This pull request has been mentioned on Nix community. There might be relevant details there:

https://discourse.nixos.org/t/cypress-ui-testing-issue/2208/3

@acyuta108 Do you have interest in adapting the code from @erosennin as described in the discourse discussion? I think it would be great to get cypress packaged for NixOS.

I am tempted to use https://discourse.nixos.org/t/cypress-ui-testing-issue/2208/5?u=acyuta108 from @thorstenweber83

@acyuta108 That sounds great.

Rebased and upgraded to latest version of Cypress (3.2.0)

#58423

Thanks for your help @thorstenweber83, @erosennin and @aanderse

@acyuta108 I misunderstood the new solution you adopted. The solution @erosenin provided actually included the cypress binary and was reproducible. The solution you have chosen requires more work from the user and is not as reproducible as it depends on user steps and environment variables, correct?

The solution @erosenin provided actually included the cypress binary and was reproducible

Both solutions install the binary and depend on the npm steps. It is the standard way to install cypress in any project/system. I'll explain why.
In fact if you look closely, @erosennin mentions the env vars in his comment as described in my pull request.

The standard way is to npm i cypress which downloads the binary and caches it in the home directory. See here. Then the user runs it (cypress run or cypress open) from their project directory which includes the tests/specs to be run by the app (cypress).
For nix our solution is: The user (pre) installs the cypress binary using nix, and then when the user starts working on her project, she tells npm installer . not to install the binary (CYPRESS_INSTALL_BINARY=0) and 2. run cypress from the nix-installed version, opposed to the one expected to be cached by npm install cypress (CYPRESS_RUN_BINARY=./result/Cypress/Cypress`).

These ENV options are documented by Cypress as well, which I am glad are available, otherwise we would have to inject/override somehow our binary (path).

Ah I see. So could you use makeWrapper to make this seamless to the user?

I'd rather not. I don't see the use and I actually foresee issues.
For a UI developer it is straightforward to set these flags/envs and be aware that they are running the nix installed version of cypress. I am afraid that if we start "tucking under" the details, users might run into trouble when they need to override the settings and/or they might be confused if they use nix package manager along with the regular installed cypress (non-Nixos users) and lead to unexpected behavior.
I think it's better to keep it clean as it is. I can in fact create a pull request in the Cypress repo, to document this simple process for Nix users.

Besides, how would you even achieve that? When you run cypress run, this command calls the cypress in the .node_modules/ folder which in turn decides where and how to run the actual binary. I would have to override the ./node_modules/.bin/cypress to wrap the CYPRESS_RUN_BINARY env.
Basically, it gets very messy.

I've never used cypress so I'll defer to your experience.

My interest in cypress is that I'd like to start using a web testing framework inside nixos tests for web modules to provide more thorough tests. Currently most nixos tests simply ensure a web application can boot without testing functionality of the web application at all.

This sounds achievable with what you've described, right?

Absolutely. Any Nix/NixOS user can now use Cypress which is a pretty fantastic UI and end-to-end testing tool. Specs are written and javascript and run natively on a chrome browser. You can test VueJs (my fav), Reach, Angular and pretty much anything that has a UI that runs on the browser. I've been writing end-to-end specs for many years now and nothing compares to Cypress in speed, reliability and ease of use.