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The best way to test Redux Sagas

Phil Herbert on December 10, 2018

tl;dr: to test a saga, it's way, way better to run it as a whole (using runSaga()) than to do it step-by-step (using gen.next()) In my team, we're...

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Roy • Mar 6 '19

This is a great article! Thank you for this!

How would you throw exceptions to test for api call failures?

For example, if you had a try / catch block for,

const profile = yield call(getProfile, action.profileId);

When you test the saga as a whole, how do you force an error?

I tried using to mockImplementation to return a new Error(), but that doesn't go into the catch block.

Stefano Magni • Feb 12 '20

I don't know if the APIs are changed, at the moment (Feb, 2020) runSaga returns a Task that has a toPromise() method.
So await runSaga(...).done does not make sense, you need to do

const task = runSaga(...)
const result = await task.toPromise()
expect(result).toStrictEqual({...})

anyway: thank you so much for the article 😊

worc • Dec 10 '18

we have to check that every yield is not a call to getProfile()

i'm not sure that's entirely true. when i've gone down the path of testing sagas step-by-step, i've thrown gen.next()s into the body of the test without any assertions to get to the step i'm actually trying to test. it's not great and it's pretty brittle if you change your order of operations, but it's not silly either. i don't need to assert that the saga isn't calling a function.

Robert Jones • Feb 1 '19

Phil, this is brilliant- thanks for writing the article. I'm trying to (roughly) define a testing approach for sagas that'll encourage a more maintainable test suite, and I think you've just nailed a decent way of going about it without introducing another dependency!