undefined, done: true });
+    });
+});
+
+test("item added to array before exhaustion is accessible", () => {
+    const a = [1, 2];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    a.push(3);
+    expect(it.next()).toEqual({ value: 3, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});
+
+test("item added to array after exhaustion is inaccesible", () => {
+    const a = [1, 2];
+    const it = a[Symbol.iterator]();
+    expect(it.next()).toEqual({ value: 1, done: false });
+    expect(it.next()).toEqual({ value: 2, done: false });
+    expect(it.next()).toEqual({ value: undefined, done: true });
+    a.push(3);
+    expect(it.next()).toEqual({ value: undefined, done: true });
+});