let elementBeforeUpgrade,

    elementAfterUpgrade;

​

class Element extends HTMLElement {

  set foo(value) {

    console.log(`foo setter called with ${value}`);

    this._foo = value;

  }

  get foo() {

    console.log('foo getter called');

    return this._foo; 

  }

}

​

​

elementBeforeUpgrade = document.createElement('my-element');

elementBeforeUpgrade.foo = 'a';

​

customElements.define('my-element', Element);

elementBeforeUpgrade.foo = 'b';

​

elementAfterUpgrade = document.createElement('my-element');

elementAfterUpgrade.foo = 'c';