An alternative Side Effect model for Redux applications. Instead of dispatching thunks which get handled by the redux-thunk middleware. You create Sagas to gather all your Side Effects logic in a central place.

This means the logic of the application lives in 2 places

Reducers are responsible of handling state transitions between actions
Sagas are responsible of orchestrating complex/asynchronous operations (side effects or actions). This includes simple side effects which react to one action (e.g. send a request on each button click), but also complex operations that span across multiple actions (e.g. User onBoarding, Wizard dialogs, asynchronous Game rules ...).

A Saga is a generator function that takes user actions as inputs and may yield Side Effects (e.g. server updates, navigation, store actions ...) as output.

Getting started
Declarative Effects
Error handling
Effect Combinators
Sequencing Sagas via yield*
Composing Sagas
Building from sources

#Getting started

Install

npm install redux-saga

Create the Saga (using the counter example from Redux)

// sagas/index.js
function* incrementAsync(io) {

  while(true) {

    // wait for each INCREMENT_ASYNC action  
    const nextAction = yield io.take(INCREMENT_ASYNC)

    // call delay : Number -> Promise
    yield delay(1000)

    // dispatch INCREMENT_COUNTER
    yield io.put( increment() )
  }

}

export default [incrementAsync]

Plug redux-saga in the middleware pipeline

// store/configureStore.js
import sagaMiddleware from 'redux-saga'
import sagas from '../sagas'

const createStoreWithSaga = applyMiddleware(
  // ...,
  sagaMiddleware(...sagas)
)(createStore)

export default function configureStore(initialState) {
  return createStoreWithSaga(reducer, initialState)
}

In the above example we created an incrementAsync Saga to handle all INCREMENT_ASYNC actions. The Generator function uses yield io.take(action) to wait asynchronously for the next action. The middleware handles action queries from the Saga by pausing the Generator until an action matching the query happens. Then it resumes the Generator with the matching action.

After receiving the wanted action, the Saga triggers a call to delay(1000), which in our example returns a Promise that will be resolved after 1 second. Again, the middleware will pause the Generator until the yielded promise is resolved.

After the 1 second delay, the Saga dispatches an INCREMENT_COUNTER action using the io.put(action) method. And as for the 2 precedent statements, the Saga is resumed after resolving the result of the action dispatch (which will happen immediately if the Store's dispatch function returns a normal value, but may be later if the dispatch result is a Promise).

The Generator uses an infinite loop while(true) which means it will stay alive for all the application lifetime. But you can also create Sagas that last only for a limited amount of time. For example, the following Saga will wait for the first 3 INCREMENT_COUNTER actions, triggers a showCongratulation() action and then finishes.

function* onBoarding(io) {

  for(let i = 0; i < 3; i++)
    yield io.take(INCREMENT_COUNTER)

  yield io.put( showCongratulation() )
}

The basic idea, is that you use the yield operator every time you want to trigger a Side Effect. So to be more accurate, A Saga is a Generator function that yields Side Effects, wait for their results then resume with the responses. Everything you yield is considered an Effect: waiting for an action, triggering a server request, dispatching an action to the store...

#Declarative Effects

Sagas Generators can yield Effects in multiple forms. The simplest way is to yield a Promise

function* fetchSaga(io) {

  // returns a Promise that will resolve with the GET response
  const products = yield fetch('/products') 
  
  // dispatch a RECEIVE_PRODUCTS action 
  yield io.put( receiveProducts(products) ) 
}

In the example above, fetch('/url') returns a Promise that will resolve with the GET response. So the 'fetch effect' will be executed immediately . Simple and idiomatic but ...

Suppose we want to test generator above

import io from 'redux-saga/io' // exposed for testing purposes

const iterator = fetchSaga(io)
assert.deepEqual( iterator.next().value, ?? ) // what do we expect ?

