Jest: Use difftool to resolve snapshot differences

[JakeGinnivan]

I maintain an assertion library for .net which has a very similar feature to snapshot testing in Jest and have been a huge fan of this style of testing for ages. The docs for the feature is at http://shouldly.readthedocs.io/en/latest/assertions/shouldMatchApproved.html, I took inspiration from ApprovalTests which also has a NodeJS version. https://github.com/approvals/Approvals.NodeJS

For Jest, in addition to the update snapshot parameter it would be great to have a --resolve or --merge switch which would write out a .received.snap file next to the actual then launch a difftool in merge mode, the user can easily merge the received.snap file into the .snap file. Shouldly and ApprovalTests just have a collection of supported difftools and will simply use the first one it finds.

I started to spike this out tonight as an extension to expect (expect-jest-snapshot), then I noticed #1679 so I thought I would hold off and start a conversation about the idea first. What do you think?

[gertjvr]

I have used both ApprovalTests and shouldly and this style of testing is great when your dealing with message contracts or api responses, but the use-cases doesn't stop there. making use of visual compare tool would be an amazing feature.

[cpojer]

We were hoping to build an interactive mode for Jest where it will ask to update each snapshot individually. Would that solve your problem?

[JakeGinnivan]

It would, I wonder if just using diff tools would be a more flexible first pass? How would it work?

[thymikee]

@JakeGinnivan since Jest 17 larger diffs are shown in a form of a patch, so I think we're on a good track.
@cpojer Should we create a separate issue for interactive snapshots update mode and maybe continue there?

[cpojer]

Yeah, let's put that in a separate issue. It's on our plans for Jest but I don't know if we'll get to it anytime soon.

[JakeGinnivan]

That patch form is awesome!

I still think just launching a difftool would be a really simple way to do interactive snapshot updates for the time being. They already do everything you need very well, and people have their preferred tools to do merging.

[TazmanianD]

@cpojer Did this issue go anywhere? It seems like it got closed but with no resolution. I want to second the OP's original request to provide a mechanism for using an external diff tool to resolve snapshot failures. I personally hate the unified style diffs that we get now when there are some great diff tools like Beyond Compare that can make it much easier to see what changed.

[SimenB]

Interactive snapshots are available: https://facebook.github.io/jest/docs/en/snapshot-testing.html#interactive-snapshot-mode

[TazmanianD]

@SimenB Thanks for that but that doesn't quite answer my question. The request is for making use of an external diff tool, not just a CLI based interactive tool.

[flcdrg]

@JakeGinnivan did you ever get something working with Beyond Compare (or similar)?

[JakeGinnivan]

Never dug in and sorted it out unfortunately @flcdrg