test.js

// thunk
export function save(filename, notebook) {
  return (subject) => {
    // If there isn't a filename, save-as it instead
    if (!filename) {
      throw new Error('save needs a filename');
    }

    subject.next({
      type: constants.START_SAVING,
    });
    writeFile(filename, JSON.stringify(commutable.toJS(notebook), null, 2), (err) => {
      if (err) {
        console.error(err);
        throw err;
      }
      subject.next({
        type: constants.DONE_SAVING,
      });
    });
  };
}

//test
it('emits a done action', (done) => {
    let subject = new Rx.Subject();
    let thunk = actions.save('test.js');
    subject
    .skip(1)
    .subscribe((action) => {
      expect(action).to.deep.equal({
        type: constants.DONE_SAVING
      });
      done();
    });
    thunk(subject);
  });