Migrate ESLint to flat config so ESLint 9 can land - #1325
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Replace `.eslintrc` and `.eslintignore` with a single `eslint.config.js`,
porting the existing extends, plugins, parser, env, rules and settings to
their flat-config equivalents at the already-installed plugin versions:
- `eslint:recommended` -> `@eslint/js` `js.configs.recommended`
- `plugin:jest/recommended` -> `configs['flat/recommended']`
- `plugin:unicorn/recommended` -> `configs['flat/recommended']`
- `plugin:react/recommended` -> `configs.flat.recommended`
- `plugin:@typescript-eslint/recommended` -> `configs['flat/recommended']`
- `plugin:prettier/recommended` -> `eslint-plugin-prettier/recommended`
`env: { node, browser, es6 }` becomes `languageOptions.globals`, and the
seven `.eslintignore` entries become an `ignores` block.
Bump `eslint` to ^9, promote `@eslint/js` to a direct devDependency, and
add `globals`.
The migration is behaviour-preserving -- `pnpm run lint` reports the same
(zero) problems before and after across the same files:
- Flat config lints dotfiles that `.eslintrc` skipped by default, so the
jsdom stub `.empty_module.js` is added to `ignores` to stay unlinted.
- Flat config defaults `reportUnusedDisableDirectives` to "warn"; pin it
back to "off" to match the previous default.
- Drop the now-unsupported `/* eslint-env es2020 */` comment from
`jest.setup.js`; its globals are covered by `languageOptions.globals`.
`pnpm run lint` and `pnpm test` (43 suites / 903 passed) are green on
ESLint 9.
tbsvttr
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The ESLint major hold was added in boardgameio#1323 because .eslintrc could not survive ESLint 9, and boardgameio#1324 asked for it to come out once flat config landed. boardgameio#1325 landed flat config, but taking the hold out now would let ESLint 10 in, and that is a harder stop than the one it replaces: eslint-plugin-react 7.37.5 is the plugin's latest release, peers `^9.7`, and throws on 10 — TypeError: Error while loading rule 'react/display-name': contextOrFilename.getFilename is not a function So the hold stays and its rationale is rewritten to say what actually blocks it, and what would unblock it. Two neighbours of eslint were never covered and can each redden the weekly grouped pull request on their own, since a group is only as mergeable as its worst member: * @eslint/js 10.0.1 resolves against eslint 9 without complaint, then turns on rules 9 ships no baseline for — `eslint .` goes from 0 problems to 7 (preserve-caught-error x4, no-useless-assignment x3). * eslint-plugin-unicorn 65.0.0, the highest that resolves against ESLint 9, reports 24 problems. Several autofixes are not cosmetic: unicorn/prefer-https rewrites `http://example.com` to `https://` inside the CORS fixtures whose tests assert those origins are rejected, and unicorn/prefer-queue-microtask swaps `process.nextTick` for `queueMicrotask`, which schedules differently. Both are deliberate, documented holds with an exit condition rather than silent pins.
tbsvttr
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Aug 18, 2026
boardgameio#1325 deleted .eslintrc, but the commands section still sends agents there to find out how linting is configured.
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Closes #1324.
pnpm run lintstill ran ESLint 8.57.1, which is no longer supported, and #1302 (the Dependabot bump to ESLint 9) cannot land because ESLint 9 removed.eslintrcand.eslintignore.This replaces both with a single
eslint.config.js, porting the existingextends,plugins,parser,env,rulesandsettingsto their flat-config equivalents at the plugin versions already installed — nothing needed upgrading first.eslintmoves to^9,@eslint/jsis promoted from a transitive dependency to a direct devDependency, andglobalsis added.Behaviour is preserved
pnpm run lintreports zero problems both before and after, over the same files:.eslintrc.js, 99.ts, 9.tsx)eslint.config.js.js, 1.cjs, 99.ts, 9.tsx)No file loses coverage. The two added are
eslint.config.jsitself andjest-svelte-transformer.cjs— flat config lints.cjsby default, where ESLint 8'seslint .did not.Three deliberate adjustments keep the reported problem set unchanged:
.eslintrcskipped by default, so the jsdom stub.empty_module.jsis added toignores.reportUnusedDisableDirectivestowarn; it is pinned back tooff./* eslint-env es2020 */is no longer supported, so it is dropped fromjest.setup.js— those globals now come fromlanguageOptions.globals.Testing
pnpm run lintis green, andpnpm testpasses 43 suites / 903 tests (1 todo).