What is the correct way to test Reducer in Jest?

Question

JavaScript

I'm trying to put into practice Jest tests that are built into create react app
reducer

function reducer(state = initialState, action) {
  switch (action.type) {
// унивирсальный onChange
    case patientActions.ON_CHANGE_PATIENT:
      return {
        ...state,
        [action.payload.field]: action.payload.value,
      };
// если нашли, то открываем модал с информацией
      case patientActions.ON_SEARCH_PATIENT_FOUND:
          return {
              ...state,
              showModal: true,
              modalBodyContainer: <PatientDetails {...action.payload} />,
          };
...

action

// Universal onChange
export const onChangePatient = createAction(patientActions.ON_CHANGE_PATIENT, (value, field) => ({
    value,
    field,
}));

// Search Patient
export const onSearchPatient = (ur, token) => dispatch => (
    callToApiWithToken(`patient/by/ur/${ur}`, token)
        .then(response =>
            (response.data === null) ? dispatch(onSearchPatientNotFound(ur)) : dispatch(onSearchPatientFound(response.data))
        )
        .catch((error) => ({type: patientActions.ON_SUBMIT_PATIENT_ERROR}) )
);

export const onSearchPatientFound = createAction(patientActions.ON_SEARCH_PATIENT_FOUND);
export const onSearchPatientNotFound = createAction(patientActions.ON_SEARCH_PATIENT_NOT_FOUND);

test

describe('Request reducer', ()=>{
    it('has a default states', ()=>{
        expect(reducer(undefined, {type: 'unexpected'})).toEqual(defaultStates)
    }); // Проходит этот тест 

    it(`handle action ${patientActions.ON_SEARCH_PATIENT_FOUND}`, ()=>{
        expect(reducer(undefined,
            {
                type: patientActions.ON_SEARCH_PATIENT_FOUND,
                payload: {
                    showModal: true,
                    modalBodyContainer: {} // Нужно ли сюда писать <PatientDetails {...action.payload} />?
                }

            })
        )
            .toEqual({...defaultStates,
                showModal: true,
                modalBodyContainer: ???
            })
    }) // не проходит

it(`handle action ${patientActions.ON_CHANGE_PATIENT}`, ()=>{
        expect(reducer(undefined, {
            type: patientActions.PATIENT_INIT_STATE,
            payload: {
                value: '11122',
                field: 'patient_ur'
            }
        })).toEqual({...defaultStates, patient_ur: '11122'})
    }) // не проходит

The task so far is to test the reducers. How to make it all work?
PS The code all works.

Answer 1

redux.js.org/docs/recipes/WritingTests.html
There are some decent examples here. should help you

Answer 2

it(`handle action ${patientActions.ON_CHANGE_PATIENT}`, ()=>{
        expect(reducer(undefined, {
            type: patientActions.PATIENT_INIT_STATE, // тут должно быть patientActions.ON_CHANGE_PATIENT
            payload: {
                value: '11122',
                field: 'patient_ur'
            }
        })).toEqual({...defaultStates, patient_ur: '11122'})
    })

correct the error in this test, if it "passes", then I think everything will work out further. The essence of unit tests is that you write: it was A, applied some function (in your case, a reducer), it became B. Compare with C.
And you describe "C" yourself.