Biome: Replace ESLint + Prettier With One Rust-Powered Tool

pnpm add -D --save-exact @biomejs/biome

# Initialize config
npx @biomejs/biome init

{
  "$schema": "https://biomejs.dev/schemas/1.9.0/schema.json",
  "organizeImports": { "enabled": true },
  "linter": {
    "enabled": true,
    "rules": { "recommended": true }
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 100
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "double",
      "trailingCommas": "all"
    }
  }
}

# Check (lint + format) without fixing
npx @biomejs/biome check .

# Check and auto-fix
npx @biomejs/biome check --write .

# Format only
npx @biomejs/biome format --write .

# Lint only
npx @biomejs/biome lint .

{
  "linter": {
    "rules": {
      "recommended": true,
      "correctness": {
        "noUnusedVariables": "error",
        "useExhaustiveDependencies": "warn"
      },
      "suspicious": {
        "noExplicitAny": "warn",
        "noConsoleLog": "warn"
      },
      "style": {
        "noNonNullAssertion": "warn",
        "useConst": "error"
      }
    }
  }
}

# Migrate from Prettier
npx @biomejs/biome migrate prettier --write

# Migrate from ESLint
npx @biomejs/biome migrate eslint --write

# Remove old tools
pnpm remove eslint prettier eslint-config-prettier @typescript-eslint/eslint-plugin @typescript-eslint/parser

# Update package.json scripts
# "lint": "eslint ." → "lint": "biome check ."
# "format": "prettier --write ." → (handled by biome check --write)

# .github/workflows/ci.yml
- name: Lint and format check
  run: npx @biomejs/biome ci .
  # ci mode: exits with error on any issue, no auto-fix

// packages/ui/biome.json
{
  "extends": ["../../biome.json"],
  "linter": {
    "rules": {
      "style": { "noDefaultExport": "off" }
    }
  }
}