We want to check the result of the first value yielded by the generator, which is in our case the result of running fetch('/products'). Executing the real service during tests is not a viable nor a practical approach, so we have to mock the fetch service, i.e. we'll have to replace the real fetch method with a fake one which doesn't actually run the GET request but only checks that w've called fetch with the right arguments ('/products' in our case).

Mocks makes testing more difficult and less reliable. On the other hand, functions which returns simple values are easier to test, we can use a simple equal() to check the result.This is the way to write the most relibale tests.

Not convinced ? I encourage you to read this [Eric Elliott' article] (https://medium.com/javascript-scene/what-every-unit-test-needs-f6cd34d9836d#.4ttnnzpgc)

(...)equal(), by nature answers the two most important questions every unit test must answer, but most don’t:

What is the actual output?
What is the expected output?

If you finish a test without answering those two questions, you don’t have a real unit test. You have a sloppy, half-baked test.

What we need actually, is just to make sure the fetchSaga yields a call with the right function and the right arguments. For this reason, the library provides some declarative ways to yield Side Effects while still making it easy to test the Saga logic

function* fetchSaga(io) {
  const products = yield io.call( fetch, '/products' ) // don't run the effect 
}

We're using now io.call(fn, ...args) function. The difference from the precedent example is that now we're not executing the fetch call immedieately, instead, io.call creates a description of the effect. Just as in Redux you use action creators to create a plain object descibing the action that will get executed by the Store, io.call creates a plain object describing the function call. The redux-saga middleware takes care of executing the function call and resuming the generator with the resolved response.

This allows us to easily test the Generator outside the Redux environment.

import io from 'redux-saga/io' // exposed for testing purposes

const iterator = fetchSaga(io)
assert.deepEqual(iterator.next().value, io.call(fetch, '/products') // expects a io.call(...) value

Now, we don't need to mock anything, a simple equality test will suffice.

The advantage of declarative effects is that we can test all the logic inside a Saga/Generator by simply iterating over the resulting iterator and doing a simple equality tests on the values yielded successively. This is a real benefit, as your complex asynchronous operations are no longer black boxes, you can test in detail their logic of operation no matter how complex it is.

The io.call method is well suited for functions which return Promise results. Another function io.cps can be used to handle Node style functions (e.g. fn(...args, callback) where callback is of the form (error, result) => ()). For example

const content = yield io.cps(readFile, '/path/to/file')

and of course you can test it just like you test io.call

import io from 'redux-saga/io' // exposed for testing purposes

const iterator = fetchSaga(io)
assert.deepEqual(iterator.next().value, io.cps(readFile, '/path/to/file') )

#Error handling

You can catch errors inside the Generator using the simple try/catch syntax. In the following example, the Saga catch errors from the api.buyProducts call (i.e. a rejected Promise)

function* checkout(io, getState) {

  while( yield io.take(types.CHECKOUT_REQUEST) ) {
    try {
      const cart = getState().cart
      yield io.call(api.buyProducts, cart)
      yield io.put(actions.checkoutSuccess(cart))
    } catch(error) {
      yield io.put(actions.checkoutFailure(error))
    }
  }
}

#Effect Combinators

The yield statements are great for representing asynchronous control flow in a simple and linear style. But we also need to do things in parallel. We can't simply write

// Wrong, effects will be executed in sequence
const users  = yield io.call(fetch, '/users'),
      repose = yield io.call(fetch, '/repose')

Because the 2nd Effect will not get executed until the first call resolves. Instead we have to write

// correct, effects will get executed in parallel
const [users, repose]  = yield [
  io.call(fetch, '/users'),
  io.call(fetch, '/repose')
]

When we yield an array of effects, the Generator is paused until all the effects are resolved (or as soon as one is rejected, just like specified by the Promise.all method).

Sometimes we also need a behavior similar to Promise.race:getting the first resolved effect from multiple ones. The method io.race offers a declarative way of triggering a race between effects.

