LibJS: Add Function() and Function.prototype

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/2944039d6b2
Pull-request: https://github.com/SerenityOS/serenity/pull/1622
Reviewed-by: https://github.com/awesomekling ✅

Libraries/LibJS/AST.cpp

@@ -333,6 +333,8 @@ Value UnaryExpression::execute(Interpreter& interpreter) const
         case Value::Type::String:
             return js_string(interpreter, "string");
         case Value::Type::Object:
+            if (lhs_result.as_object().is_function())
+                return js_string(interpreter, "function");
             return js_string(interpreter, "object");
         case Value::Type::Boolean:
             return js_string(interpreter, "boolean");

Libraries/LibJS/Interpreter.cpp

@@ -31,6 +31,7 @@
 #include <LibJS/Runtime/DatePrototype.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/ErrorPrototype.h>
+#include <LibJS/Runtime/FunctionPrototype.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/Object.h>
@@ -47,6 +48,7 @@ Interpreter::Interpreter()
     m_empty_object_shape = heap().allocate<Shape>();
 
     m_object_prototype = heap().allocate<ObjectPrototype>();
+    m_function_prototype = heap().allocate<FunctionPrototype>();
     m_string_prototype = heap().allocate<StringPrototype>();
     m_array_prototype = heap().allocate<ArrayPrototype>();
     m_error_prototype = heap().allocate<ErrorPrototype>();
@@ -171,6 +173,7 @@ void Interpreter::gather_roots(Badge<Heap>, HashTable<Cell*>& roots)
     roots.set(m_array_prototype);
     roots.set(m_error_prototype);
     roots.set(m_date_prototype);
+    roots.set(m_function_prototype);
 
     roots.set(m_exception);
 

Libraries/LibJS/Interpreter.h

@@ -136,6 +136,7 @@ public:
     Object* array_prototype() { return m_array_prototype; }
     Object* error_prototype() { return m_error_prototype; }
     Object* date_prototype() { return m_date_prototype; }
+    Object* function_prototype() { return m_function_prototype; }
 
     Exception* exception() { return m_exception; }
     void clear_exception() { m_exception = nullptr; }
@@ -168,6 +169,7 @@ private:
     Object* m_array_prototype { nullptr };
     Object* m_error_prototype { nullptr };
     Object* m_date_prototype { nullptr };
+    Object* m_function_prototype { nullptr };
 
     Exception* m_exception { nullptr };
 

Libraries/LibJS/Makefile

@@ -18,6 +18,8 @@ OBJS = \
     Runtime/ErrorPrototype.o \
     Runtime/Exception.o \
     Runtime/Function.o \
+    Runtime/FunctionConstructor.o \
+    Runtime/FunctionPrototype.o \
     Runtime/GlobalObject.o \
     Runtime/MathObject.o \
     Runtime/NativeFunction.o \

Libraries/LibJS/Runtime/Function.cpp

@@ -24,16 +24,14 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <AK/FlyString.h>
-#include <LibJS/Heap/Heap.h>
+#include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/Function.h>
-#include <LibJS/Runtime/Value.h>
 
 namespace JS {
 
 Function::Function()
 {
-    put("prototype", heap().allocate<Object>());
+    set_prototype(interpreter().function_prototype());
 }
 
 Function::~Function()

