I guess I've been on a bit of a testing kick recently, I promise this newsletterwill delve into more stuff later. But I sawthis tweet fromJustin Searls (a developer who I admire with agreat deal of experience in testing) and thought it would be a great subject towrite about.
In that tweet, Justin shares a screenshot of a bunch of his thoughts on snapshottesting. For the sake of accessibility, I've typed out the entirety of what hewrote below. I'll then talk about some of my thoughts on what he says.
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They are tests you don't understand, so when they fail, you don't usuallyunderstand why or how to fix it. That means you have to do true/false negativeanalysis & then suffer indirection as you debug how to resolve the issue
Good tests encode the developer's intention, they don't only lock in thetest's behavior without editorialization of what's important and why. Snapshottests lack (or at least, fail to encourage) expressing the author's intent asto what the code does (much less why)
They are generated files, and developers tend to be undisciplined aboutscrutinizing generated files before committing them, if not at first thendefinitely over time. Most developers, upon seeing a snapshot test fail, willsooner just nuke the snapshot and record a fresh passing one instead ofagonizing over what broke it.
Because they're more integrated and try to serialize an incomplete system(e.g. one with some kind of side effects: from browser/library/runtimeversions to environment to database/API changes), they will tend to have highfalse-negatives (failing test for which the production code is actually fineand the test just needs to be changed). False negatives quickly erode theteam's trust in a test to actually find bugs and instead come to be seen as achore on a checklist they need to satisfy before they can move on to the nextthing.
Instead, when the code changes, the tests will surely fail, but determiningwhether and what is actually "broken" by that failure is the more painful paththan simply re-recording & committing a fresh snapshot. (After all, it's notlike the past snapshot was well understood or carefully expressed authorialintent.) As a result, if a snapshot test fails because some intended behaviordisappeared, then there's little stated intention describing it and we'd muchrather regenerate the file than spend a lot of time agonizing over how to getthe same test green again.
If you know me, you'll know that I really appreciate theJest's snapshot testing feature.(For those of you subscribed on egghead.iowatch this).That said, I share Justin's feelings about them on many levels. The quote abovefrom Justin is just full of golden insights that we should not ignore. I'vepersonally experienced many of the pitfalls with snapshot testing that Justincalls out (with myself and with others). So thanks for sharing your thoughtsJustin!
One thing I want to make clear before continuing is that snapshot testing is anassertion, just like the toBe in: expect('foo').toBe('foo'). I think there'ssometimes confusion on this point, so I just wanted to clear that up.
Despite Justin's arguments against snapshots, I'd suggest that there is value inthem if you use them effectively. With that in mind, I thought I'd share a fewcases where snapshot testing really shines, things to avoid with snapshots, andthings you can do to make your snapshots more effective:
If you're writing a tool for developers, it's a really common case that you wantto write a test to ensure that a good error or warning message is logged to theconsole for the developers using your tool. Before snapshot testing I wouldalways write a silly regex that got the basic gist of what the message shouldsay, but with snapshot testing it's so much easier.
I honestly don't know how I'd test babel plugins with anything but Jestsnapshot testing. It would be prohibitively difficult to attempt asserting onthe resulting AST. babel-plugin-tester uses snapshots for its assertion andthey're fantastic. They avoid a lot of the pitfalls that Justin mentions becauseof the way the results are serialized. Here'san examplefrom import-all.macro:
Have you ever shipped code that busted your app user experience because stylingwasn't applied properly? I have. Writing tests to ensure this kind of confidenceis really difficult. Even E2E tests can't reliably test this kind of thing.There are tools that will take visual snapshots and do visual diff comparisons.But these tools are difficult to set up, run, and are often quite flaky. On topof that, they're basically snapshot tests, so they suffer from many of the samethings Justin calls out about snapshot tests too!
