Replace ESLint + Prettier with Biome

Single Rust-native tool for linting + formatting (27x faster).
Migrated config matches Prettier style + ESLint rules.

New rules enabled: noUnusedImports, noUnusedVariables,
noUnusedFunctionParameters (warn), noRedeclare,
noFallthroughSwitchClause, useGetterReturn (error).

Removed: eslint, @eslint/js, typescript-eslint, prettier, globals
Added: @biomejs/biome
CI: single `biome ci .` replaces Prettier + ESLint steps

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

.github/workflows/lint-and-typecheck.yml

@@ -21,11 +21,8 @@ jobs:
       - name: Install dependencies
         run: bun install --frozen-lockfile
 
-      - name: Check Prettier
-        run: bun run format
-
-      - name: Check ESLint
-        run: bun run lint
+      - name: Check Biome (lint + format)
+        run: bunx @biomejs/biome ci .
 
       - name: Build (required for tsc)
         run: bun run build

.prettierignore

@@ -38,10 +38,11 @@ All commands use Bun. Run from the repo root:
 bun install               # Install all dependencies (uses Bun workspaces)
 bun run build             # Build all packages (ESM, CommonJS, MJS, browser)
 bun run test              # Run all tests (Vitest, headless Chromium via Playwright)
-bun run lint              # Run ESLint
-bun run lint:fix          # Run ESLint with auto-fix
-bun run format            # Check Prettier formatting
-bun run format:fix        # Fix Prettier formatting
+bun run check             # Run Biome (lint + format)
+bun run lint              # Run Biome lint only
+bun run lint:fix          # Run Biome lint with auto-fix
+bun run format            # Check Biome formatting
+bun run format:fix        # Fix Biome formatting
 bun run tsc               # TypeScript type-check (no emit)
 bun run dts               # Generate TypeScript declaration files
 bun run clean             # Clean dist/ and coverage/ dirs
@@ -62,12 +63,11 @@ because TKO does low-level DOM manipulation, MutationObserver, and event handlin
 
 ## Code Style
 
-- **Formatter**: Prettier — no semicolons, single quotes, trailing commas: none, 120 char width
-- **Linter**: ESLint with typescript-eslint (flat config)
-- **Editor**: 2-space indentation for JS/TS, LF line endings
-- See `.prettierrc` and `eslint.config.js` for full config
+- **Linter + Formatter**: Biome — single Rust-native tool replacing ESLint + Prettier
+- **Style**: no semicolons, single quotes, trailing commas: none, 120 char width, 2-space indent, LF line endings
+- See `biome.json` for full config
 
-Run `bun run format:fix && bun run lint:fix` before committing.
+Run `bun run lint:fix` before committing.
 
 ## TypeScript
 
@@ -102,7 +102,7 @@ GitHub Actions workflows (`.github/workflows/`):
 |----------|---------|---------|
 | `main-build.yml` | Push to main | Build + audit + headless test |
 | `test-headless.yml` | PRs | Matrix test (Chrome, Firefox, jQuery) |
-| `lint-and-typecheck.yml` | PRs | Prettier + ESLint + tsc (combined) |
+| `lint-and-typecheck.yml` | PRs | Biome + tsc (lint, format, typecheck) |
 | `publish-check.yml` | PRs | Verify packages are publishable |
 | `release.yml` | Tag push (`v*`) | Changeset version PRs + npm publish + GitHub release creation |
 | `github-release.yml` | Manual fallback | Backfill a GitHub release/tag for a published `main` commit if automatic release creation needs a retry |

.prettierrc

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://biomejs.dev/schemas/2.4.12/schema.json",
+  "$schema": "https://biomejs.dev/schemas/2.4.11/schema.json",
   "vcs": { "enabled": true, "clientKind": "git", "useIgnoreFile": true },
   "files": {
     "ignoreUnknown": true,
@@ -18,31 +18,47 @@
     "rules": {
       "recommended": true,
       "complexity": {
-        "noUselessEscapeInRegex": "off"
+        "noUselessEscapeInRegex": "off",
+        "noArguments": "off",
+        "noBannedTypes": "off",
+        "useArrowFunction": "off",
+        "noThisInStatic": "off"
       },
       "correctness": {
-        "noUnusedVariables": "off",
-        "noUnusedImports": "off"
+        "noUnusedVariables": "warn",
+        "noUnusedImports": "warn",
+        "noUnusedFunctionParameters": "warn"
       },
       "style": {
         "useConst": "off",
         "useArrayLiterals": "off",
-        "noNamespace": "off"
+        "noNamespace": "off",
+        "noNonNullAssertion": "off"
       },
       "suspicious": {
         "noExplicitAny": "off",
         "noAssignInExpressions": "off",
         "noMisleadingCharacterClass": "off",
         "noDoubleEquals": "off",
-        "noRedeclare": "off",
+        "noRedeclare": "error",
         "noShadowRestrictedNames": "off",
         "noControlCharactersInRegex": "off",
         "noEmptyBlockStatements": "off",
         "noPrototypeBuiltins": "off",
-        "noFallthroughSwitchClause": "off"
+        "noFallthroughSwitchClause": "error",
+        "noImplicitAnyLet": "off",
+        "noTemplateCurlyInString": "off",
+        "useIterableCallbackReturn": "off",
+        "noGlobalIsNan": "off",
+        "noGlobalIsFinite": "off",
+        "noThenProperty": "off",
+        "useGetterReturn": "error"
       },
       "performance": {
         "noDelete": "off"
+      },
+      "security": {
+        "noGlobalEval": "off"
       }
     }
   },
@@ -55,11 +71,6 @@
     }
   },
   "assist": {
-    "enabled": true,
-    "actions": {
-      "source": {
-        "organizeImports": "on"
-      }
-    }
+    "enabled": false
   }
 }