Libraries/LibJS/Runtime/FunctionConstructor.cpp

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * 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 <AK/StringBuilder.h>
+#include <LibJS/AST.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Parser.h>
+#include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/FunctionConstructor.h>
+#include <LibJS/Runtime/ScriptFunction.h>
+
+namespace JS {
+
+FunctionConstructor::FunctionConstructor()
+{
+    put("prototype", interpreter().function_prototype());
+    put("length", Value(1));
+}
+
+FunctionConstructor::~FunctionConstructor()
+{
+}
+
+Value FunctionConstructor::call(Interpreter& interpreter)
+{
+    return construct(interpreter);
+}
+
+Value FunctionConstructor::construct(Interpreter& interpreter)
+{
+    String parameters_source = "";
+    String body_source = "";
+    if (interpreter.argument_count() == 1)
+        body_source = interpreter.argument(0).to_string();
+    if (interpreter.argument_count() > 1) {
+        Vector<String> parameters;
+        for (size_t i = 0; i < interpreter.argument_count() - 1; ++i)
+            parameters.append(interpreter.argument(i).to_string());
+        StringBuilder parameters_builder;
+        parameters_builder.join(',', parameters);
+        parameters_source = parameters_builder.build();
+        body_source = interpreter.argument(interpreter.argument_count() - 1).to_string();
+    }
+    auto source = String::format("function (%s) { %s }", parameters_source.characters(), body_source.characters());
+    auto parser = Parser(Lexer(source));
+    auto function_expression = parser.parse_function_node<FunctionExpression>();
+    if (parser.has_errors()) {
+        // FIXME: The parser should expose parsing error strings rather than just fprintf()'ing them
+        return interpreter.heap().allocate<Error>("SyntaxError", "");
+    }
+    return function_expression->execute(interpreter);
+}
+
+}

Libraries/LibJS/Runtime/FunctionConstructor.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * 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/NativeFunction.h>
+
+namespace JS {
+
+class FunctionConstructor final : public NativeFunction {
+public:
+    FunctionConstructor();
+    virtual ~FunctionConstructor() override;
+
+    virtual Value call(Interpreter&) override;
+    virtual Value construct(Interpreter&) override;
+
+private:
+    virtual bool has_constructor() const override { return true; }
+    virtual const char* class_name() const override { return "FunctionConstructor"; }
+};
+
+}

Libraries/LibJS/Runtime/FunctionPrototype.cpp

@@ -0,0 +1,123 @@
+/*
+ * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * 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 <AK/Function.h>
+#include <AK/StringBuilder.h>
+#include <LibJS/AST.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/Function.h>
+#include <LibJS/Runtime/FunctionPrototype.h>
+#include <LibJS/Runtime/ScriptFunction.h>
+
+namespace JS {
+
+FunctionPrototype::FunctionPrototype()
+{
+    put_native_function("apply", apply, 2);
+    put_native_function("bind", bind, 1);
+    put_native_function("call", call, 1);
+    put_native_function("toString", to_string);
+    put("length", Value(0));
+}
+
+FunctionPrototype::~FunctionPrototype()
+{
+}
+
+Value FunctionPrototype::apply(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    if (!this_object->is_function())
+        return interpreter.throw_exception<Error>("TypeError", "Not a Function object");
+    auto function = static_cast<Function*>(this_object);
+    auto this_arg = interpreter.argument(0);
+    auto arg_array = interpreter.argument(1);
+    if (arg_array.is_null() || arg_array.is_undefined())
+        return interpreter.call(function, this_arg);
+    if (!arg_array.is_object())
+        return interpreter.throw_exception<Error>("TypeError", "argument array must be an object");
+    size_t length = 0;
+    auto length_property = arg_array.as_object().get("length");
+    if (length_property.has_value())
+        length = length_property.value().to_number().to_i32();
+    Vector<Value> arguments;
+    for (size_t i = 0; i < length; ++i)
+        arguments.append(arg_array.as_object().get(String::number(i)).value_or(js_undefined()));
+    return interpreter.call(function, this_arg, arguments);
+}
+
+Value FunctionPrototype::bind(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    // FIXME: Implement me :^)
+    ASSERT_NOT_REACHED();
+}
+
+Value FunctionPrototype::call(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    if (!this_object->is_function())
+        return interpreter.throw_exception<Error>("TypeError", "Not a Function object");
+    auto function = static_cast<Function*>(this_object);
+    auto this_arg = interpreter.argument(0);
+    Vector<Value> arguments;
+    if (interpreter.argument_count() > 1) {
+        for (size_t i = 1; i < interpreter.argument_count(); ++i)
+            arguments.append(interpreter.argument(i));
+    }
+    return interpreter.call(function, this_arg, arguments);
+}
+
+Value FunctionPrototype::to_string(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return {};
+    if (!this_object->is_function())
+        return interpreter.throw_exception<Error>("TypeError", "Not a Function object");
+    // FIXME: Functions should be able to know their name, if any
+    if (this_object->is_native_function()) {
+        auto function_source = String::format("function () {\n  [%s]\n}", this_object->class_name());
+        return js_string(interpreter, function_source);
+    }
+    auto parameters = static_cast<ScriptFunction*>(this_object)->parameters();
+    StringBuilder parameters_builder;
+    parameters_builder.join(", ", parameters);
+    // FIXME: ASTNodes should be able to dump themselves to source strings - something like this:
+    // auto& body = static_cast<ScriptFunction*>(this_object)->body();
+    // auto body_source = body.to_source();
+    auto function_source = String::format("function (%s) {\n  %s\n}", parameters_builder.build().characters(), "???");
+    return js_string(interpreter, function_source);
+}
+
+}