That said, we still get these bugs and it'd be nice to avoid them. If you'reusing CSS-in-JS, there's a great way to use snapshot testing to reduce some ofthe difficulty of testing these kinds of changes. If you use a tool likejest-glamor-reactthen you can include the applicable CSS with whatever yourendered.For example:
This is probably the biggest cause for all the things that Justin's talkingabout. When your snapshot is more than a few dozen lines it's going to suffermajor maintenance issues and slow you and your team down. Remember that testsare all about giving you confidence that you wont ship things that are brokenand you're not going to be able to ensure that very well if you have hugesnapshots that nobody will review carefully. I've personally experienced thiswith a snapshot that's over 640 lines long. Nobody reviews it, the only careanyone puts into it is to nuke it and retake it whenever there's a change (likeJustin mentioned).
So, avoid huge snapshots and take smaller, more focused ones. While you're atit, see if you can actually change it from a snapshot to a more explicitassertion (because you probably can ?).
I should add that even huge snapshots aren't entirely useless. Because if thesnapshot changes unexpectedly it can (and has) inform us that we've made achange with further reaching impacts than anticipated.
jest-glamor-react is in fact a custom serializer. This has made our snapshotsmuch more effective. Writing a custom serializer is actually quite simple.Here's one that I wrotewhich normalizes paths so any path in the snapshot is relative to the projectdirectory and looks the same on windows and mac:
One of the most useful things that I've found with test maintainability is whenyou have many tests that look the same, try to make their differences stand out.This makes it easier for people coming into your codebase to know what theimportant pieces are. So you can try to split out the common setup/teardown intoa small helper function to make the test have more differences and fewercommonalities to each other.
I've seen some tests where you take one snapshot of a react component before theuser interacts and another after the user interacts. What you're trying toassert on is the difference between the before and after, but you get much morethan you bargain for and this results in more false negatives that Justin'stalking about. However, if you could serialize the difference between the twostates, that would be much more helpful. And that's what snapshot-diff can dofor you:
A typical snapshot test case renders a UI component, takes a snapshot, then compares it to a reference snapshot file stored alongside the test. The test will fail if the two snapshots do not match: either the change is unexpected, or the reference snapshot needs to be updated to the new version of the UI component.
A similar approach can be taken when it comes to testing your React components. Instead of rendering the graphical UI, which would require building the entire app, you can use a test renderer to quickly generate a serializable value for your React tree. Consider this example test for a Link component:
The snapshot artifact should be committed alongside code changes, and reviewed as part of your code review process. Jest uses pretty-format to make snapshots human-readable during code review. On subsequent test runs, Jest will compare the rendered output with the previous snapshot. If they match, the test will pass. If they don't match, either the test runner found a bug in your code (in the component in this case) that should be fixed, or the implementation has changed and the snapshot needs to be updated.
More information on how snapshot testing works and why we built it can be found on the release blog post. We recommend reading this blog post to get a good sense of when you should use snapshot testing. We also recommend watching this egghead video on Snapshot Testing with Jest.
It's straightforward to spot when a snapshot test fails after a bug has been introduced. When that happens, go ahead and fix the issue and make sure your snapshot tests are passing again. Now, let's talk about the case when a snapshot test is failing due to an intentional implementation change.
Since we just updated our component to point to a different address, it's reasonable to expect changes in the snapshot for this component. Our snapshot test case is failing because the snapshot for our updated component no longer matches the snapshot artifact for this test case.
Go ahead and accept the changes by running the above command. You may also use the equivalent single-character -u flag to re-generate snapshots if you prefer. This will re-generate snapshot artifacts for all failing snapshot tests. If we had any additional failing snapshot tests due to an unintentional bug, we would need to fix the bug before re-generating snapshots to avoid recording snapshots of the buggy behavior.
Inline snapshots behave identically to external snapshots (.snap files), except the snapshot values are written automatically back into the source code. This means you can get the benefits of automatically generated snapshots without having to switch to an external file to make sure the correct value was written.
By default, Jest handles the writing of snapshots into your source code. However, if you're using prettier in your project, Jest will detect this and delegate the work to prettier instead (including honoring your configuration). 152ee80cbc
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