Use Oxlint instead of ESLint for JavaScript/TypeScript linting and add tests (#2226)

.claude/rules/coding-style.md

@@ -11,7 +11,7 @@ Before writing new logic, decide which layer it belongs to. Run this check at pl
 | DB schema      | `prisma/`                                              | Migrations are immutable after apply                                       |
 | DB access      | `src/lib/server/`                                      | Server-only; never import in client code                                   |
 | Validation     | `src/**/zod/`                                          | `z.number().int()` for Int fields; comment dual-enforcement with SQL CHECK |
-| Domain types   | `src/**/types/` (`_types/` inside `src/routes/`)       | Plural aliases; TSDoc on every export; no `any`                            |
+| Domain types   | `src/**/types/` (`_types/` inside `src/routes/`)       | Plural aliases; TSDoc on every export; avoid `any`; see alternatives       |
 | Test data      | `src/**/fixtures/` (`_fixtures/` inside `src/routes/`) | Write before implementation (TDD); use realistic values                    |
 | Business logic | `src/**/services/`                                     | Return pure values or `null`; no `Response`/`json()`                       |
 | Pure utilities | `src/**/utils/` (`_utils/` inside `src/routes/`)       | No side effects; adjacent unit test required                               |
@@ -26,7 +26,16 @@ Before writing new logic, decide which layer it belongs to. Run this check at pl
 - **Abbreviations**: avoid non-standard abbreviations (`res` → `response`, `btn` → `button`). When in doubt, spell it out.
 - **Lambda parameters**: no single-character names (e.g., use `placement`, `workbook`). Iterator index `i` is the only exception.
 - **`upsert`**: only use when the implementation performs both insert and update. For insert-only, use `initialize`, `seed`, or another accurate verb.
-- **`any`**: before using `any`, check the value's origin — adding a missing `@types/*` or `devDependency` often provides the correct type.
+- **`any`**: before using `any`, check the value's origin — adding a missing `@types/*` or `devDependency` often provides the correct type. When `any` seems unavoidable, use the narrowest alternative:
+
+  | Situation                                                  | Alternative                                                              |
+  | ---------------------------------------------------------- | ------------------------------------------------------------------------ |
+  | Assign to a property not on the type                       | `obj as T & { prop: U }` (intersection cast)                             |
+  | Return type too complex to write manually                  | `ReturnType<typeof fn>`                                                  |
+  | Partial mock: only specific properties matter              | `Partial<T>`, `Pick<T, 'a' \| 'b'>`, or `satisfies` — prefer these first |
+  | Partial mock: none of the above narrow the type far enough | `as unknown as T` — last resort; bypasses type checking entirely         |
+  | Inline `: any` annotation where inference reaches          | Delete the annotation                                                    |
+
 - **UI labels**: if a label does not match actual behavior, update it or add an inline comment explaining the intentional mismatch.
 - **Constant names**: reflect what the value IS (content), not what it is used for (purpose). e.g., a set holding all enum tab values is `EXISTING_TABS`, not `VALID_TABS`.
 - **New files**: before naming a new file or directory, grep the relevant `src/` directory to confirm existing conventions. Confirm at plan time, not during implementation:
@@ -39,23 +48,44 @@ Before writing new logic, decide which layer it belongs to. Run this check at pl
 
 - **Braces**: always use braces for single-statement `if` blocks. Never `if () return;` — write `if () { return; }`.
 - **Plural type aliases**: define `type Placements = Placement[]` instead of using `Placement[]` directly in signatures and variables.
+- **Empty `catch` blocks**: never use `catch { }` or `catch (_e)` to silence errors. Every `catch` must re-throw, log, or contain an explanatory comment justifying the suppression. Silent swallowing hides bugs and makes failures untraceable.
+
+```typescript
+// Bad: silently discards the error
+try { ... } catch { }
+try { ... } catch (_e) { }
+
+// Good: log and re-throw (adds context before propagating)
+try { ... } catch (error) {
+  console.error('Operation failed:', error);
+  throw error;
+}
+
+// Good: intentional suppression with explanation
+try {
+  localStorage.setItem(key, value);
+} catch {
+  // localStorage may be unavailable (private browsing) — fall back to in-memory store
+}
+```
 
 ### No Hard-Coded Values
 
 Extract magic numbers and strings to named constants. Never embed literal values whose meaning is not self-evident from the type or immediate context.
 
 ```typescript
-// Bad
+// Bad: magic literals embedded inline
 if (grade >= 11) { ... }
 
-const url = '/api/workbooks/submit';
+const response = await fetch('/api/workbooks/submit', options);
 
 // Good
 const MIN_GRADE = 11;
+const SUBMIT_URL = '/api/workbooks/submit';
 
 if (grade >= MIN_GRADE) { ... }
 
-const SUBMIT_URL = '/api/workbooks/submit';
+const response = await fetch(SUBMIT_URL, options);
 ```
 
 Place constants at the top of the file, or in a dedicated `constants/` module when shared across files.
@@ -67,7 +97,7 @@ Within a file, order declarations as follows:
 1. Exported functions and classes (public API first)
 2. Internal helper functions (supporting the exports above)
 
-Shared helper functions (used by two or more exports) should be grouped at the end of the file.
+Place a private helper immediately after the single export that uses it. Place helpers shared by two or more exports at the end of the file.
 
