Biome Setup Skill

This skill covers Biome configuration for TypeScript projects as an alternative to ESLint + Prettier.

When to Use

Use this skill when:

• Setting up a new project with modern tooling
• Replacing ESLint + Prettier with a single tool
• Need faster linting and formatting
• Want simpler configuration

Core Principle

ONE TOOL, ZERO DEPENDENCIES - Biome replaces both ESLint and Prettier with 10-100x better performance.

Installation

npm install -D @biomejs/biome

Basic Configuration

biome.json

{
  "$schema": "https://biomejs.dev/schemas/1.9.0/schema.json",
  "organizeImports": {
    "enabled": true
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "suspicious": {
        "noExplicitAny": "error",
        "noImplicitAnyLet": "error"
      },
      "correctness": {
        "noUnusedVariables": "error",
        "noUnusedImports": "error"
      },
      "style": {
        "noNonNullAssertion": "warn",
        "useConst": "error",
        "useTemplate": "error"
      },
      "complexity": {
        "noBannedTypes": "error",
        "noUselessTypeConstraint": "error"
      }
    }
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 100
  },
  "javascript": {
    "formatter": {
      "semicolons": "always",
      "quoteStyle": "single",
      "trailingCommas": "all",
      "arrowParentheses": "always"
    }
  },
  "files": {
    "ignore": ["node_modules", "dist", "coverage"]
  }
}

Package.json Scripts

{
  "scripts": {
    "lint": "biome check .",
    "lint:fix": "biome check --write .",
    "format": "biome format --write .",
    "format:check": "biome format ."
  }
}

Key Rules

Type Safety Rules

{
  "linter": {
    "rules": {
      "suspicious": {
        "noExplicitAny": "error",
        "noImplicitAnyLet": "error",
        "noConfusingVoidType": "error",
        "noAsyncPromiseExecutor": "error"
      }
    }
  }
}

noExplicitAny:

// ❌ Error
function process(data: any) { }

// ✅ OK
function process(data: unknown) { }

noImplicitAnyLet:

// ❌ Error
let value;  // implicit any

// ✅ OK
let value: string;
let value: unknown;

Code Quality Rules

{
  "linter": {
    "rules": {
      "correctness": {
        "noUnusedVariables": "error",
        "noUnusedImports": "error",
        "noConstantCondition": "error",
        "noUndeclaredVariables": "error"
      }
    }
  }
}

Style Rules

{
  "linter": {
    "rules": {
      "style": {
        "useConst": "error",
        "useTemplate": "error",
        "noVar": "error",
        "useShorthandFunctionType": "error",
        "useExportType": "error",
        "useImportType": "error"
      }
    }
  }
}

useExportType / useImportType:

// ❌ Error - using value import for type
import { User } from './types';
export { User };

// ✅ OK - type-only imports/exports
import type { User } from './types';
export type { User };

Formatter Configuration

JavaScript/TypeScript Formatting

{
  "javascript": {
    "formatter": {
      "semicolons": "always",
      "quoteStyle": "single",
      "jsxQuoteStyle": "double",
      "trailingCommas": "all",
      "arrowParentheses": "always",
      "bracketSpacing": true,
      "bracketSameLine": false
    }
  }
}

JSON Formatting

{
  "json": {
    "formatter": {
      "trailingCommas": "none"
    }
  }
}

File-Specific Overrides

{
  "overrides": [
    {
      "include": ["**/*.test.ts", "**/*.test.tsx"],
      "linter": {
        "rules": {
          "suspicious": {
            "noExplicitAny": "off"
          }
        }
      }
    },
    {
      "include": ["*.config.js", "*.config.ts"],
      "linter": {
        "rules": {
          "style": {
            "noDefaultExport": "off"
          }
        }
      }
    }
  ]
}

VS Code Integration

Install Extension

Install "Biome" extension from VS Code marketplace.

settings.json

{
  "editor.defaultFormatter": "biomejs.biome",
  "editor.formatOnSave": true,
  "editor.codeActionsOnSave": {
    "quickfix.biome": "explicit",
    "source.organizeImports.biome": "explicit"
  },
  "[javascript]": {
    "editor.defaultFormatter": "biomejs.biome"
  },
  "[typescript]": {
    "editor.defaultFormatter": "biomejs.biome"
  },
  "[json]": {
    "editor.defaultFormatter": "biomejs.biome"
  }
}

CLI Commands

Linting

# Check all files
biome check .

# Check and fix
biome check --write .

# Check specific files
biome check src/

# Check with specific rules
biome check --diagnostic-level=error .

Formatting

# Check formatting
biome format .

# Apply formatting
biome format --write .

# Format specific files
biome format --write src/**/*.ts

CI Integration

# Check only (exit non-zero on issues)
biome ci .

Migration from ESLint + Prettier

Step 1: Install Biome

npm uninstall eslint prettier eslint-config-prettier @typescript-eslint/eslint-plugin @typescript-eslint/parser
npm install -D @biomejs/biome

Step 2: Create biome.json

npx biome init

Step 3: Update Scripts

{
  "scripts": {
    "lint": "biome check .",
    "format": "biome format --write ."
  }
}

Step 4: Remove Old Config Files

rm .eslintrc.json .eslintrc.js .prettierrc .prettierignore

Rule Mapping

ESLint Rule	Biome Rule
@typescript-eslint/no-explicit-any	suspicious/noExplicitAny
@typescript-eslint/no-unused-vars	correctness/noUnusedVariables
no-var	style/noVar
prefer-const	style/useConst
prefer-template	style/useTemplate

Limitations

Biome does not support:

• Custom rules (plugin ecosystem)
• Some ESLint-specific rules
• JSON configuration with comments

Use ESLint if you need:

• eslint-plugin-react-hooks
• eslint-plugin-jsx-a11y
• Custom rule development

Pre-commit Hook

{
  "lint-staged": {
    "*.{ts,tsx,js,jsx,json,md}": [
      "biome check --write --no-errors-on-unmatched"
    ]
  }
}

GitHub Actions

name: Lint

on: [push, pull_request]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '22'
      - run: npm ci
      - run: npx biome ci .

Best Practices Summary

1. Use biome ci in CI pipelines
2. Enable organizeImports
3. Set noExplicitAny to error
4. Use overrides for test files
5. Configure VS Code for format on save
6. Use lint-staged for pre-commit

Code Review Checklist

• biome.json present in project root
• noExplicitAny set to error
• noUnusedVariables enabled
• Formatter configured consistently
• VS Code extension installed
• Pre-commit hook configured
• CI uses biome ci