[diggan]

> rely more on snapshot testing are spot on

Never quite liked "snapshot testing" which I think has a better name under "golden master testing" or similar anyways.

Reason for the dislike, is that it's basically a codified "Trust me bro, it's correct" without actually making clear what you are asserting with that test. I haven't found any team that used snapshot testing and didn't also need to change the snapshots for every little change, which obviously defeats the purpose.

The only things snapshot testing seems to be good for, is when you've written something and you know it'll never change again, for any reason. Beyond that, unit tests and functional/integration tests are much easier to structure in a way so you don't waste so much time reviewing changes.

[hansvm]

The purpose of snapshot testing is to not have observable changes if you think there shouldn't be observable changes. To that end, a pattern I like is:

- Don't store/commit the snapshot and have an "update" command. Your CI/CD should run both versions of the software and diff them. That eliminates a lot of the toil.

- You should have a completely trivial way to mark that a given PR intends to have observable changes. That could be a tag on GitHub, a square-bracket thing in a commit message, etc. Details don't matter a ton. The point is that the test just catches if things have changed, and a person still needs to determine if that change is appropriate, but that happens often enough that you should make that process easy.

- Culturally, you should split out PRs which change golden-affecting behavior from those which don't. A bundle of bug fixes, style changes, and a couple new features is not a good thing to commit to the repo as a whole.

The net effect is that:

1. Performance improvements, unrelated features, etc are tested exactly as you expect. If your perf enhancement changes behavior, that was likely wrong, and the test caught it. If it doesn't, the test gives you confidence that it really doesn't.

2. Legitimate changes to the golden behavior are easy to institute. Just toggle a flag somewhere and say that you do intend for there to be a new button or new struct field or whatever you're testing.

3. You have a historical record of the commits which actually changed that behavior, and because of the cultural shift I proposed they're all small changes. Bisecting or otherwise diagnosing tricky prod bugs becomes trivial.

[MoreQARespect]

>I haven't found any team that used snapshot testing and didn't also need to change the snapshots for every little change, which obviously defeats the purpose

I dont see how this even defeats the point, let alone obviously.

If a UI changes I appreciate being notified. If a REST API response changes I like to see a diff.

If somebody changes some CSS and it changes 50 snapshots, it isnt a huge burden to approve them all and sometimes it highlights a bug.

[diggan]

> I dont see how this even defeats the point, let alone obviously.

You generally don't want to have to change all the tests for every change, particularly implementation details. Usually when people do snapshot testing of for example UI components, they serialize the entire component then assert the full component is the same as the snapshot, so any change requires the snapshot to be updated.

> If somebody changes some CSS and it changes 50 snapshots, it isnt a huge burden to approve them all and sometimes it highlights a bug.

Lets say person A initially created all these snapshots, person B did a change that shows 50 snapshots changed, who's responsibility is it to make sure the snapshots are correct? Person A doesn't have the context of the change, so less ideal. Person B doesn't know the initial conditions Person A had in mind, so also less ideal.

When you have unit tests and functional tests you can read through the test and know what the person who wrote it wanted to test. With snapshots, you only know that "This was good", sometimes only with the context of the name of the test itself, but no assertions you can read and say "Ah, X still shows Y so all good".

[MoreQARespect]

>You generally don't want to have to change all the tests for every change

You generally don't want every change to result in a lot of work. If changing a lot of tests means looking at a table of 30 images and diffs, scanning for problems and clicking an "approve button", that isn't a lot of work though.

>Lets say person A initially created all these snapshots, person B did a change that shows 50 snapshots changed, who's responsibility is it to make sure the snapshots are correct?

The person who made the change.

>Person B doesn't know the initial conditions Person A had in mind, so also less ideal.

Yes they will because the initial conditions also had a snapshot attached. If your snapshot testing is even mildly fancy it will come with a diff too.

>When you have unit tests and functional tests you can read through the test and know what the person who wrote it wanted to test. With snapshots, you only know that "This was good",

If you made a change and you can see the previous snapshot, current snapshot and a diff and you never know if the change was ok then you probably shouldn't be working on the project in the first place.

And no, the same isn't necessarily true of unit or functional tests - I've seen hundreds of unit tests that assert things about objects and properties which are tangentially related to the end user and come with zero context attached and I have to try and figure out wtf the test writer meant by "assert xyz_obj.transitional is None". With a user facing snapshot it's obvious.

[tharkun__]

No regular developer will carefully review 50 changed snapshots. They'll stop doing a proper job of it after the third or fourth that looks like the same trivial unimportant change and miss the bug found by snapshot 37.

I do agree that a lot of people write bad tests meaning the test name does not properly describe what the test is supposed to be about so I can check the test implementation and assertions against intent. They also like you say assert on superfluous things.

The problem with snapshots is that it's doing the exact same thing. It asserts on lots of completely unimportant stuff. Unlike proper unit tests however I can't make it better. In a unit test I can make an effort to educate my peers and personally do a good job of only asserting relevant things and writing individual tests where the test name explains why "transitional has to be None".

Snapshots are a blunt no-effort tool for the lazy dev that then later requires constant vigilence and overcoming things humans are bad at (like carefully checking 50 snapshots) by many different humans VS a unit test that I can make good and easy to comprehend and check by putting in effort once when I write it. A good one will also be easy to adjust when needed if it comes time to actually need to change an assertion.

[MoreQARespect]

>No regular developer will carefully review 50 changed snapshots.

If you follow a habit of making small, incremental changes in your pull requests (which you should anyway), those 50 snapshots will generally all change in the same way. A glance across all of them to see that (for example) a box moved to the left in all of them.

>The problem with snapshots is that it's doing the exact same thing. It asserts on lots of completely unimportant stuff.

