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, 18 insertions, 4 deletions

diff --git a/Userland/Libraries/LibJS/MarkupGenerator.cpp b/Userland/Libraries/LibJS/MarkupGenerator.cpp
index 6766b1b858..fb8e8fb6e5 100644
--- a/Userland/Libraries/LibJS/MarkupGenerator.cpp
+++ b/Userland/Libraries/LibJS/MarkupGenerator.cpp
@@ -53,6 +53,14 @@ String MarkupGenerator::html_from_value(Value value)
     return output_html.to_string();
 }
 
+String MarkupGenerator::html_from_error(Object& object)
+{
+    StringBuilder output_html;
+    HashTable<Object*> seen_objects;
+    error_to_html(object, output_html, seen_objects);
+    return output_html.to_string();
+}
+
 void MarkupGenerator::value_to_html(Value value, StringBuilder& output_html, HashTable<Object*> seen_objects)
 {
     if (value.is_empty()) {
@@ -156,10 +164,16 @@ void MarkupGenerator::date_to_html(const Object& date, StringBuilder& html_outpu
 
 void MarkupGenerator::error_to_html(const Object& object, StringBuilder& html_output, HashTable<Object*>&)
 {
-    auto& error = static_cast<const Error&>(object);
-    html_output.append(wrap_string_in_style(String::formatted("[{}]", error.name()), StyleType::Invalid));
-    if (!error.message().is_empty()) {
-        html_output.appendff(": {}", escape_html_entities(error.message()));
+    auto name = object.get_without_side_effects("name").value_or(JS::js_undefined());
+    auto message = object.get_without_side_effects("message").value_or(JS::js_undefined());
+    if (name.is_accessor() || name.is_native_property() || message.is_accessor() || message.is_native_property()) {
+        html_output.append(wrap_string_in_style(JS::Value(&object).to_string_without_side_effects(), StyleType::Invalid));
+    } else {
+        auto name_string = name.to_string_without_side_effects();
+        auto message_string = message.to_string_without_side_effects();
+        html_output.append(wrap_string_in_style(String::formatted("[{}]", name_string), StyleType::Invalid));
+        if (!message_string.is_empty())
+            html_output.appendff(": {}", escape_html_entities(message_string));
     }
 }