🛠 Repo: Bump eslint from 8 to 9

[JoshuaKGoldberg]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Mocha has a devDependency right now on a 2-year-old, deprecated version of eslint:

mocha/package.json

Line 139 in b2985b3

"eslint": "^8.56.0",

This isn't a problem on its own. eslint@8 works as-is. But it pulls in old dependencies that bloat our local node_modules size. For example, in #5484, we see it pulling in strip-json-comments@3:

$ npm ls strip-json-comments@3
mocha@11.7.4 /Users/josh/repos/mocha
├─┬ eslint@8.56.0
│ └─┬ @eslint/eslintrc@2.1.4
│   └── strip-json-comments@3.1.1
├─┬ jsdoc@3.6.11
│ └── strip-json-comments@3.1.1
└─┬ markdownlint-cli@0.30.0
  └─┬ run-con@1.2.11
    └── strip-json-comments@3.1.1

I think it'd be a good idea in general to bump it to the latest major version.

Additional Info

In theory it'd be nice to automatically upgrade all devDependencies. #5055 is blocked for now: #5055 (comment).

Also note that this issue is not about significantly changing the list of rules we use. It's just about bumping the version of ESLint (and, if necessary, plugins / other related devDependencies). Rule & plugin changes are tracked separately.

See also ESLint 9 Migration Guide

💖

[Lightning00Blade]

Hey @JoshuaKGoldberg,

If you have not started working on this (given the above PR in progress), I would be interested in helping out, considering I have done that migration for two other repos already.

[JoshuaKGoldberg]

Hey! I haven't done anything with this. The only blocker to a PR is that this is still status: in triage, waiting for other Mocha team members to confirm we're open to this.

[Lightning00Blade]

Hey,

Just a quick update and a FYI regarding this it seems that to do this rollup needs to be update first,
or at least remove rollup-plugin-node-globals and use rollup-plugin-polyfill-node only,

I did make it work here - Lightning00Blade#1,
but it seems I need to fix my local checkout formatting and git settings.

[JoshuaKGoldberg]

@Lightning00Blade forgive my not diving deep personally, but - why does rollup need to be updated / or at least the plugins switched first?

[Lightning00Blade]

No worries @JoshuaKGoldberg, I already have the context so it easy for me.

So EsLint v9 no correctly reports ingored in try {} catch (ignored) {}. The issue is that rollup-plugin-node-global and Rollup v2 (If I recall correctly) don't parse the try {} catch {} at all and throw errors.
rollup-plugin-node-global does not have a version that works with Rollup v3 so using rollup-plugin-polyfill-node unconditionally (see linked PR) migrates that.

All an all we can just migrate to v3 to rollup and to all new version of the plugins but v4 update is seamless, so when with it.

[JoshuaKGoldberg]

Oh, I think I see: you're referring to the no-unused-vars caughtErrors option (https://eslint.org/docs/latest/rules/no-unused-vars#caughterrors)? That option's default changed from "none" to "all" in ESLint v9: https://eslint.org/docs/latest/use/migrate-to-9.0.0#-no-unused-vars-now-defaults-caughterrors-to-all. We can always manually set that option to 'none' pending tooling such as Rollup supporting the new syntax.

I suppose it makes sense to update Rollup in general, but it doesn't block ESLint 9.