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Testing Elixir (PragProg)
by @whatyouhide and @idlehands
Elixir offers new paradigms, and challenges you to test in unconventional ways. Start with ExUnit: almost everything you need to write tests covering all levels of detail, from unit to integration, but only if you know how to use it to the fullest—we’ll show you how. Explore testing Elixir-specific challenges such as OTP-based modules, asynchronous code, Ecto-based applications, and Phoenix applications. Explore new tools like Mox for mocks and StreamData for property-based testing. Armed with this knowledge, you can create test suites that add value to your production cycle and guard you from regressions.
Write Elixir tests that you can be proud of. Dive into Elixir’s test philosophy and gain mastery over the terminology and concepts that underlie good tests. Create and structure a comprehensive ExUnit test suite, starting from the basics, and build comprehensive test coverage that will provide safety for refactoring and confidence that your code performs as designed. Use tests to make your software more reliable and fault tolerant.
Explore the basic tool set provided by ExUnit and Mix to write and organize your test suite. Test code built around different OTP functionality. Isolate your code through dependency injection and by using Mox. Write comprehensive tests for Ecto projects, covering Ecto as a database tool as well as a standalone data validation tool. Test Phoenix channels from end to end, including authentication and joining topics. Write Phoenix controller tests and understand the concepts of integration testing in Elixir. Learn property-based testing with StreamData from the author who wrote the library.
Code with high confidence that you are getting the most out of your test suite, with the right tools that make testing your code a pleasure and a valuable part of your development cycle.
Don’t forget you can get 35% off the ebook with our Devtalk discount, use code ‘devtalk.com’ 
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kokolegorille
There is a new discount available now… devtalk.com
Update: it is not a link, it is the new discount code!
idlehands
That’s not what I am saying, sorry if there is confusion. We show several ways to isolate code and I specifically mentioned Mox, so if you want me to pick, I’d say we lean more towards London/Mockist but also don’t find value in forcing people to declare their style. Not every problem is the same and I don’t solve them all the same way.
There is plenty in the book about isolating code and you’re likely find that while our definition of a unit is intentionally vague (that section is about thinking about your unit and understanding the concept of a black box), our examples focus on small parts of an application, often just a single module.
As I mentioned, I have a feeling that you might pick up some useful stuff, but my sense is that you’re solid enough in your opinions that you either will know most of it and disagree with plenty.
We really did try to take a fairly neutral approach on a lot of things, though, so if you’re looking for “how do I test X?” you might be able to find answers.
idlehands
Sorry, I never actually answered your question. I personally would likely test the modules but have an endpoint test that also proved everything works all the way through (happy path) and at least one error case. A login endpoint is pretty straight forward but most of the endpoints I’ve ever written have plenty of branching possibilities below the hood. So, instead of writing outside tests to hit every one of those branches, I isolate that code using whatever is the correct tool and exercise the branching logic. Then, the endpoint test would be to make sure that all of the pieces connect.
As far as which tests I start with, it 100% depends on the scenario every time. Sometimes I start really low-level and sometimes I start on the endpoint. The only thing that I’m dogmatic about is that I write my tests first.
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