redux header reducer - https://medium.com/@manojdarshana/reactjs-testing-with-jest-101-81bb17dd6c65

header.reducer.ts

import axios from 'axios';

export const ACTION_TYPES = {
  SET_HEADERDETAILS: 'SET_HEADERDETAILS',
};

const initialState = {
  headerTitle: '',
};

export type HeaderState = Readonly<typeof initialState>;

// Reducer

export default (state: HeaderState = initialState, action): HeaderState => {
  switch (action.type) {
    case ACTION_TYPES.SET_HEADERDETAILS:
      return {
        ...state,
        headerTitle: action.payload.header,
      };
    default:
      return state;
  }
};

export const loadHeader: (header: string) => void = header => async (dispatch, getState) => {
  await dispatch({
    type: ACTION_TYPES.SET_HEADERDETAILS,
    payload: { header },
  });
};