Testing the Filereader.onload callback function in Angular/Jasmine

In the ngOnInit of my component I am defining the Filereader.onload callback function. I found several solutions that say to mock the filereader and the onload but I think this defeats my purpose to test if the callback is correctly executed when onload is being called. So I tried to mock the onload Event and then test if the functions inside the callback get called.

it("should import data on filereader onload event", () => {
 const fileReader = new FileReader();
      let loadEvent: ProgressEvent<FileReader>;
     
      spyOn(fileReader, "onload").and.callThrough();

      spyOn(mockServiceOne, "convertData").and.returnValue(
        mockData
      );
      spyOn(mockServiceTwo, "importData").and.callThrough();

      fileReader.onload(loadEvent);
      fixture.detectChanges();

      expect(fileReader.result).toBeDefined();

      expect(mockServiceOne.convertData).toHaveBeenCalled();
      expect(mockServiceTwo.importData).toHaveBeenCalledTimes(1);
    });
  });

The function I am trying to test is this:

this.fileReader.onload = (event) => {
      const jsonData = JSON.parse(event.target.result.toString());
      const data = this.serviceOne.convertData(
        jsonData
      );
      this.serviceTwo.importData(data);
    };

I mocked the two services and methods inside them. But in the test spec they never get called. However the first assertion that the result of the onload event (event.target.result) needs to be defined seems to check true.

Maybe there is a problem with the first line in the function that converts the data to json, since I am not actually giving the function a real file. However when I try to give anything other than the mocked ProgressEvent it gives an error.

Please help me with testing the onload callback. Is it correct that I can't mock the filereader onload for this?