From 2e4832c3da770357c1a45a5f4c5af5ee4b32c7ad Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 29 Nov 2020 18:01:59 +0100
Subject: [PATCH] LibJS: Constructor function's "prototype" property should be
 writable

This matches other engines.
---
 Libraries/LibJS/AST.cpp                                |  2 +-
 Libraries/LibJS/Runtime/ScriptFunction.cpp             |  2 +-
 .../Tests/functions/function-prototype-writable.js     | 10 ++++++++++
 3 files changed, 12 insertions(+), 2 deletions(-)
 create mode 100644 Libraries/LibJS/Tests/functions/function-prototype-writable.js

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index a78ce9192f..a6cb719120 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -759,7 +759,7 @@ Value ClassExpression::execute(Interpreter& interpreter, GlobalObject& global_ob
         prototype->define_property(vm.names.constructor, class_constructor, 0);
         if (interpreter.exception())
             return {};
-        class_constructor->define_property(vm.names.prototype, prototype, 0);
+        class_constructor->define_property(vm.names.prototype, prototype, Attribute::Writable);
         if (interpreter.exception())
             return {};
         class_constructor->set_prototype(super_constructor.is_null() ? global_object.function_prototype() : &super_constructor.as_object());
diff --git a/Libraries/LibJS/Runtime/ScriptFunction.cpp b/Libraries/LibJS/Runtime/ScriptFunction.cpp
index 5b751be423..55bfb9f11e 100644
--- a/Libraries/LibJS/Runtime/ScriptFunction.cpp
+++ b/Libraries/LibJS/Runtime/ScriptFunction.cpp
@@ -71,7 +71,7 @@ void ScriptFunction::initialize(GlobalObject& global_object)
     if (!m_is_arrow_function) {
         Object* prototype = vm.heap().allocate<Object>(global_object, *global_object.new_script_function_prototype_object_shape());
         prototype->define_property(vm.names.constructor, this, Attribute::Writable | Attribute::Configurable);
-        define_property(vm.names.prototype, prototype, 0);
+        define_property(vm.names.prototype, prototype, Attribute::Writable);
     }
     define_native_property(vm.names.length, length_getter, nullptr, Attribute::Configurable);
     define_native_property(vm.names.name, name_getter, nullptr, Attribute::Configurable);
diff --git a/Libraries/LibJS/Tests/functions/function-prototype-writable.js b/Libraries/LibJS/Tests/functions/function-prototype-writable.js
new file mode 100644
index 0000000000..ba8be23f43
--- /dev/null
+++ b/Libraries/LibJS/Tests/functions/function-prototype-writable.js
@@ -0,0 +1,10 @@
+test("a function's prototype property should be writable", () => {
+    function x() {}
+    var desc = Object.getOwnPropertyDescriptor(x, "prototype");
+    expect(desc.writable).toBe(true);
+    expect(desc.enumerable).toBe(false);
+    expect(desc.configurable).toBe(false);
+
+    x.prototype = 1;
+    expect(x.prototype).toBe(1);
+});