LibJS: Add Object::get_without_side_effects()

Similar to Value::to_string_without_side_effects() this is mostly a
regular object property lookup, but with the guarantee that it will be
side-effect free, i.e. no accessors or native property functions will
be called. This is needed when we want to access user-controlled object
properties for debug logging, for example. The specific use case will be
error objects which will soon no longer have internal name/message
properties, so we need to guarantee that printing an error, which may
already be the result of an exception, won't blow up in our face :^)

4 files changed, 23 insertions, 11 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Object.cpp b/Userland/Libraries/LibJS/Runtime/Object.cpp
index 0e9bbaac85..634340c0ae 100644
--- a/Userland/Libraries/LibJS/Runtime/Object.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Object.cpp
@@ -234,7 +234,7 @@ bool Object::test_integrity_level(IntegrityLevel level)
     return true;
 }
 
-Value Object::get_own_property(const PropertyName& property_name, Value receiver) const
+Value Object::get_own_property(const PropertyName& property_name, Value receiver, bool without_side_effects) const
 {
     VERIFY(property_name.is_valid());
     VERIFY(!receiver.is_empty());
@@ -254,10 +254,12 @@ Value Object::get_own_property(const PropertyName& property_name, Value receiver
     }
 
     VERIFY(!value_here.is_empty());
-    if (value_here.is_accessor())
-        return value_here.as_accessor().call_getter(receiver);
-    if (value_here.is_native_property())
-        return call_native_property_getter(value_here.as_native_property(), receiver);
+    if (!without_side_effects) {
+        if (value_here.is_accessor())
+            return value_here.as_accessor().call_getter(receiver);
+        if (value_here.is_native_property())
+            return call_native_property_getter(value_here.as_native_property(), receiver);
+    }
     return value_here;
 }
 
@@ -769,7 +771,7 @@ Value Object::get_by_index(u32 property_index) const
     return {};
 }
 
-Value Object::get(const PropertyName& property_name, Value receiver) const
+Value Object::get(const PropertyName& property_name, Value receiver, bool without_side_effects) const
 {
     VERIFY(property_name.is_valid());
 
@@ -788,7 +790,7 @@ Value Object::get(const PropertyName& property_name, Value receiver) const
 
     const Object* object = this;
     while (object) {
-        auto value = object->get_own_property(property_name, receiver);
+        auto value = object->get_own_property(property_name, receiver, without_side_effects);
         if (vm().exception())
             return {};
         if (!value.is_empty())
@@ -800,6 +802,11 @@ Value Object::get(const PropertyName& property_name, Value receiver) const
     return {};
 }
 
+Value Object::get_without_side_effects(const PropertyName& property_name) const
+{
+    return get(property_name, {}, true);
+}
+
 bool Object::put_by_index(u32 property_index, Value value)
 {
     VERIFY(!value.is_empty());
diff --git a/Userland/Libraries/LibJS/Runtime/Object.h b/Userland/Libraries/LibJS/Runtime/Object.h
index 23f5f53e5f..d3f08f6094 100644
--- a/Userland/Libraries/LibJS/Runtime/Object.h
+++ b/Userland/Libraries/LibJS/Runtime/Object.h
@@ -94,14 +94,15 @@ public:
 
     GlobalObject& global_object() const { return *shape().global_object(); }
 
-    virtual Value get(const PropertyName&, Value receiver = {}) const;
+    virtual Value get(const PropertyName&, Value receiver = {}, bool without_side_effects = false) const;
+    Value get_without_side_effects(const PropertyName&) const;
 
     virtual bool has_property(const PropertyName&) const;
     bool has_own_property(const PropertyName&) const;
 
     virtual bool put(const PropertyName&, Value, Value receiver = {});
 
-    Value get_own_property(const PropertyName&, Value receiver) const;
+    Value get_own_property(const PropertyName&, Value receiver, bool without_side_effects = false) const;
     MarkedValueList get_own_properties(PropertyKind, bool only_enumerable_properties = false, GetOwnPropertyReturnType = GetOwnPropertyReturnType::All) const;
     MarkedValueList get_enumerable_own_property_names(PropertyKind) const;
     virtual Optional<PropertyDescriptor> get_own_property_descriptor(const PropertyName&) const;
diff --git a/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp b/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
index 59c3201b5d..ac5ea45fb3 100644
--- a/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
@@ -325,9 +325,13 @@ bool ProxyObject::has_property(const PropertyName& name) const
     return trap_result.to_boolean();
 }
 
-Value ProxyObject::get(const PropertyName& name, Value receiver) const
+Value ProxyObject::get(const PropertyName& name, Value receiver, bool without_side_effects) const
 {
     auto& vm = this->vm();
+    if (without_side_effects) {
+        // Sorry, we're not going to call anything on this proxy.
+        return js_string(vm, "<ProxyObject>");
+    }
     if (m_is_revoked) {
         vm.throw_exception<TypeError>(global_object(), ErrorType::ProxyRevoked);
         return {};
diff --git a/Userland/Libraries/LibJS/Runtime/ProxyObject.h b/Userland/Libraries/LibJS/Runtime/ProxyObject.h
index 046c985c14..a64fd57237 100644
--- a/Userland/Libraries/LibJS/Runtime/ProxyObject.h
+++ b/Userland/Libraries/LibJS/Runtime/ProxyObject.h
@@ -55,7 +55,7 @@ public:
     virtual Optional<PropertyDescriptor> get_own_property_descriptor(const PropertyName&) const override;
     virtual bool define_property(const StringOrSymbol& property_name, const Object& descriptor, bool throw_exceptions = true) override;
     virtual bool has_property(const PropertyName& name) const override;
-    virtual Value get(const PropertyName& name, Value receiver) const override;
+    virtual Value get(const PropertyName& name, Value receiver, bool without_side_effects = false) const override;
     virtual bool put(const PropertyName& name, Value value, Value receiver) override;
     virtual bool delete_property(const PropertyName& name) override;