Libraries/LibJS/Runtime/FunctionPrototype.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * 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 FunctionPrototype final : public Object {
+public:
+    FunctionPrototype();
+    virtual ~FunctionPrototype() override;
+
+private:
+    virtual const char* class_name() const override { return "FunctionPrototype"; }
+
+    static Value apply(Interpreter&);
+    static Value bind(Interpreter&);
+    static Value call(Interpreter&);
+    static Value to_string(Interpreter&);
+};
+
+}

Libraries/LibJS/Runtime/GlobalObject.cpp

@@ -5,6 +5,7 @@
 #include <LibJS/Runtime/ConsoleObject.h>
 #include <LibJS/Runtime/DateConstructor.h>
 #include <LibJS/Runtime/ErrorConstructor.h>
+#include <LibJS/Runtime/FunctionConstructor.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/MathObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
@@ -26,6 +27,7 @@ GlobalObject::GlobalObject()
     put("console", heap().allocate<ConsoleObject>());
     put("Date", heap().allocate<DateConstructor>());
     put("Error", heap().allocate<ErrorConstructor>());
+    put("Function", heap().allocate<FunctionConstructor>());
     put("Math", heap().allocate<MathObject>());
     put("Object", heap().allocate<ObjectConstructor>());
 }

Libraries/LibJS/Runtime/ScriptFunction.cpp

@@ -37,6 +37,7 @@ ScriptFunction::ScriptFunction(const ScopeNode& body, Vector<FlyString> paramete
     : m_body(body)
     , m_parameters(move(parameters))
 {
+    put("prototype", heap().allocate<Object>());
     put_native_property("length", length_getter, length_setter);
 }
 

Libraries/LibJS/Tests/Function.js

@@ -0,0 +1,29 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    assert(Function.length === 1);
+    assert(Function.prototype.length === 0);
+    assert(typeof Function() === "function");
+    assert(typeof new Function() === "function");
+    assert(Function()() === undefined);
+    assert(new Function()() === undefined);
+    assert(Function("return 42")() === 42);
+    assert(new Function("return 42")() === 42);
+    assert(new Function("foo", "return foo")(42) === 42);
+    assert(new Function("foo,bar", "return foo + bar")(1, 2) === 3);
+    assert(new Function("foo", "bar", "return foo + bar")(1, 2) === 3);
+    assert(new Function("foo", "bar,baz", "return foo + bar + baz")(1, 2, 3) === 6);
+    assert(new Function("foo", "bar", "baz", "return foo + bar + baz")(1, 2, 3) === 6);
+    assert(new Function("foo", "if (foo) { return 42; } else { return 'bar'; }")(true) === 42);
+    assert(new Function("foo", "if (foo) { return 42; } else { return 'bar'; }")(false) === "bar");
+    assert(new Function("return typeof Function()")() === "function");
+    // FIXME: This is equivalent to
+    //   (function (x) { return function (y) { return x + y;} })(1)(2)
+    // and should totally work, but both currently fail with
+    //   Uncaught exception: [ReferenceError]: 'x' not known
+    // assert(new Function("x", "return function (y) { return x + y };")(1)(2) === 3);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e.message);
+}

