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Testing Sagas

There are two main ways to test Sagas: testing the saga generator function step-by-step or running the full saga and asserting the side effects.

Testing the Saga Generator Function

Suppose we have the following actions:

const CHOOSE_COLOR = 'CHOOSE_COLOR';
const CHANGE_UI = 'CHANGE_UI';

const chooseColor = (color) => ({
  type: CHOOSE_COLOR,
  payload: {
    color,
  },
});

const changeUI = (color) => ({
  type: CHANGE_UI,
  payload: {
    color,
  },
});

We want to test the saga:

function* changeColorSaga() {
  const action = yield take(CHOOSE_COLOR);
  yield put(changeUI(action.payload.color));
}

Since Sagas always yield an Effect, and these effects have basic factory functions (e.g. put, take etc.) a test may inspect the yielded effect and compare it to an expected effect. To get the first yielded value from a saga, call its next().value:

  const gen = changeColorSaga();

  assert.deepEqual(
    gen.next().value,
    take(CHOOSE_COLOR),
    'it should wait for a user to choose a color'
  );

A value must then be returned to assign to the action constant, which is used for the argument to the put effect:

  const color = 'red';
  assert.deepEqual(
    gen.next(chooseColor(color)).value,
    put(changeUI(color)),
    'it should dispatch an action to change the ui'
  );

Since there are no more yields, then next time next() is called, the generator will be done:

  assert.deepEqual(
    gen.next().done,
    true,
    'it should be done'
  );

Branching Saga

Sometimes your saga will have different outcomes. To test the different branches without repeating all the steps that lead to it you can use the utility function cloneableGenerator

This time we add two new actions, CHOOSE_NUMBER and DO_STUFF, with a related action creators:

const CHOOSE_NUMBER = 'CHOOSE_NUMBER';
const DO_STUFF = 'DO_STUFF';

const chooseNumber = (number) => ({
  type: CHOOSE_NUMBER,
  payload: {
    number,
  },
});

const doStuff = () => ({
  type: DO_STUFF,
});

Now the saga under test will put two DO_STUFF actions before waiting for a CHOOSE_NUMBER action and then putting either changeUI('red') or changeUI('blue'), depending on whether the number is even or odd.

function* doStuffThenChangeColor() {
  yield put(doStuff());
  yield put(doStuff());
  const action = yield take(CHOOSE_NUMBER);
  if (action.payload.number % 2 === 0) {
    yield put(changeUI('red'));
  } else {
    yield put(changeUI('blue'));
  }
}

The test is as follows:

import { put, take } from 'redux-saga/effects';
import { cloneableGenerator } from '@redux-saga/testing-utils';

test('doStuffThenChangeColor', assert => {
  const gen = cloneableGenerator(doStuffThenChangeColor)();
  gen.next(); // DO_STUFF
  gen.next(); // DO_STUFF
  gen.next(); // CHOOSE_NUMBER

  assert.test('user choose an even number', a => {
    // cloning the generator before sending data
    const clone = gen.clone();
    a.deepEqual(
      clone.next(chooseNumber(2)).value,
      put(changeUI('red')),
      'should change the color to red'
    );

    a.equal(
      clone.next().done,
      true,
      'it should be done'
    );

    a.end();
  });

  assert.test('user choose an odd number', a => {
    const clone = gen.clone();
    a.deepEqual(
      clone.next(chooseNumber(3)).value,
      put(changeUI('blue')),
      'should change the color to blue'
    );

    a.equal(
      clone.next().done,
      true,
      'it should be done'
    );

    a.end();
  });
});

See also: Task cancellation for testing fork effects

Testing the full Saga

Although it may be useful to test each step of a saga, in practise this makes for brittle tests. Instead, it may be preferable to run the whole saga and assert that the expected effects have occurred.

