Add eslint-config package

[wormat]

Opening as a draft to get feedback before I add it to all the packages.

This defines a default ESLint config for TS and an additional config for JS-only projects. The idea is we can then update ESLint rules in one place.

I'll add to the UI app in a separate PR because it will need to be published first.

Checklist

• Github project and label have been assigned
• Tests have been added or are unnecessary
• Docs have been updated or are unnecessary
• Preview deployment works (visit the Cloudflare preview URL)
• Manual testing in #product-testing completed or unnecessary

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[nicomiicro]

if we go for typescript only projects can't we include these as dependencies (and the typescript specific ones). Wouldn't that be more convenient?

[wormat]

Unfortunately not (I went down several tunnels in this rabbit hole). First of all plugins and configs are treated very differently and get resolved differently. ESLint says this:

If your shareable config depends on a plugin, you should also specify it as a peerDependency (plugins will be loaded relative to the end user’s project, so the end user is required to install the plugins they need). However, if your shareable config depends on a third-party parser or another shareable config, you can specify these packages as dependencies.

However, we can't actually do that with the prettier config because it's an optional dependency of eslint-plugin-prettier: prettier/eslint-plugin-prettier#451. Which means we have to include it as a dependency of whatever loads the plugin if I understand correctly (also didn't get it to work any other way).

I also couldn't get the TS parser to work as a dependency, and I didn't work out why.

[wormat]

I think they're working on changing all of this but this issue has been open since 2015: eslint/eslint#3458

[nicomiicro]

I have an example of a similar package we used in my previous job, see https://github.com/denis-sokolov/eslint-plugin/blob/master/package.json but I didn't set it up myself so I'm missing the little details

[nicomiicro]

But I see airbnb also lists that peer dependencies step https://github.com/airbnb/javascript/tree/master/packages/eslint-config-airbnb#eslint-config-airbnb-1

[wormat]

This was somewhat helpful: https://zaicevas.com/blog/how-eslint-resolves-plugins-and-shareable-configs
It might well be that it's yarn that won't let us break the rules.

[fisprak]

What if we'll add postinstall script that will automatically install peer dependencies?

[wormat]

As of Yarn v2, Yarn won't run this automatically: https://yarnpkg.com/getting-started/migration#explicitly-call-the-pre-and-post-scripts
But that could be a good thing! I think it's also OK as it is, since we only need to add them once per package.

[wormat]

Oh actually maybe it does do that for dependencies otherwise everything would probably break.

[wormat]

Not sure this is even possible on reflection. The postinstall script will run in the scope of the dependency, but we actually need to add them to the consuming package.

[fisprak]

What if we'll add a postinstall script to that will install peer dependencies of @swim-io/eslint-config? To package.json in @swim-io/eslint-config.

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Just mentioning that there's a single ESLint config in Elastic Kibana. Kibana is probably the biggest open-source TypeScript monorepo.

Same for Vue and Next.js.

[fisprak]

What if we'll add postinstall script that will automatically install peer dependencies?

[nicomiicro]

LGTM 👍