There's an observable difference with your getter based approach and the "normal property values" approach in the article:

    const t = new Translation
    // property value that
    // is initialized once
    t.translations === t.translations
    // getters which return
    // new objects
    t.translations !== t.translations

so at the very least the browser can't optimize that behavior away, it needs to create new objects. Besides, ensuring that say, call to `Object.keys` is side-effect free is challenging.