code: function counter(state = 0, action = {}) {
    switch (action.type) {
      case INCREMENT_COUNTER:
        return state + 1;
      case DECREMENT_COUNTER:
        return state - 1;
      default:
        return state;
    }
  }
docs: reducer for the counter
  @param  {number} state  input state
  @param  {object} action object containing the action type
  @param  {string} action.type action type
  @return {number}