Suppose we have a basic saga which calls an HTTP API:

function* callApi(url) {
  const someValue = yield select(somethingFromState);
  try {
    const result = yield call(myApi, url, someValue);
    yield put(success(result.json()));
    return result.status;
  } catch (e) {
    yield put(error(e));
    return -1;
  }
}

We can run the saga with mocked values:

const dispatched = [];

const saga = runSaga({
  dispatch: (action) => dispatched.push(action),
  getState: () => ({ value: 'test' }),
}, callApi, 'http://url');

A test could then be written to assert the dispatched actions and mock calls:

import sinon from 'sinon';
import * as api from './api';

test('callApi', async (assert) => {
  const dispatched = [];
  sinon.stub(api, 'myApi').callsFake(() => ({
    json: () => ({
      some: 'value'
    })
  }));
  const url = 'http://url';
  const result = await runSaga({
    dispatch: (action) => dispatched.push(action),
    getState: () => ({ state: 'test' }),
  }, callApi, url).toPromise();

  assert.true(myApi.calledWith(url, somethingFromState({ state: 'test' })));
  assert.deepEqual(dispatched, [success({ some: 'value' })]);
});

See also: Repository Examples:

https://github.com/redux-saga/redux-saga/blob/master/examples/counter/test/sagas.js

https://github.com/redux-saga/redux-saga/blob/master/examples/shopping-cart/test/sagas.js

Testing libraries

While both of the above testing methods can be written natively, there exist several libraries to make both methods easier. Additionally, some libraries can be used to test sagas in a third way: recording specific side-effects (but not all).

Sam Hogarth's (@sh1989) article summarizes the different options well.

For testing each generator yield step-by-step there is redux-saga-test and redux-saga-testing. redux-saga-test-engine is for recording and testing for specific side effects. For an integration test, redux-saga-tester. And redux-saga-test-plan can actually cover all three bases.

redux-saga-test and redux-saga-testing for step-by-step testing

The redux-saga-test library provides syntactic sugar for your step-by-step tests. The fromGenerator function returns a value that can be iterated manually with .next() and have an assertion made using the relevant saga effect method.

import fromGenerator from 'redux-saga-test';

test('with redux-saga-test', () => {
  const generator = callApi('url');
  /*
  * The assertions passed to fromGenerator
  * requires a `deepEqual` method
  */
  const expect = fromGenerator(assertions, generator);

  expect.next().select(somethingFromState);
  expect.next(selectedData).call(myApi, 'url', selectedData);
  expect.next(result).put(success(result.json));
});

redux-saga-testing library provides a method sagaHelper that takes your generator and returns a value that works a lot like Jest's it() function, but also advances the generator being tested. The result parameter passed into the callback is the value yielded by the generater

import sagaHelper from 'redux-saga-testing';

test('with redux-saga-testing', () => {
  const it = sagaHelper(callApi());

  it('should select from state', selectResult => {
    // with Jest's `expect`
    expect(selectResult).toBe(value);
  });

  it('should select from state', apiResponse => {
    // without tape's `test`
    assert.deepEqual(apiResponse.json(), jsonResponse);
  });

  // an empty call to `it` can be used to skip an effect
  it('', () => {});
});

redux-saga-test-plan

This is the most versatile library. The testSaga API is used for exact order testing and expectSaga is for both recording side-effects and integration testing.

import { expectSaga, testSaga } from 'redux-saga-test-plan';

test('exact order with redux-saga-test-plan', () => {
  return testSaga(callApi, 'url')
    .next()
    .select(selectFromState)
    .next()
    .call(myApi, 'url', valueFromSelect);

    ...
});

test('recorded effects with redux-saga-test-plan', () => {
  /*
  * With expectSaga, you can assert that any yield from
  * your saga occurs as expected, *regardless of order*.
  * You must call .run() at the end.
  */
  return expectSaga(callApi, 'url')
    .put(success(value)) // last effect from our saga, first one tested

    .call(myApi, 'url', value)
    .run();
    /* notice no assertion for the select call */
});

test('test only final effect with .provide()', () => {
  /*
  * With the .provide() method from expectSaga
  * you can by pass in all expected values
  * and test only your saga's final effect.
  */
  return expectSaga(callApi, 'url')
    .provide([
      [select(selectFromState), selectedValue],
      [call(myApi, 'url', selectedValue), response]
    ])
    .put(success(response))
    .run();
});

test('integration test with withReducer', () => {
  /*
  * Using `withReducer` allows you to test
  * the state shape upon completion of your reducer -
  * a true integration test for your Redux store management.
  */

  return expectSaga(callApi, 'url')
    .withReducer(myReducer)
    .provide([
      [call(myApi, 'url', value), response]
    ])
    .hasFinalState({
      data: response
    })
    .run();
});

redux-saga-test-engine

This library functions very similarly in setup to redux-saga-test-plan, but is best used to record effects. Provide a collection of saga generic effects to be watched by createSagaTestEngine function which in turn returns a function. Then provide your saga and specific effects and their arguments.

