LibJS: Make Errors fully spec compliant

The previous handling of the name and message properties specifically
was breaking websites that created their own error types and relied on
the error prototype working correctly - not assuming an JS::Error this
object, that is.

The way it works now, and it is supposed to work, is:

- Error.prototype.name and Error.prototype.message just have initial
  string values and are no longer getters/setters
- When constructing an error with a message, we create a regular
  property on the newly created object, so a lookup of the message
  property will either get it from the object directly or go though the
  prototype chain
- Internal m_name/m_message properties are no longer needed and removed

This makes printing errors slightly more complicated, as we can no
longer rely on the (safe) internal properties, and cannot trust a
property lookup either - get_without_side_effects() is used to solve
this, it's not perfect but something we can revisit later.

I did some refactoring along the way, there was some really old stuff in
there - accessing vm.call_frame().arguments[0] is not something we (have
to) do anymore :^)

Fixes #6245.

1 files changed, 7 insertions, 15 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Error.h b/Userland/Libraries/LibJS/Runtime/Error.h
index 3605825f1d..0c7e9ac66c 100644
--- a/Userland/Libraries/LibJS/Runtime/Error.h
+++ b/Userland/Libraries/LibJS/Runtime/Error.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Linus Groh <mail@linusgroh.de>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -35,19 +36,10 @@ class Error : public Object {
     JS_OBJECT(Error, Object);
 
 public:
-    static Error* create(GlobalObject&, const FlyString& name, const String& message);
+    static Error* create(GlobalObject&, const String& message = {});
 
-    Error(const FlyString& name, const String& message, Object& prototype);
-    virtual ~Error() override;
-
-    const FlyString& name() const { return m_name; }
-    const String& message() const { return m_message; }
-
-    void set_name(const FlyString& name) { m_name = name; }
-
-private:
-    FlyString m_name;
-    String m_message;
+    explicit Error(Object& prototype);
+    virtual ~Error() override = default;
 };
 
 #define DECLARE_ERROR_SUBCLASS(ClassName, snake_name, PrototypeName, ConstructorName) \
@@ -55,10 +47,10 @@ private:
         JS_OBJECT(ClassName, Error);                                                  \
                                                                                       \
     public:                                                                           \
-        static ClassName* create(GlobalObject&, const String& message);               \
+        static ClassName* create(GlobalObject&, const String& message = {});          \
                                                                                       \
-        ClassName(const String& message, Object& prototype);                          \
-        virtual ~ClassName() override;                                                \
+        explicit ClassName(Object& prototype);                                        \
+        virtual ~ClassName() override = default;                                      \
     };
 
 #define __JS_ENUMERATE(ClassName, snake_name, PrototypeName, ConstructorName, ArrayType) \