LibJS: Implement spec-compliant OrdinaryToPrimitive

This renames Object::to_primitive() to Object::ordinary_to_primitive()
for two reasons:

- No confusion with Value::to_primitive()
- To match the spec's name

Also change existing uses of Object::to_primitive() to
Value::to_primitive() when the spec uses the latter (which will still
call Object::ordinary_to_primitive()). Object::to_string() has been
removed as it's not needed anymore (and nothing the spec uses).

This makes it possible to overwrite an object's toString and valueOf and
have them provide results for anything that uses to_primitive() - e.g.:

    const o = { toString: undefined, valueOf: () => 42 };
    Number(o) // 42, previously NaN
    ["foo", o].toString(); // "foo,42", previously "foo,[object Object]"
    ++o // 43, previously NaN

etc.

4 files changed, 49 insertions, 41 deletions

diff --git a/Libraries/LibJS/Runtime/Object.cpp b/Libraries/LibJS/Runtime/Object.cpp
index 5ed464624c..331f3b3704 100644
--- a/Libraries/LibJS/Runtime/Object.cpp
+++ b/Libraries/LibJS/Runtime/Object.cpp
@@ -836,48 +836,30 @@ bool Object::has_own_property(const PropertyName& property_name) const
     return shape().lookup(property_name.to_string_or_symbol()).has_value();
 }
 
-Value Object::to_primitive(Value::PreferredType preferred_type) const
+Value Object::ordinary_to_primitive(Value::PreferredType preferred_type) const
 {
-    Value result = js_undefined();
+    ASSERT(preferred_type == Value::PreferredType::String || preferred_type == Value::PreferredType::Number);
 
-    switch (preferred_type) {
-    case Value::PreferredType::Default:
-    case Value::PreferredType::Number: {
-        result = value_of();
-        if (result.is_object()) {
-            result = to_string();
-        }
-        break;
-    }
-    case Value::PreferredType::String: {
-        result = to_string();
-        if (result.is_object())
-            result = value_of();
-        break;
-    }
-    }
+    auto& vm = this->vm();
 
-    ASSERT(!result.is_object());
-    return result;
-}
+    Vector<FlyString, 2> method_names;
+    if (preferred_type == Value::PreferredType::String)
+        method_names = { vm.names.toString, vm.names.valueOf };
+    else
+        method_names = { vm.names.valueOf, vm.names.toString };
 
-Value Object::to_string() const
-{
-    auto& vm = this->vm();
-    auto to_string_property = get(vm.names.toString);
-    if (to_string_property.is_function()) {
-        auto& to_string_function = to_string_property.as_function();
-        auto to_string_result = vm.call(to_string_function, const_cast<Object*>(this));
-        if (to_string_result.is_object())
-            vm.throw_exception<TypeError>(global_object(), ErrorType::Convert, "object", "string");
+    for (auto& method_name : method_names) {
+        auto method = get(method_name);
         if (vm.exception())
             return {};
-        auto* string = to_string_result.to_primitive_string(global_object());
-        if (vm.exception())
-            return {};
-        return string;
+        if (method.is_function()) {
+            auto result = vm.call(method.as_function(), const_cast<Object*>(this));
+            if (!result.is_object())
+                return result;
+        }
     }
-    return js_string(vm, String::formatted("[object {}]", class_name()));
+    vm.throw_exception<TypeError>(global_object(), ErrorType::Convert, "object", preferred_type == Value::PreferredType::String ? "string" : "number");
+    return {};
 }
 
 Value Object::invoke(const StringOrSymbol& property_name, Optional<MarkedValueList> arguments)
diff --git a/Libraries/LibJS/Runtime/Object.h b/Libraries/LibJS/Runtime/Object.h
index a1d76957a3..93e9daac69 100644
--- a/Libraries/LibJS/Runtime/Object.h
+++ b/Libraries/LibJS/Runtime/Object.h
@@ -140,8 +140,7 @@ public:
     virtual bool prevent_extensions();
 
     virtual Value value_of() const { return Value(const_cast<Object*>(this)); }
-    virtual Value to_primitive(Value::PreferredType preferred_type = Value::PreferredType::Default) const;
-    virtual Value to_string() const;
+    virtual Value ordinary_to_primitive(Value::PreferredType preferred_type) const;
 
     Value get_direct(size_t index) const { return m_storage[index]; }
 
diff --git a/Libraries/LibJS/Runtime/Value.cpp b/Libraries/LibJS/Runtime/Value.cpp
index 953a3b70ad..a0b6a4f96d 100644
--- a/Libraries/LibJS/Runtime/Value.cpp
+++ b/Libraries/LibJS/Runtime/Value.cpp
@@ -168,7 +168,7 @@ String Value::to_string(GlobalObject& global_object) const
     case Type::BigInt:
         return m_value.as_bigint->big_integer().to_base10();
     case Type::Object: {
-        auto primitive_value = as_object().to_primitive(PreferredType::String);
+        auto primitive_value = to_primitive(PreferredType::String);
         if (global_object.vm().exception())
             return {};
         return primitive_value.to_string(global_object);
@@ -205,8 +205,12 @@ bool Value::to_boolean() const
 
 Value Value::to_primitive(PreferredType preferred_type) const
 {
-    if (is_object())
-        return as_object().to_primitive(preferred_type);
+    if (is_object()) {
+        // FIXME: Also support @@toPrimitive
+        if (preferred_type == PreferredType::Default)
+            preferred_type = PreferredType::Number;
+        return as_object().ordinary_to_primitive(preferred_type);
+    }
     return *this;
 }
 
@@ -277,7 +281,7 @@ Value Value::to_number(GlobalObject& global_object) const
         global_object.vm().throw_exception<TypeError>(global_object, ErrorType::Convert, "BigInt", "number");
         return {};
     case Type::Object: {
-        auto primitive = m_value.as_object->to_primitive(PreferredType::Number);
+        auto primitive = to_primitive(PreferredType::Number);
         if (global_object.vm().exception())
             return {};
         return primitive.to_number(global_object);
diff --git a/Libraries/LibJS/Tests/ordinary-to-primitive.js b/Libraries/LibJS/Tests/ordinary-to-primitive.js
new file mode 100644
index 0000000000..176beadb76
--- /dev/null
+++ b/Libraries/LibJS/Tests/ordinary-to-primitive.js
@@ -0,0 +1,23 @@
+test("object with custom toString", () => {
+    const o = { toString: () => "foo" };
+    expect(o + "bar").toBe("foobar");
+    expect([o, "bar"].toString()).toBe("foo,bar");
+});
+
+test("object with uncallable toString and custom valueOf", () => {
+    const o = { toString: undefined, valueOf: () => "foo" };
+    expect(o + "bar").toBe("foobar");
+    expect([o, "bar"].toString()).toBe("foo,bar");
+});
+
+test("object with custom valueOf", () => {
+    const o = { valueOf: () => 42 };
+    expect(Number(o)).toBe(42);
+    expect(o + 1).toBe(43);
+});
+
+test("object with uncallable valueOf and custom toString", () => {
+    const o = { valueOf: undefined, toString: () => "42" };
+    expect(Number(o)).toBe(42);
+    expect(o + 1).toBe("421");
+});