const collectedEffects  = createSagaTestEngine(['SELECT', 'CALL', 'PUT']);
const actualEffects = collectEffects(mySaga, [ [myEffect(arg), value], ... ], argsToMySaga);

The value of actualEffects is an array containing elements equal to the yielded values from all collected effects, in order of occurence.

import createSagaTestEngine from 'redux-saga-test-engine';

test('testing with redux-saga-test-engine', () => {
  const collectEffects = createSagaTestEngine(['CALL', 'PUT']);

  const actualEffects = collectEffects(
    callApi,
    [
      [select(selectFromState), selectedValue],
      [call(myApi, 'url', selectedValue), response]
    ],
    // Any further args are passed to the saga
    // Here it is our URL, but typically would be the dispatched action
    'url'
  );

  // assert that the effects you care about occurred as expected, in order
  assert.equal(actualEffects[0], call(myApi, 'url', selectedValue));
  assert.equal(actualEffects[1], put(success, response));

  // assert that your saga does nothing unexpected
  assert.true(actualEffects.length === 2);
});

redux-saga-tester

A final library to consider for integration testing. this library provides a sagaTester class, to which you instantiate with your store's initial state and your reducer.

To test your saga, the sagaTester instance start() method with your saga and its argument(s). This runs your saga to its end. Then you may assert that effects occured, actions were dispatched and the state was updated as expected.

import SagaTester from 'redux-saga-tester';

test('with redux-saga-tester', () => {
  const sagaTester = new SagaTester({
    initialState: defaultState,
    reducers: reducer
  });

  sagaTester.start(callApi);

  sagaTester.dispatch(actionToTriggerSaga());

  await sagaTester.waitFor(success);

  assert.true(sagaTester.wasCalled(success(response)));

  assert.deepEqual(sagaTester.getState(), { data: response });
});

effectMiddlwares

Provides a native way to perform integration like testing without one of the above libraries.

The idea is that you can create a real redux store with saga middleware in your test file. The saga middlware takes an object as an argument. That object would have an effectMiddlewares value: a function where you can intercept/hijack any effect and resolve it on your own - passing it very redux-style to the next middleware.

In your test, you would start a saga, intercept/resolve async effects with effectMiddlewares and assert on things like state updates to test integration between your saga and a store.

Here's an example from the docs:

test('effectMiddleware', assert => {
  assert.plan(1);

  let actual = [];

  function rootReducer(state = {}, action) {
    return action;
  }

  const effectMiddleware = next => effect => {
    if (effect === apiCall) {
      Promise.resolve().then(() => next('injected value'));
      return;
    }
    return next(effect);
  };

  const middleware = sagaMiddleware({ effectMiddlewares: [effectMiddleware] });
  const store = createStore(rootReducer, {}, applyMiddleware(middleware));

  const apiCall = call(() => new Promise(() => {}));

  function* root() {
    actual.push(yield all([call(fnA), apiCall]));
  }

  function* fnA() {
    const result = [];
    result.push((yield take('ACTION-1')).val);
    result.push((yield take('ACTION-2')).val);
    return result;
  }

  const task = middleware.run(root)

  Promise.resolve()
    .then(() => store.dispatch({ type: 'ACTION-1', val: 1 }))
    .then(() => store.dispatch({ type: 'ACTION-2', val: 2 }));

  const expected = [[[1, 2], 'injected value']];

  task
    .toPromise()
    .then(() => {
      assert.deepEqual(
        actual,
        expected,
        'effectMiddleware must be able to intercept and resolve effect in a custom way',
      )
    })
    .catch(err => assert.fail(err));
});
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