redux-saga-worker-and-watcher.js

import { takeEvery } from 'redux-saga';
import { call, put } from 'redux-saga/effects';

// Watcher
function* watchFetchData() {
  yield* takeEvery('FETCH_REQUESTED', fetchData);
}

// Worker
function* fetchData(action) {
   try {
      const data = yield call(fetch, action.payload.url);
      yield put({type: 'FETCH_SUCCEEDED', data});
   } catch (error) {
      yield put({type: 'FETCH_FAILED', error});
   }
}

+export default function* rootSaga() {
+  yield all([
+    fork(watchFetchData),
+  ]);
+}