 ## Documentation
 

.claude/rules/testing.md

@@ -24,6 +24,25 @@ If a task description does not mention tests, add them anyway for any non-trivia
 - Never delete, comment out, or weaken assertions (e.g. `toEqual` → `toBeDefined`) to make tests pass
 - Fix the implementation, not the test; if the test itself is wrong, explain why in a comment or commit message
 
+## Unused Imports in Test Files
+
+An unused import in a test file is a signal that a test was planned but not yet written — not dead code to remove.
+
+Before deleting such an import, check whether the corresponding test case is missing and add it:
+
+```typescript
+// Bad: remove the import because it's unused
+import { ABCLikeProvider } from './contest_table_provider';
+
+// Good: the import is unused because the test is missing — add the test
+test('expects to create ABCLike preset correctly', () => {
+  const group = prepareContestProviderPresets().ABCLike();
+  expect(group.getProvider(ContestType.ABC_LIKE)).toBeInstanceOf(ABCLikeProvider);
+});
+```
+
+Removing the import silences the linter but leaves a coverage gap. Adding the test both satisfies the linter and improves coverage.
+
 ## Test Types
 
 | Type | Tool       | Location                                                          | Run Command      |

.oxlintrc.json

@@ -0,0 +1,29 @@
+{
+  "ignorePatterns": ["**/*.svelte"],
+  "env": {
+    "browser": true,
+    "node": true
+  },
+  "globals": {
+    "$state": "readonly",
+    "$derived": "readonly",
+    "$effect": "readonly",
+    "$props": "readonly",
+    "$bindable": "readonly",
+    "$inspect": "readonly"
+  },
+  "rules": {
+    "@typescript-eslint/ban-ts-comment": [
+      "error",
+      {
+        "ts-expect-error": "allow-with-description",
+        "ts-ignore": false
+      }
+    ],
+    "@typescript-eslint/no-unused-vars": [
+      "error",
+      { "argsIgnorePattern": "^_", "varsIgnorePattern": "^_" }
+    ],
+    "@typescript-eslint/no-explicit-any": "warn"
+  }
+}

AGENTS.md

@@ -21,7 +21,7 @@ Always prefer simplicity over pathological correctness. YAGNI, KISS, DRY. No bac
 
 ## Tech Stack
 
-SvelteKit 2 + Svelte 5 (Runes) + TypeScript | PostgreSQL + Prisma | Flowbite Svelte + Tailwind 4 | Vitest + Playwright
+SvelteKit 2 + Svelte 5 (Runes) + TypeScript | PostgreSQL + Prisma | Flowbite Svelte + Tailwind 4 | Vitest + Playwright | oxlint (JS/TS) + ESLint (Svelte)
 
 ## Commands
 
@@ -32,7 +32,7 @@ pnpm test             # Run all tests
 pnpm test:unit        # Vitest unit tests
 pnpm test:e2e         # Playwright E2E tests
 pnpm coverage         # Report test coverage
-pnpm lint             # ESLint check
+pnpm lint             # Prettier + oxlint (JS/TS) + ESLint (.svelte) check
 pnpm format           # Prettier format
 pnpm check            # Svelte type check
 pnpm exec prisma generate           # Generate Prisma client
@@ -78,7 +78,7 @@ prisma/schema.prisma # Database schema
 - **Forms**: Superforms + Zod validation
 - **Tests**: Write tests before implementation (TDD). Use `@quramy/prisma-fabbrica` for factories only in `prisma/seed.ts`. For service-layer unit tests, mock the DB with `vi.mock('$lib/server/database', ...)` — do not use fabbrica there. Use Nock for HTTP mocking
 - **Naming**: `camelCase` variables, `PascalCase` types/components, `snake_case` files/routes, `kebab-case` directories
-- **Pre-commit**: Lefthook runs Prettier + ESLint (bypass: `LEFTHOOK=0 git commit`)
+- **Pre-commit**: Lefthook runs Prettier + oxlint (JS/TS) + ESLint (.svelte only) (bypass: `LEFTHOOK=0 git commit`)
 
