Use flat config for the project's eslint configuration (#615)

* chore: use flat config for the project's eslint configuration

This shall let us dogfood the recommended eslint config when we add it

* chore: skip perf step in node 14 as eslint plugin isn't compatible with v14

Author: BPScott

.eslintignore

@@ -36,4 +36,7 @@ jobs:
         run: pnpm mocha
 
       - name: Perf
+        # Skip on node 14, as eslint-plugin-n contains syntax that doesn't work
+        # with node 14
+        if: ${{ matrix.node-version != 14 }}
         run: TIMING=1 pnpm lint

.eslintrc

@@ -0,0 +1,52 @@
+const { configs: eslintConfigs } = require('@eslint/js');
+const eslintPluginN = require('eslint-plugin-n');
+const eslintPluginEslintComments = require('@eslint-community/eslint-plugin-eslint-comments');
+const eslintPluginEslintPluginRecommended = require('eslint-plugin-eslint-plugin/configs/recommended');
+const eslintPluginMdx = require('eslint-plugin-mdx');
+const eslintConfigPrettier = require('eslint-config-prettier');
+const eslintPluginPrettier = require('./eslint-plugin-prettier');
+
+module.exports = [
+  eslintConfigs.recommended,
+  eslintPluginN.configs['flat/recommended'],
+  // eslint-community/eslint-comments does not expose a reusable flat config,
+  // so create one from its legacy config
+  {
+    plugins: {
+      '@eslint-community/eslint-comments': eslintPluginEslintComments,
+    },
+    rules: eslintPluginEslintComments.configs.recommended.rules,
+  },
+  eslintPluginEslintPluginRecommended,
+  eslintPluginMdx.flat,
+  eslintPluginMdx.flatCodeBlocks,
+  eslintConfigPrettier,
+  // No built-in flat recommended config yet
+  {
+    plugins: { prettier: eslintPluginPrettier },
+    rules: eslintPluginPrettier.configs.recommended.rules,
+  },
+  {
+    rules: {
+      'eslint-plugin/report-message-format': ['error', '^[^a-z].*\\.$'],
+    },
+  },
+  {
+    files: ['**/*.md'],
+    rules: { 'prettier/prettier': ['error', { parser: 'markdown' }] },
+  },
+  {
+    files: ['**/*.mdx'],
+    rules: { 'prettier/prettier': ['error', { parser: 'mdx' }] },
+  },
+  // Global ignores
+  // If a config block only contains an `ignores` key, then the globs are
+  // ignored globally
+  {
+    ignores: [
+      'CHANGELOG.md',
+      '.github/ISSUE_TEMPLATE.md',
+      'test/fixtures/**/*',
+    ],
+  },
+];

.github/workflows/ci.yml

@@ -59,6 +59,7 @@
     "@changesets/cli": "^2.26.2",
     "@commitlint/config-conventional": "^18.4.3",
     "@eslint-community/eslint-plugin-eslint-comments": "^4.1.0",
+    "@eslint/js": "^8.55.0",
     "@graphql-eslint/eslint-plugin": "^3.20.0",
     "@types/eslint": "^8.44.7",
     "@types/prettier-linter-helpers": "^1.0.1",
@@ -68,8 +69,8 @@
     "eslint-formatter-friendly": "^7.0.0",
     "eslint-mdx": "^2.1.0",
     "eslint-plugin-eslint-plugin": "^5.1.0",
-    "eslint-plugin-mdx": "^2.1.0",
-    "eslint-plugin-n": "^16.0.1",
+    "eslint-plugin-mdx": "^2.2.0",
+    "eslint-plugin-n": "^16.3.1",
     "eslint-plugin-prettier": "link:.",
     "eslint-plugin-svelte": "^2.32.2",
     "eslint-plugin-svelte3": "^4.0.0",