Upgrade to ember-typescript 3 complains about default parameters

Our Ember components often use the following pattern which is working as expected with ember-typescript 2:

export default class DatePicker extends Component {
  format: string = this.format || 'dd/LL/yyyy';
}

If format is not passed via the template it is initialized with 'dd/LL/yyyy'.

With ember-typescript 3 and the newer TS version the compiler is not happy anymore. It complains with

error TS2729: Property 'format' is used before its initialization.

as it is not happy with the usage of this.format.

What should I do to solve this problem? Is our code (which works fine so far) invalid and should be changed? If yes how?