biome-linting

What this skill does

# Biome Linting

Expert knowledge of Biome's linting capabilities, rule categories, and code quality enforcement for JavaScript and TypeScript projects.

## Overview

Biome's linter provides fast, comprehensive code quality checks with a focus on correctness, performance, security, and best practices. It's designed to catch common bugs and enforce consistent code patterns.

## Core Commands

### Basic Linting

```bash
# Check files without fixing
biome check .

# Check and auto-fix
biome check --write .

# Check specific files
biome check src/**/*.ts

# CI mode (strict, fails on warnings)
biome ci .
```

### Command Options

```bash
# Verbose output
biome check --verbose .

# JSON output
biome check --json .

# Only lint (skip formatting)
biome lint .

# Apply safe fixes only
biome check --write --unsafe=false .
```

## Rule Categories

### Accessibility (a11y)

Rules for web accessibility and WCAG compliance:

```json
{
  "linter": {
    "rules": {
      "a11y": {
        "recommended": true,
        "noAccessKey": "error",
        "noAriaHiddenOnFocusable": "error",
        "noAutofocus": "warn",
        "noBlankTarget": "error",
        "noPositiveTabindex": "error",
        "useAltText": "error",
        "useAnchorContent": "error",
        "useButtonType": "error",
        "useValidAriaProps": "error"
      }
    }
  }
}
```

Common violations:

- Missing alt text on images
- Autofocus on form elements
- Positive tabindex values
- Links without content
- Invalid ARIA properties

### Complexity

Rules to reduce code complexity:

```json
{
  "linter": {
    "rules": {
      "complexity": {
        "recommended": true,
        "noBannedTypes": "error",
        "noExcessiveCognitiveComplexity": "warn",
        "noExtraBooleanCast": "error",
        "noMultipleSpacesInRegularExpressionLiterals": "error",
        "noUselessCatch": "error",
        "noUselessConstructor": "error",
        "noUselessEmptyExport": "error",
        "noUselessFragments": "error",
        "noUselessLabel": "error",
        "noUselessRename": "error",
        "noUselessSwitchCase": "error",
        "noWith": "error"
      }
    }
  }
}
```

### Correctness

Rules for code correctness and bug prevention:

```json
{
  "linter": {
    "rules": {
      "correctness": {
        "recommended": true,
        "noChildrenProp": "error",
        "noConstAssign": "error",
        "noConstantCondition": "error",
        "noConstructorReturn": "error",
        "noEmptyPattern": "error",
        "noGlobalObjectCalls": "error",
        "noInnerDeclarations": "error",
        "noInvalidConstructorSuper": "error",
        "noNewSymbol": "error",
        "noNonoctalDecimalEscape": "error",
        "noPrecisionLoss": "error",
        "noSelfAssign": "error",
        "noSetterReturn": "error",
        "noSwitchDeclarations": "error",
        "noUndeclaredVariables": "error",
        "noUnreachable": "error",
        "noUnreachableSuper": "error",
        "noUnsafeFinally": "error",
        "noUnsafeOptionalChaining": "error",
        "noUnusedLabels": "error",
        "noUnusedVariables": "error",
        "useIsNan": "error",
        "useValidForDirection": "error",
        "useYield": "error"
      }
    }
  }
}
```

Critical rules:

- `noUndeclaredVariables`: Catch undeclared variables
- `noUnusedVariables`: Remove unused code
- `noConstAssign`: Prevent const reassignment
- `noUnreachable`: Detect unreachable code
- `useIsNan`: Use isNaN() for NaN checks

### Performance

Rules for performance optimization:

```json
{
  "linter": {
    "rules": {
      "performance": {
        "recommended": true,
        "noAccumulatingSpread": "warn",
        "noDelete": "error"
      }
    }
  }
}
```

### Security

Rules for security best practices:

```json
{
  "linter": {
    "rules": {
      "security": {
        "recommended": true,
        "noDangerouslySetInnerHtml": "error",
        "noDangerouslySetInnerHtmlWithChildren": "error",
        "noGlobalEval": "error"
      }
    }
  }
}
```

