How to define a computed property in ViewModel, KnockoutJS, TypeScript

KnockoutJS-ComputedField.ts

module SampleModule {
export class SampleComponent {
  isValid: KnockoutComputed<boolean>;
  //...
  
  constructor(public element) {
    //...
    this.isValidConnectAs = ko.computed<boolean>({
                owner: this,
                read: () => {
                    return this.connectAs().duser().length === 7;
                }
            });
    ko.applyBindings(this, element);
  }
  //...
}
}