Replace ESLint + Prettier with Biome

.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

AGENTS.md

@@ -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 |

biome.json

@@ -0,0 +1,76 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.4.11/schema.json",
+  "vcs": { "enabled": true, "clientKind": "git", "useIgnoreFile": true },
+  "files": {
+    "ignoreUnknown": true,
+    "includes": ["packages/**", "builds/**", "tools/**", "global.d.ts", "vitest.config.ts"]
+  },
+  "formatter": {
+    "enabled": true,
+    "indentStyle": "space",
+    "indentWidth": 2,
+    "lineWidth": 120,
+    "lineEnding": "lf"
+  },
+  "linter": {
+    "enabled": true,
+    "includes": ["**/*.ts"],
+    "rules": {
+      "recommended": true,
+      "complexity": {
+        "noUselessEscapeInRegex": "off",
+        "noArguments": "off",
+        "noBannedTypes": "off",
+        "useArrowFunction": "off",
+        "noThisInStatic": "off"
+      },
+      "correctness": {
+        "noUnusedVariables": "warn",
+        "noUnusedImports": "warn",
+        "noUnusedFunctionParameters": "warn"
+      },
+      "style": {
+        "useConst": "off",
+        "useArrayLiterals": "off",
+        "noNamespace": "off",
+        "noNonNullAssertion": "off"
+      },
+      "suspicious": {
+        "noExplicitAny": "off",
+        "noAssignInExpressions": "off",
+        "noMisleadingCharacterClass": "off",
+        "noDoubleEquals": "off",
+        "noRedeclare": "error",
+        "noShadowRestrictedNames": "off",
+        "noControlCharactersInRegex": "off",
+        "noEmptyBlockStatements": "off",
+        "noPrototypeBuiltins": "off",
+        "noFallthroughSwitchClause": "error",
+        "noImplicitAnyLet": "off",
+        "noTemplateCurlyInString": "off",
+        "useIterableCallbackReturn": "off",
+        "noGlobalIsNan": "off",
+        "noGlobalIsFinite": "off",
+        "noThenProperty": "off",
+        "useGetterReturn": "error"
+      },
+      "performance": {
+        "noDelete": "off"
+      },
+      "security": {
+        "noGlobalEval": "off"
+      }
+    }
+  },
+  "javascript": {
+    "formatter": {
+      "semicolons": "asNeeded",
+      "trailingCommas": "none",
+      "quoteStyle": "single",
+      "arrowParentheses": "asNeeded"
+    }
+  },
+  "assist": {
+    "enabled": false
+  }
+}

builds/knockout/helpers/mocha-test-helpers.js

@@ -49,12 +49,14 @@ function detectIEVersion() {
   const div = document.createElement('div')
   const iElems = div.getElementsByTagName('i')
 
-  while ((div.innerHTML = '<!--[if gt IE ' + ++version + ']><i></i><![endif]-->'), iElems[0]) {}
+  while (((div.innerHTML = '<!--[if gt IE ' + ++version + ']><i></i><![endif]-->'), iElems[0])) {}
   return version > 4 ? version : undefined
 }
 
 function expectEqualOneOf(actual, expectedPossibilities) {
-  const matches = expectedPossibilities.some(function (expected) { return chai.util.eql(actual, expected) })
+  const matches = expectedPossibilities.some(function (expected) {
+    return chai.util.eql(actual, expected)
+  })
   expect(matches, 'expected value to deeply equal one of the provided possibilities').to.equal(true)
 }
 
@@ -93,18 +95,31 @@ function expectHaveTexts(actual, expectedTexts) {
 }
 
 function expectHaveValues(actual, expectedValues) {
-  const values = ko.utils.arrayFilter(ko.utils.arrayMap(actual.childNodes, function (node) { return node.value }), function (value) { return value !== undefined })
+  const values = ko.utils.arrayFilter(
+    ko.utils.arrayMap(actual.childNodes, function (node) {
+      return node.value
+    }),
+    function (value) {
+      return value !== undefined
+    }
+  )
   expect(values).to.deep.equal(expectedValues)
 }
 
 function expectHaveCheckedStates(actual, expectedValues) {
-  const values = ko.utils.arrayMap(actual.childNodes, function (node) { return node.checked })
+  const values = ko.utils.arrayMap(actual.childNodes, function (node) {
+    return node.checked
+  })
   expect(values).to.deep.equal(expectedValues)
 }
 
 function expectHaveSelectedValues(actual, expectedValues) {
-  const selectedNodes = ko.utils.arrayFilter(actual.childNodes, function (node) { return node.selected })
-  const selectedValues = ko.utils.arrayMap(selectedNodes, function (node) { return ko.selectExtensions.readValue(node) })
+  const selectedNodes = ko.utils.arrayFilter(actual.childNodes, function (node) {
+    return node.selected
+  })
+  const selectedValues = ko.utils.arrayMap(selectedNodes, function (node) {
+    return ko.selectExtensions.readValue(node)
+  })
   expect(selectedValues).to.deep.equal(expectedValues)
 }