Calling undefined parameter - Jest tesing

I had a problem with testing in jest and i've found solution like this:

test('the data is peanut butter', (done) => {
  function callback(data) {
    expect(data).toBe('peanut butter');
   done();
  }

  fetchData(callback);
})

It's working but... In documentation is written that jest will wait until done callback is called before finishing the test.

Could you explain why it's working like this?