A problem more than made up for by the fact that rewriting it, eyeballing it and approving it is very quick.

>Unlike proper unit tests however I can't make it better

If the "then" part of a unit test "expects" a particular error message or json snippet, for instance, it's a giant waste of time to craft the expected message yourself if the test can simply snapshot the text from the code and you can eyeball it to see if the diff looks correct.

I have written thousands of unit tests like this and saved a ton of time doing so. I've also worked with devs who did it your way (e.g. craft a whole or part of the json output and put it in the assert) and the only difference was that they took longer.

>and personally do a good job of only asserting relevant things

If, for example, you're testing a dashboard and the spec is a hand scribbled design that shows roughly what it looks like, all the other ways to assert the relevant things are vastly more expensive and impractical to implement.

In practice most devs (and you too i expect) will simply leave the design untested and design regressions by, say, some gnarly css issue would be undetected except by manual test.

>Snapshots are a blunt no-effort tool for the lazy dev

95% of devs don't use snapshot tests because they're super sensitive to flakiness and because ruthlessly eliminating flakiness from code and tests requires engineering discipline and ability most devs don't have.

For those who can, it massively accelerates test writing.

[tharkun__]

Regrettably we still have some snapshot tests in our code base, yes. I cringe every time one goes red and I'm supposed to check them. Like you say, I eyeball them and after the fourth that's the same pattern I give up and just regenerate them. Meaning they might as well not be there coz they won't catch any actual bugs for me.

We try to replace them every time we come across one that needed adjusting actually. Quick is bad here. And yes they're flaky as hell if you use them for everything. Even a tiny change to just introduce a new element that's supposed to be there can change unrelated parts of snapshots because of generated names in many places.

Asserting on the important parts of some json output is not generally more expensive at all. You let the code run to generate the equivalent of a snapshot and then paste it in the assertion(s) and adjust as necessary. Yes it takes more time than a snapshot. But optimizing for time at that end is the wrong optimization. You're optimizing one devs time expenditure while making both the reviewers' and later devs' and reviewers' time expenditure larger (if they want to do a proper job instead of eyeballing and then YOLOing it).

As I see it, devs using snapshots are the opposite of a 10x dev. It's being a 0.1x dev. Thanks but no thanks.

[diggan]

> scanning for problems and clicking an "approve button", that isn't a lot of work though.

But you're actually mentally listing requirements for each one of those snapshots you check, which hopefully is the same as the previous person who run it, but who knows?

> Yes they will because the initial conditions also had a snapshot attached. If your snapshot testing is even mildly fancy it will come with a diff too.

Maybe I didn't explain properly. Say I create a component, and use snapshot testing to verify that "This is how it should look". Now next person changes something that makes that snapshot "old", and the person needs to look at the diff and new component, and say "Yeah, this is now how it should look". But there is a lot of things that are implicitly correct in that situation, instead of explicitly correct. How can we be sure the next person is mentally checking the same requirements as I did?

> If you made a change and you can see the previous snapshot, current snapshot and a diff and you never know if the change was ok then you probably shouldn't be working on the project in the first place.

It seems to work fine for very small and obvious things, but for people make changes that affect a larger part of the codebase (which happens from time to time if you're multiple people working on a big codebase), it's hard to needing to implicitly understand what's correct everywhere. That's why unit/functional tests are so helpful, they're telling us what results we should expect, explicitly.

> I've seen hundreds of unit tests that [...] With a user facing snapshot it's obvious.

I agree that people generally don't treat test code with as much thought as other "production" code, which is a shame I suppose. I guess we need to compare "well done snapshot testing" with "well done unit/functional testing" for it to be a fair comparison.

For that last part, I guess we're just gonna have to agree to disagree, most snapshot test cases I come across aren't obvious at all.

[MoreQARespect]

>Maybe I didn't explain properly. Say I create a component, and use snapshot testing to verify that "This is how it should look". Now next person changes something that makes that snapshot "old", and the person needs to look at the diff and new component, and say "Yeah, this is now how it should look". But there is a lot of things that are implicitly correct in that situation, instead of explicitly correct. How can we be sure the next person is mentally checking the same requirements as I did?

This is a problem that applies equally to code and all other tests to an even greater extent. For them it is a problem dealt with with code reviews.

The great thing about snapshots which doesn't apply to code and functional tests is that they can be reviewed by UX and the PM as well. In this respect they are actually more valuable - PM and UX can spot issues in the PR. The PM doesn't spot when you made a mistake interpreting the requirements that is only visible in the code of the functional test or when it's something the functional test didn't check.

>It seems to work fine for very small and obvious things, but for people make changes that affect a larger part of the codebase (which happens from time to time if you're multiple people working on a big codebase), it's hard to needing to implicitly understand what's correct everywhere

It should not be hard to ascertain if what you see is what should be expected. E.g. if text disappears from a window where there was text before and you only made a styling change then that's obviously a bug.

>I guess we need to compare "well done snapshot testing" with "well done unit/functional testing"

Snapshots are not a replacement for functional tests they are a part of good functional tests. You can write a functional test that logs in and cheaply checks some arbitrary quality of a dashboard (e.g. the div it goes in exists) or you can write a functional test that cheaply snapshots the dashboard.

The latter functional test can give confidence that nothing broke when you refactor the code underneath it and the snapshot hasn't changed. The former can give you confidence that there is still a div present where the dashboard was before. This is significantly less useful.

[necovek]

Are you really going to be reviewing all those 50 snapshots carefully?

The bound of testing on the "other side" is to test just enough not to increase the maintenance burden too much.

[parthdesai]

In the talk, he also talks about passing a flag which would actually update the snapshots/golden files if needed