eslint 9 migration with flat config

[thomasvargiu]

• Migrate to flat config and ES modules
• Migrate to @smarttools/eslint-plugin-rxjs for eslint 9 compatibility

Resolves #10

[thomasvargiu]

@siuvdlec can we release an alpha version then?

[siuvdlec]

@siuvdlec can we release an alpha version then?

yep, 1.0.0-rc.0

[giacomoforlani]

Hi @thomasvargiu and @siuvdlec,
I’ve just pushed some changes to this branch to enable rules that weren’t applied by default. I’ve also installed eslint-plugin-fp-ts@0.4.0, since the previous version isn’t compatible with eslint@9.

One thing I noticed: when trying to use this setup in another project, I had to explicitly define all the ignore paths. With eslint@8, folders like dist, build, and .next were ignored by default.

Do you think it makes sense to include these ignore paths directly in the plugin configuration?

[thomasvargiu]

@giacomoforlani I'm not experiencing something like that. Probably you need to update the command with something like this eslint src test

[giacomoforlani]

Hey @thomasvargiu,

I made a few tweaks to the ESLint config to make it work. Since ESLint 9 doesn’t support the --ext flag anymore, I had to specify the file types directly in the config using files: ["**/*.{ts,tsx}"]. This way, ESLint knows exactly which files to lint without relying on CLI options. Despite this change, I had to update the ignore list anyway by adding "**/.next/**" and "**/*.*js" paths.

Even though .js files aren’t included in the files pattern, ESLint still scans them unless you explicitly ignore them—so I added **/*.*js to avoid unnecessary processing or config warnings.

As for the .next folder: before, ESLint 8 used to ignore dotfolders by default. But with ESLint 9 and the new flat config (eslint.config.js), that doesn’t happen anymore—you have to ignore them manually now.

I also added the --flag v10_config_lookup_from_file option. This changes how ESLint finds its config: instead of starting from the working directory, it starts from the file being linted and walks up the folder tree until it finds a config file.
This works better in monorepos where each package might have its own ESLint setup.

We need to report these adjustments in a migration guide. Other than that, the MR is ready for review, merge, and release by @siuvdlec.