Javascript裡有如下的寫法，可以用在定義方法時，或者定義方法時用在方法的引數上，非常靈活

// What does the following mean as a type: () => void = null
https://stackoverflow.com/questions/41868693/what-does-the-following-mean-as-a-type-void-null

• 定義方法時

export class ExcelOperationA {
  constructor() {}
  public exportExcelA: ()
 
 => void;
}

import { createXlsx } from './create-xlsx';
import { ExcelOperationA } from './ExcelOperationA';
@Component({
  selector: 'app-classNameB',
  templateUrl: './ClassNameB.html'
})
export class ClassNameB {
  constructor(
    public excelOperationA: ExcelOperationA
  ) {
    this.excelOperationA.exportExcelA
 
= () => createXlsx(parameterA);
  }
}

@Component({
  selector: 'rms-pt',
  templateUrl: `<app-classNameB [datas]="datas"></app-classNameB>`
})
export class RmsPtComponent {
  public datas: Data[] = [];
  constructor(private public excelOperationA: ExcelOperationA) {}
  public excelDownloadHandle() {
    this
 
.excelOperationA.exportToExcel();
  }
}