class A extends HTMLElement {

    connectedCallback() {

        console.log("A", this);

        console.log("this.attr", this.attr); // <= undefined

        console.log("this.observedAttributes", this.observedAttributes); // <= undefined

        console.log("A.observedAttributes", A.observedAttributes); // <= undefined

        console.log("HTMLElement.observedAttributes", HTMLElement.observedAttributes); // <= undefined

    }

}

​

class B extends A {

    static get observedAttributes() {

      super.attr = ['bar'];

      return ['bar'];

    }

}

​

customElements.define('y-bar', B);