Asynchronous api calls with redux-saga

Api.fetchUser is a function, where should be performed api ajax call and it should return promise.

In your case, this promise should resolve user data variable.

For example:

// services/api.jsexport function fetchUser(userId) {  // `axios` function returns promise, you can use any ajax lib, which can  // return promise, or wrap in promise ajax call  return axios.get('/api/user/' + userId);};

Then is sagas:

function* fetchUserSaga(action) {  // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.  // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable  const userData = yield call(api.fetchUser, action.userId);  // Instructing middleware to dispatch corresponding action.  yield put({    type: 'FETCH_USER_SUCCESS',    userData  });}

call, put are effects creators functions. They not have something familiar with GET or POST requests.

call function is used to create effect description, which instructs middleware to call the promise.put function creates effect, in which instructs middleware to dispatch an action to the store.

Things like call, put, take, race are effects creator functions. The Api.fetchUser is a placeholder for your own function that handles API requests.

Here’s a full example of a loginSaga:

export function* loginUserSaga() {  while (true) {    const watcher = yield race({      loginUser: take(USER_LOGIN),      stop: take(LOCATION_CHANGE),    });    if (watcher.stop) break;    const {loginUser} = watcher || {};    const {username, password} = loginUser || {};    const data = {username, password};    const login = yield call(SessionService.login, data);    if (login.err === undefined || login.err === null && login.response) {      yield put(loginSuccess(login.response));    } else {      yield put(loginError({message: 'Invalid credentials. Please try again.'}));    }  }}

In this snippet, the SessionService is a class that implements a login method which handles the HTTP request to the API. The redux-saga call will call this method and apply the data parameter to it. In the snippet above, we can then evaluate the result of the call and dispatch loginSuccess or loginError actions accordingly using put.

A side note: The snippet above is a loginSaga that continuously listens for the USER_LOGIN event, but breaks when a LOCATION_CHANGE happens. This is thanks to the race effect creator.