The following shows a Saga that triggers a showCongratulation message if the user triggers 3 INCREMENT_COUNTER actions with less than 5 seconds between 2 actions.

function* onBoarding(io) {
  let nbIncrements = 0
  while(nbIncrements < 3) {
    // wait for INCREMENT_COUNTER with a timeout of 5 seconds
    const winner = yield io.race({
      increment : io.take(INCREMENT_COUNTER),
      timeout   : io.call(delay, 5000)
    })

    if(winner.increment)
      nbIncrements++
    else
      nbIncrements = 0
  }

  yield io.put(showCongratulation())
}

Note the Saga has an internal state, but that has nothing to do with the state inside the Store, this is a local state to the Saga function used to control the flow of actions.

If by some means you want to view this state (e.g. shows the user progress) then you have to move it into the store and create a dedicated action/reducer for it

function* onBoarding(io, getState) {

  while( getState().nbIncrements < 3 ) {
    // wait for INCREMENT_COUNTER with a timeout of 5 seconds
    const winner = yield io.race({
      increment : io.take(INCREMENT_COUNTER),
      timeout   : io.call(delay, 5000)
    })

    if(winner.increment)
      yield io.put( actions.incNbIncrements() )
    else
      yield io.put( actions.resetNbIncrements() )
  }

  yield io.put(showCongratulation())
}

#Sequencing Sagas via yield*

You can use the builtin yield* operator to compose multiple sagas in a sequential way. This allows you to sequence your macro-operations in a simple procedural style.

function* playLevelOne(io, getState) { ... }

function* playLevelTwo(io, getState) { ... }

function* playLevelThree(io, getState) { ... }

function* game(io, getState) {

  const score1 = yield* playLevelOne(io, getState)
  io.put(showScore(score1))

  const score2 = yield* playLevelTwo(io, getState)
  io.put(showScore(score2))

  const score3 = yield* playLevelThree(io, getState)
  io.put(showScore(score3))

}

Note that using yield* will cause the JavaScript runtime to flatten the whole sequence. i.e. the resulting iterator (game() return value) will yield all values from the nested iterators. A more powerful alternative is to use the generic middleware composition mechanism.

#Composing Sagas

While using yield* provides an idiomatic way of compositing Sagas. The approach has some limits:

You'll likely to test nested generators separately. This leads to test duplication because when testing the main generator you'll have to iterate again on all the nested generators. This can be achieved by making the nested tests reusable but the tests will still take more time to execute.
More importantly, yield* allows only for sequential composition of generators, you can only yield* to one generator at a time. But there can be use cases when you want to launch multiple operations in parallel. You spawn multiple Sagas in the background and resumes when all the spawned Sagas are done.

The Saga middleware offers an alternative way of composition using the simple yield statement. When yielding a generator (more accurately an iterator) the middleware will convert the resulting sub-iterator into a promise that will resolve to that sub-iterator return value (or a rejected with an eventual error thrown from it).

This effectively lets you compose nested generators with other effects, like future actions, timeouts, ...

For example you may want the user finish some game in a limited amount of time

function* game(io, getState) {

  let finished
  while(!finished) {
    // has to finish in 60 seconds
    const {score, timeout}  = yield io.race({
      score  : play(io, getState),
      timeout : delay(60000)
    })

    if(!timeout) {
      finished = true
      io.put( showScore(score) )
    }
  }

}

Or you may want to spawn multiple Sagas in parallel to monitor user actions and congratulate the user when he accomplishes all the desired tasks.

function* monitorTask1(io, getState) {...}
...

function* game(io, getState) {

  const tasks = yield [ monitorTask1(io, getState), ... ]

  yield io.put( showCongratulation() )
}

#Building from sources

git clone https://github.com/yelouafi/redux-saga.git
cd redux-saga
npm install
npm test

There are 2 examples ported from the Redux repos

Counter example

// build the example
npm run build-counter

// test sample for the generator
npm run test-counter

Shopping Cart example

// build the example
npm run build-shop

// test sample for the generator
npm run test-shop

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