Reducer test - https://medium.com/@manojdarshana/reactjs-testing-with-jest-101-81bb17dd6c65

header,spec.ts

import configureStore from 'redux-mock-store';
import thunk from 'redux-thunk';
import header, { loadHeader, ACTION_TYPES } from 'app/shared/reducers/header';

describe('Header reducer tests', () => {
  it('should return the initial state', () => {
    const headerState = header(undefined, {});
    expect(headerState).toMatchObject({
      headerTitle: '',
    });
  });

  it('should correctly detect update in current Header state', () => {
    const headerState = header(undefined, { type: ACTION_TYPES.SET_HEADERDETAILS, payload: { header: 'header title' } });
    expect(headerState).toMatchObject({ headerTitle: headerState.headerTitle });
    const updatedHeaderState = header(
      {
        loading: false,
        headerTitle: 'header title',
      },
      { type: ACTION_TYPES.SET_HEADERDETAILS, payload: { header: 'header title updated' } }
    );
    expect(updatedHeaderState).toMatchObject({ headerTitle: updatedHeaderState.headerTitle });
  });

  describe('Header actions', () => {
    it('dispatches SET_HEADERDETAILS actions', () => {
      const expectedActions = [
        {
          type: ACTION_TYPES.SET_HEADERDETAILS,
          payload: { header: 'header title' },
        },
      ];
      store.dispatch(loadHeader('header title'));
      expect(store.getActions()).toEqual(expectedActions);
    });
  });
});