Critical security rules:

- Prevent XSS via innerHTML
- Block eval() usage
- Detect security vulnerabilities

### Style

Code style and consistency rules:

```json
{
  "linter": {
    "rules": {
      "style": {
        "recommended": true,
        "noArguments": "error",
        "noCommaOperator": "error",
        "noImplicitBoolean": "warn",
        "noNegationElse": "warn",
        "noNonNullAssertion": "warn",
        "noParameterAssign": "error",
        "noRestrictedGlobals": "error",
        "noShoutyConstants": "warn",
        "noUnusedTemplateLiteral": "error",
        "noVar": "error",
        "useBlockStatements": "warn",
        "useCollapsedElseIf": "warn",
        "useConst": "error",
        "useDefaultParameterLast": "error",
        "useEnumInitializers": "warn",
        "useExponentiationOperator": "error",
        "useFragmentSyntax": "error",
        "useNumericLiterals": "error",
        "useSelfClosingElements": "error",
        "useShorthandArrayType": "error",
        "useSingleVarDeclarator": "error",
        "useTemplate": "warn",
        "useWhile": "error"
      }
    }
  }
}
```

Key style rules:

- `noVar`: Use let/const instead of var
- `useConst`: Prefer const for immutable bindings
- `noNonNullAssertion`: Avoid ! assertions in TypeScript
- `useTemplate`: Prefer template literals

### Suspicious

Rules for suspicious code patterns:

```json
{
  "linter": {
    "rules": {
      "suspicious": {
        "recommended": true,
        "noArrayIndexKey": "warn",
        "noAssignInExpressions": "error",
        "noAsyncPromiseExecutor": "error",
        "noCatchAssign": "error",
        "noClassAssign": "error",
        "noCommentText": "error",
        "noCompareNegZero": "error",
        "noConsoleLog": "warn",
        "noControlCharactersInRegex": "error",
        "noDebugger": "error",
        "noDoubleEquals": "error",
        "noDuplicateCase": "error",
        "noDuplicateClassMembers": "error",
        "noDuplicateObjectKeys": "error",
        "noDuplicateParameters": "error",
        "noEmptyBlockStatements": "warn",
        "noExplicitAny": "warn",
        "noExtraNonNullAssertion": "error",
        "noFallthroughSwitchClause": "error",
        "noFunctionAssign": "error",
        "noGlobalAssign": "error",
        "noImportAssign": "error",
        "noLabelVar": "error",
        "noMisleadingCharacterClass": "error",
        "noPrototypeBuiltins": "error",
        "noRedeclare": "error",
        "noSelfCompare": "error",
        "noShadowRestrictedNames": "error",
        "noUnsafeNegation": "error",
        "useDefaultSwitchClauseLast": "error",
        "useGetterReturn": "error",
        "useValidTypeof": "error"
      }
    }
  }
}
```

Critical suspicious patterns:

- `noExplicitAny`: Avoid any in TypeScript
- `noConsoleLog`: Remove console.log in production
- `noDebugger`: Remove debugger statements
- `noDoubleEquals`: Use === instead of ==
- `noDuplicateObjectKeys`: Catch duplicate keys

## Rule Configuration Patterns

### Strict Configuration

Maximum strictness for high-quality codebases:

```json
{
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "a11y": { "recommended": true },
      "complexity": { "recommended": true },
      "correctness": { "recommended": true },
      "performance": { "recommended": true },
      "security": { "recommended": true },
      "style": { "recommended": true },
      "suspicious": {
        "recommended": true,
        "noExplicitAny": "error",
        "noConsoleLog": "error"
      }
    }
  }
}
```

### Gradual Adoption

Start lenient and progressively tighten:

```json
{
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "suspicious": {
        "noExplicitAny": "warn",
        "noConsoleLog": "warn"
      },
      "style": {
        "noVar": "error",
        "useConst": "warn"
      }
    }
  }
}
```

### Framework-Specific

React configuration example:

```json
{
  "