Redux-Saga

Redux-Saga is a library for managing application state. It installs as middleware to Redux similarly to Redux-Thunk, but the syntax with which it is called is different.

Redux-Saga uses a rather old concept of generators, which appeared together with promises at the JS conference in 2013, but until now was not and still is not a popular solution. It seems quite difficult to understand at first, but it solves a lot of problems in the form of at least the so-called Callback Hell, i.e. nested multiple queries returning results.

Keywords in Redux-Saga

Redux-Saga uses keywords in its syntax, such as:

call, put, takeEvery, takeLatest, all 

Each of these keywords is used in a function that is a generator, returning objects called Effects.

The simplest example would be an API query. In this query, we will retrieve a list of products for our e-commerce store, so that the component that uses the variable with the list of products will automatically reload.

// saga.js
import { call, takeLatest, all } from 'redux-saga/effects'

function* fetchProducts() {
  const products = yield call(Api.fetch, '/products')
}

function* actionWatcher() {
     yield takeLatest('GET_PRODUCTS', fetchProducts)
}

export default function* rootSaga() {
   yield all([
   actionWatcher(),
   ]);
}

// app.js
const sagaMiddleware = createSagaMiddleware();

const store = createStore(
   reducer,
   applyMiddleware(sagaMiddleware),
);
sagaMiddleware.run(rootSaga);

The operation of Redux Saga consists of the following:

1. A Redux store is created and we connect our sagaMiddleware to it.
2. The RootSaga using the keyword all accepts a list of other saga objects to which it listens.
3. ActionWatcher using takeLatest listens for changes in the Redux action named GET_PRODUCTS 
4. The component using Redux calls the redux action
5. saga generator fetchProducts is called .

Using takeLatest is interesting in the sense - when the same action is called again the previous one is canceled. There are many other elements built into Saga that help manage asynchronicity such as debounce, throttle, delay.

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When to use Redux-Saga

Saga seems to be a good replacement for Redux-Thunks, and it's worth trying it yourself in your own project. It comes in handy especially when you are dealing with a complicated interface and many actions. Keywords, or so-called saga effects, solve many problems related to asynchronicity. They can delay queries, determine their maximum frequency, cancel a query as well as many others.