suggested-username#reactive-username.ts

reactiveUsername() {
  const firstName = this.form.get('firstName');
  const lastName = this.form.get('lastName');
  const username = this.form.get('username');

  const combined$ = combineLatest(
    firstName.valueChanges,
    lastName.valueChanges
  )
   .pipe(map(([first, last]) => (first && last ? `${first}-${last}` : '')))
   .subscribe(value => username.setValue(value));
}