Migrate to ESLint 9 and ESLint flat config

.gitignore

@@ -3,6 +3,8 @@ dist/
 node_modules/
 coverage/
 
+.eslintcache
+
 # The client uses yarn rather than npm to manage the lockfile.
 package-lock.json
 

eslint.config.js

@@ -0,0 +1,94 @@
+import hypothesis from 'eslint-config-hypothesis';
+import globals from 'globals';
+import tseslint from 'typescript-eslint';
+
+export default tseslint.config(
+  {
+    ignores: [
+      '.yalc/**/*',
+      '.yarn/**/*',
+      'build/**/*',
+      'dist/**/*',
+      '**/vendor/**/*.js',
+      '**/coverage/**/*',
+      'docs/_build/*',
+      // TODO - Lint these files
+      'rollup*.config.js',
+    ],
+  },
+  ...hypothesis,
+  ...tseslint.configs.recommended,
+  {
+    rules: {
+      'prefer-arrow-callback': ['error', { allowNamedFunctions: true }],
+      'object-shorthand': ['error', 'properties'],
+
+      // Upgrade TS rules from warning to error.
+      '@typescript-eslint/no-unused-vars': 'error',
+
+      // Disable TS rules that we dislike.
+      '@typescript-eslint/ban-ts-comment': 'off',
+      '@typescript-eslint/no-empty-function': 'off',
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-non-null-assertion': 'off',
+      '@typescript-eslint/no-this-alias': 'off',
+
+      // Enforce consistency in cases where TypeScript supports old and new
+      // syntaxes for the same thing.
+      //
+      // - Require `<var> as <type>` for casts
+      // - Require `import type` for type imports. The corresponding rule for
+      //   exports is not enabled yet because that requires setting up type-aware
+      //   linting.
+      '@typescript-eslint/consistent-type-assertions': 'error',
+      '@typescript-eslint/consistent-type-imports': 'error',
+    },
+    languageOptions: {
+      globals: {
+        chrome: false,
+      },
+    },
+  },
+
+  // Entry points which get loaded as non-module scripts.
+  {
+    files: ['src/unload-client.js'],
+    rules: {
+      strict: ['error', 'global'],
+    },
+    languageOptions: {
+      parserOptions: {
+        sourceType: 'script',
+      },
+    },
+  },
+
+  // CommonJS scripts which run in Node
+  {
+    files: ['tests/karma.config.cjs'],
+    rules: {
+      strict: ['error', 'global'],
+    },
+    languageOptions: {
+      parserOptions: {
+        sourceType: 'script',
+      },
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+
+  // ESM scripts which run in Node
+  {
+    files: ['tools/*.js'],
+    rules: {
+      'no-console': 'off',
+    },
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+);

gulpfile.js

@@ -1,5 +1,3 @@
-/* eslint-disable no-console */
-/* eslint-env node */
 // @ts-nocheck
 
 import { spawn } from 'node:child_process';

package.json

@@ -19,19 +19,18 @@
     "@rollup/plugin-multi-entry": "^6.0.1",
     "@rollup/plugin-node-resolve": "^15.3.0",
     "@types/chrome": "^0.0.277",
-    "@typescript-eslint/eslint-plugin": "^7.4.0",
-    "@typescript-eslint/parser": "^7.4.0",
     "babel-plugin-istanbul": "^7.0.0",
     "babel-plugin-mockable-imports": "^2.0.1",
     "chai": "^5.1.1",
     "chrome-webstore-upload-cli": "^3.3.1",
     "diff": "^7.0.0",
-    "eslint": "^8.57.0",
-    "eslint-config-hypothesis": "^2.6.0",
-    "eslint-plugin-mocha": "^10.4.1",
-    "eslint-plugin-react": "^7.34.1",
-    "eslint-plugin-react-hooks": "^4.6.0",
+    "eslint": "^9.12.0",
+    "eslint-config-hypothesis": "^3.0.0",
+    "eslint-plugin-mocha": "^10.5.0",
+    "eslint-plugin-react": "^7.37.1",
+    "eslint-plugin-react-hooks": "^5.0.0",
     "git-describe": "^4.1.1",
+    "globals": "^15.11.0",
     "gulp": "^5.0.0",
     "hypothesis": "^1.1527.0",
     "is-equal-shallow": "^0.1.3",
@@ -47,10 +46,12 @@
     "rollup": "^4.24.0",
     "sinon": "^19.0.2",
     "typescript": "^5.6.3",
+    "typescript-eslint": "^8.9.0",
     "web-ext": "^7.12.0"
   },
   "browserslist": "chrome 85, firefox 75",
   "scripts": {
+    "lint": "eslint --cache .",
     "test": "rollup -c rollup-tests.config.js && karma start tests/karma.config.cjs --single-run",
     "typecheck": "tsc --build src/tsconfig.json"
   },

tests/karma.config.cjs

@@ -1,7 +1,6 @@
+/* eslint-disable @typescript-eslint/no-require-imports */
 'use strict';
 
-/* global __dirname */
-
 const path = require('path');
 
 module.exports = function (config) {