angular 6未选择任何选项时如何更新属性

selectedOption: string;

<select [(ngModel)]="selectedOption">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

updateProperty() {
  if (this.selectedOption) {
    this.calledUpdatedProperty = '选项已选择';
  } else {
    this.calledUpdatedProperty = '未选择任何选项';
  }
}

<select [(ngModel)]="selectedOption" (change)="updateProperty()">
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>