angular: fakeAsync cannot work with FileReader event

verison

zone.js: 0.8.26

issue

For testing the FileReader event, like onload, if with setTimeout, test case can pass, while with fakeAsync, failed. The below is the code that can reproduce this issue and can just be pasted in the current test spec and run

// function need to be tested
function fileReaderTest(blob: Blob): number {
  const reader = new FileReader();

  let num = 0;

  reader.onload = () => {
    num++;
  };

  reader.readAsText(blob);

  return num;
}

// test the above function
fdescribe('', () => {
  let blob: Blob;

  beforeEach(() => {
    const data = 'Hello, World!';
    blob = new Blob([data]);
  });

  // can pass
  it('with setTimeout', () => {
    const number = fileReaderTest(blob);

    setTimeout(() => {
      expect(number).toEqual(1);
    });
  });

  // fail
  it('with fakeAsync', () => {
    let FakeAsyncTestZoneSpec = (Zone as any)['FakeAsyncTestZoneSpec'];
    let testZoneSpec: any;
    let fakeAsyncTestZone: Zone;

    testZoneSpec = new FakeAsyncTestZoneSpec('name');
    fakeAsyncTestZone = Zone.current.fork(testZoneSpec);

    const num = fileReaderTest(blob);

    fakeAsyncTestZone.run(() => {

      testZoneSpec.tick(100);
      testZoneSpec.tick(100);
      testZoneSpec.flush(); 
      testZoneSpec.flushMicrotasks();
      expect(num).toEqual(1);
    });
  });
});

Background

Today when I tried to write some unit tests for a function with FileReader, I got this issue. But I’m not sure if it’s really a bug or just my misuse.

thanks