Libraries/LibJS/Tests/Function.prototype.apply.js

@@ -0,0 +1,53 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    function Foo(arg) {
+        this.foo = arg;
+    }
+    function Bar(arg) {
+        this.bar = arg;
+    }
+    function FooBar(arg) {
+        Foo.apply(this, [arg]);
+        Bar.apply(this, [arg]);
+    }
+    function FooBarBaz(arg) {
+        Foo.apply(this, [arg]);
+        Bar.apply(this, [arg]);
+        this.baz = arg;
+    }
+
+    assert(Function.prototype.apply.length === 2);
+
+    var foo = new Foo("test");
+    assert(foo.foo === "test");
+    assert(foo.bar === undefined);
+    assert(foo.baz === undefined);
+
+    var bar = new Bar("test");
+    assert(bar.foo === undefined);
+    assert(bar.bar === "test");
+    assert(bar.baz === undefined);
+
+    var foobar = new FooBar("test");
+    assert(foobar.foo === "test");
+    assert(foobar.bar === "test");
+    assert(foobar.baz === undefined);
+
+    var foobarbaz = new FooBarBaz("test");
+    assert(foobarbaz.foo === "test");
+    assert(foobarbaz.bar === "test");
+    assert(foobarbaz.baz === "test");
+
+    assert(Math.abs.apply(null, [-1]) === 1);
+
+    var add = (x, y) => x + y;
+    assert(add.apply(null, [1, 2]) === 3);
+
+    var multiply = function (x, y) { return x * y; };
+    assert(multiply.apply(null, [3, 4]) === 12);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/Function.prototype.call.js

@@ -0,0 +1,53 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    function Foo(arg) {
+        this.foo = arg;
+    }
+    function Bar(arg) {
+        this.bar = arg;
+    }
+    function FooBar(arg) {
+        Foo.call(this, arg);
+        Bar.call(this, arg);
+    }
+    function FooBarBaz(arg) {
+        Foo.call(this, arg);
+        Bar.call(this, arg);
+        this.baz = arg;
+    }
+
+    assert(Function.prototype.call.length === 1);
+
+    var foo = new Foo("test");
+    assert(foo.foo === "test");
+    assert(foo.bar === undefined);
+    assert(foo.baz === undefined);
+
+    var bar = new Bar("test");
+    assert(bar.foo === undefined);
+    assert(bar.bar === "test");
+    assert(bar.baz === undefined);
+
+    var foobar = new FooBar("test");
+    assert(foobar.foo === "test");
+    assert(foobar.bar === "test");
+    assert(foobar.baz === undefined);
+
+    var foobarbaz = new FooBarBaz("test");
+    assert(foobarbaz.foo === "test");
+    assert(foobarbaz.bar === "test");
+    assert(foobarbaz.baz === "test");
+
+    assert(Math.abs.call(null, -1) === 1);
+
+    var add = (x, y) => x + y;
+    assert(add.call(null, 1, 2) === 3);
+
+    var multiply = function (x, y) { return x * y; };
+    assert(multiply.call(null, 3, 4) === 12);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}

Libraries/LibJS/Tests/Function.prototype.toString.js

@@ -0,0 +1,21 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    assert((function() {}).toString() === "function () {\n  ???\n}");
+    assert((function(foo) {}).toString() === "function (foo) {\n  ???\n}");
+    assert((function(foo, bar, baz) {}).toString() === "function (foo, bar, baz) {\n  ???\n}");
+    assert((function(foo, bar, baz) {
+        if (foo) {
+            return baz;
+        } else if (bar) {
+            return foo;
+        }
+        return bar + 42;
+    }).toString() === "function (foo, bar, baz) {\n  ???\n}");
+    assert(console.log.toString() === "function () {\n  [NativeFunction]\n}");
+    assert(Function.toString() === "function () {\n  [FunctionConstructor]\n}");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}