From Qt Wiki
- Accessing that property from JS would result in that QVariantMap being converted back into a JS object.
- JS function reference, special JS values (null, undefined) could not be stored in the 'property variant' type of property.
- Support storing of anything created in JS, including JS function references.
- Only when accessed from C++ (via QObject::property() or QQmlProperty::read()) will be converted to a QVariant (same conversions rules of any other JS value to QVariant conversion apply).
- When implementing types on the C++ side, one can use the QJSValue class as a property/method parameter to transfer values between C++ and QML/JS without type/data loss.
- Includes JS functions; for example, you can assign a function to a property from QML and call it later from C++ using QJSValue::call().