chore: upgrade eslint and fix the current ruleset (#999)

__tests__/eslintConfig.test.ts

@@ -0,0 +1,78 @@
+import { execFileSync } from 'child_process';
+import path from 'path';
+
+const repoRoot = path.resolve(__dirname, '..');
+const eslintBin = path.join(repoRoot, 'node_modules', 'eslint', 'bin', 'eslint.js');
+
+type LintMessage = {
+  ruleId: string | null;
+  message: string;
+  severity: number;
+};
+
+jest.setTimeout(180000);
+
+function lint(code: string, filename: string): LintMessage[] {
+  const stdout = execFileSync(
+    process.execPath,
+    [eslintBin, '--stdin', '--stdin-filename', filename, '--format', 'json'],
+    { cwd: repoRoot, input: code, encoding: 'utf8' }
+  );
+  const [result] = JSON.parse(stdout) as { messages: LintMessage[] }[];
+  return result ? result.messages : [];
+}
+
+const ruleIds = (messages: LintMessage[]) => messages.map((message) => message.ruleId);
+
+describe('ESLint ruleset', () => {
+  it('flags console.log while allowing console.error and console.warn', () => {
+    expect(ruleIds(lint('export function f() {\n  console.log("noise");\n}\n', 'lib/sample.ts'))).toContain(
+      'no-console'
+    );
+
+    for (const method of ['error', 'warn']) {
+      const messages = lint(`export function f() {\n  console.${method}("diagnostic");\n}\n`, 'lib/sample.ts');
+      expect(messages.filter((message) => message.ruleId === 'no-console')).toEqual([]);
+    }
+  });
+
+  it('reports unused TypeScript bindings once, through the TS-aware rule', () => {
+    const messages = lint('export function f() {\n  const unusedValue = 1;\n  return 2;\n}\n', 'lib/sample.ts');
+
+    expect(ruleIds(messages)).toEqual(['@typescript-eslint/no-unused-vars']);
+    expect(ruleIds(messages)).not.toContain('no-unused-vars');
+  });
+
+  it('accepts underscore-prefixed placeholders for arguments, catches and destructuring', () => {
+    const code = [
+      'export function read({ a, b: _b }: { a: number; b: number }, _label: string) {',
+      '  const [, ...rest] = [1, 2, 3];',
+      '  try {',
+      '    return a + rest.length;',
+      '  } catch (_error) {',
+      '    return 0;',
+      '  }',
+      '}',
+    ].join('\n');
+
+    expect(lint(code, 'lib/sample.ts').filter((message) => message.severity === 2)).toEqual([]);
+  });
+
+  it('keeps checked-in and generated build output out of the lint run', () => {
+    expect(
+      lint('const stale = 1;\n', '.next/dev/stale.ts').some(
+        (message) => message.ruleId === null && /ignored/i.test(message.message)
+      )
+    ).toBe(true);
+
+    expect(
+      lint('const stale = 1;\n', 'coverage/report.ts').some((message) =>
+        /ignored/i.test(message.message)
+      )
+    ).toBe(true);
+
+    expect(
+      lint('export const live = 1;\n', 'lib/live.ts').some((message) => /ignored/i.test(message.message))
+    ).toBe(false);
+  });
+});

eslint.config.mjs

@@ -1,6 +1,11 @@
 import coreWebVitals from 'eslint-config-next/core-web-vitals';
 
+// eslint-config-next already ignores `.next`, `out`, `build`, `next-env.d.ts`
+// (and ESLint ignores `node_modules`/`.git`), so only what it misses is listed.
+const generatedOutput = ['coverage/**', '**/*.tsbuildinfo'];
+
 const config = [
+  { ignores: generatedOutput },
   ...coreWebVitals,
   {
     rules: {
@@ -17,7 +22,35 @@ const config = [
     },
   },
   {
-    ignores: ['.next/**', 'node_modules/**', 'tsconfig.tsbuildinfo'],
+    // The base rule misreports on TypeScript (types, enums and implements
+    // clauses); eslint-config-next already enables the TS-aware variant.
+    files: ['**/*.{ts,tsx,mts,cts}'],
+    rules: {
+      'no-unused-vars': 'off',
+      '@typescript-eslint/no-unused-vars': [
+        'warn',
+        {
+          varsIgnorePattern: '^_',
+          argsIgnorePattern: '^_',
+          caughtErrorsIgnorePattern: '^_',
+          destructuredArrayIgnorePattern: '^_',
+          ignoreRestSiblings: true,
+        },
+      ],
+    },
+  },
+  {
+    files: ['**/*.{js,cjs,mjs,jsx}'],
+    rules: {
+      'no-unused-vars': [
+        'warn',
+        {
+          varsIgnorePattern: '^_',
+          argsIgnorePattern: '^_',
+          caughtErrorsIgnorePattern: '^_',
+        },
+      ],
+    },
   },
 ];
 

package.json

@@ -64,7 +64,7 @@
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "autoprefixer": "^10.0.1",
-    "eslint": "^9.0.0",
+    "eslint": "^9.39.5",
     "eslint-config-next": "16.1.6",
     "jest": "^30.3.0",
     "jest-environment-jsdom": "^30.5.2",