 ## References
 

CONTRIBUTING.md

@@ -47,7 +47,8 @@
 - パッケージマネージャ
   - [pnpm](https://pnpm.io/ja/)
 - 文法およびフォーマットチェッカー
-  - [ESLint](https://eslint.org/)
+  - [oxlint](https://oxc.rs/docs/guide/usage/linter.html): JS/TS ファイルの高速リンター（50–100x 高速）
+  - [ESLint](https://eslint.org/): Svelte ファイル専用リンター（eslint-plugin-svelte）のみ使用
   - [Prettier](https://prettier.io/)
   - [lefthook](https://github.com/evilmartians/lefthook): Git hooks 管理ツール（コミット前の自動フォーマット・リント）
 - Search Engine Optimization (SEO) 対策
@@ -283,7 +284,8 @@
 
 - **Pre-commit Hook**: ステージ済みファイルのみに対して以下を実行
   - `prettier --write`: コード書式の自動修正（JavaScript、TypeScript、Markdown、Svelte）
-  - `eslint`: リント（JavaScript、TypeScript、Svelte）
+  - `oxlint`: JS/TS ファイルのリント（JavaScript、TypeScript）
+  - `eslint`: Svelte ファイルのリント（.svelte のみ）
 
 Hook は自動的にセットアップされるため、特別な操作は不要です。
 

e2e/custom_colors.spec.ts

@@ -25,7 +25,7 @@ test.describe('Custom colors for TailwindCSS v4 configuration', () => {
       if (cssFiles.length === 0) {
         cssFiles = allCssFiles;
       }
-    } catch (e) {
+    } catch {
       // True error: directory not found or inaccessible
       throw new Error(`Not found CSS directory: ${cssDir}`);
     }

e2e/signin.spec.ts

@@ -25,7 +25,7 @@ async function login(page: Page, username: string, password: string): Promise<vo
   await expect(page).toHaveURL('/', { timeout: UP_TO_ONE_MINUTE });
 }
 
-async function logout(page: Page, username: string): Promise<void> {
+async function logout(page: Page, _username: string): Promise<void> {
   // Step 1: Click user button to display dropdown
   // Use ID selector because role="presentation" due to Flowbite-Svelte NavLi + Dropdown
   const userButton = page.locator('button[id="nav-user-page"]');

eslint.config.mjs

@@ -6,7 +6,6 @@ import globals from 'globals';
 import tsParser from '@typescript-eslint/parser';
 import svelteParser from 'svelte-eslint-parser';
 import sveltePlugin from 'eslint-plugin-svelte';
-import js from '@eslint/js';
 
 export default [
   {
@@ -28,10 +27,10 @@ export default [
       '**/vite.config.js.timestamp-*',
       '**/vite.config.ts.timestamp-*',
       'prisma/.fabbrica/index.ts',
+      // oxlint handles JS/TS files
+      '**/*.{js,ts,tsx,mjs,cjs}',
     ],
   },
-  // Base JS rules first
-  js.configs.recommended,
   // Svelte rules override JS rules where appropriate (intentional)
   // This allows Svelte-specific handling of rules like no-undef, no-unused-vars
   ...sveltePlugin.configs['flat/recommended'],

lefthook.yml

@@ -7,6 +7,10 @@ pre-commit:
       run: pnpm exec prettier --write {staged_files}
       glob: '**/*.{js,jsx,ts,tsx,md,svelte}'
 
+    - name: oxlint
+      run: pnpm exec oxlint {staged_files}
+      glob: '**/*.{js,jsx,ts,tsx}'
+
     - name: eslint
       run: pnpm exec eslint {staged_files}
-      glob: '**/*.{js,jsx,ts,tsx,svelte}'
+      glob: '**/*.svelte'

package.json

@@ -13,7 +13,7 @@
     "test": "npm run test:e2e && npm run test:unit",
     "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
     "check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
-    "lint": "prettier --check . && eslint .",
+    "lint": "prettier --check . && oxlint . && eslint .",
     "format": "prettier --write .",
     "test:e2e": "playwright test",
     "test:unit": "vitest",
@@ -26,7 +26,6 @@
     "@dnd-kit/abstract": "0.3.2",
     "@dnd-kit/dom": "0.3.2",
     "@eslint/eslintrc": "3.3.5",
-    "@eslint/js": "10.0.1",
     "@playwright/test": "1.58.2",
     "@quramy/prisma-fabbrica": "2.3.3",
     "@sveltejs/adapter-vercel": "6.3.3",
@@ -47,9 +46,10 @@
     "flowbite": "3.1.2",
     "flowbite-svelte": "1.31.0",
     "globals": "17.4.0",
-    "lefthook": "2.1.4",
     "jsdom": "29.0.1",
+    "lefthook": "2.1.4",
     "nock": "14.0.11",
+    "oxlint": "^1.56.0",
     "pnpm": "10.32.1",
     "prettier": "3.8.1",
     "prettier-plugin-svelte": "3.5.1",