[SuperZen]

async/await в помощь

<script>

  class FakeDB {

    constructor() { }

    getData(data) {
      return new Promise((res, rej) => setTimeout(res, 100, data))
    }
  }

  class User extends FakeDB {

    constructor() {
      super()
    }

    getUser() {
      return super.getData({ user: 'ok' })
    }
  }

  (async function () {
    let user = new User()
    console.log(await user.getUser())
  })()

</script>