Unittest For FileReader tohavebeenCalled

I am having real issue with my tests for FileReader. i mock the FileReader and trying To test if It has been called when onChange event is triggered. I have tried to write tests for method that call on OnLoad event of File Reader.

.ts File

 reader = new FileReader();

 readFileOnChange(files: any) {
     if (files && files.length > 0) {
        const file: File = files.item(0);
        if (file.type == "text/csv") {
          this.reader.readAsText(file, "UTF-8");
          this.reader.onload = (e) => { this.parse()}
        }
     }
  }

Spec.ts

describe('AppComponent', () => {
  let mockFile = new File(['testData'], 'filename.csv', { type: 'text/csv' });
  let mockEvt = { target: { files: [mockFile] } };
   let mockReader: FileReader = jasmine.createSpyObj('FileReader', ['readAsText', 
    'onload']);

  beforeEach(waitForAsync(() => {
    TestBed.configureTestingModule({
      declarations: [ AppComponent ],
      providers: [],
      imports: [HttpClientTestingModule, BrowserDynamicTestingModule],
    }).compileComponents();
    component = fixture.componentInstance;
    fixture.detectChanges();
    spyOn(component, 'readFileOnChange').and.callThrough();
  }));

    it('should call parse on onLoad of FileReader', () => {
      spyOn(window, 'FileReader').and.returnValue(mockReader);
      component.readFileOnChange(mockEvt as any);
      expect(window.FileReader).toHaveBeenCalled();
      expect(component.parse).toHaveBeenCalled();
    });

})

gives Error:

Expected spy FileReader to have been called.

Please help Thank you