Biome

Biome is the single tool for linting and formatting. No ESLint, no Prettier.

Configuration (biome.json)

{
  "$schema": "https://biomejs.dev/schemas/2.4.8/schema.json",
  "files": {
    "includes": ["**/*.js", "**/*.ts", "**/*.mjs", "**/*.cjs",
                 "**/*.json", "**/*.astro", "**/*.svelte", "**/*.css"]
  },
  "formatter": {
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 100
  },
  "linter": {
    "rules": {
      "recommended": true,
      "correctness": { "noUnusedVariables": "warn", "noUnusedImports": "warn" },
      "suspicious": { "noExplicitAny": "warn", "noVar": "error" },
      "style": { "useConst": "error" }
    }
  },
  "javascript": {
    "formatter": { "quoteStyle": "single", "trailingCommas": "es5", "semicolons": "always" }
  },
  "json": {
    "formatter": { "trailingCommas": "none" }
  },
  "css": {
    "parser": { "cssModules": false, "tailwindDirectives": true },
    "formatter": { "enabled": true },
    "linter": { "enabled": true }
  }
}

Commands

pnpm check          # lint + format check (no changes)
pnpm check:fix      # auto-fix lint + format issues
pnpm format         # format only (writes)

Pre-commit hook

Husky + lint-staged runs Biome on staged files only:

"lint-staged": {
  "*.{js,mjs,cjs,ts,tsx,json,astro,svelte,css}": [
    "biome check --write --no-errors-on-unmatched"
  ]
}

Known quirks

Tailwind CSS directives

tailwindDirectives: true in the CSS parser config is required. Without it, Biome reports errors on @source, @theme, @apply, @custom-variant, and other Tailwind-specific at-rules.

@source ordering

Biome's noInvalidPositionAtImportRule requires @source to appear after all @import statements:

/* Correct */
@import "tailwindcss";
@import "./styles/global.css";
@import "@fontsource/inter/latin-400.css";

@source "./src/components";

/* Wrong — Biome error */
@source "./src/components";
@import "@fontsource/inter/latin-400.css";

False-positive unused imports in .astro

Biome cannot see template usage of imports in .astro files. Imports used only in the template (not in the frontmatter script) may trigger noUnusedImports. Keep the rule at "warn", not "error", to avoid blocking valid code.

.svelte file support

Biome supports .svelte for formatting and linting. Runes and {#if} blocks are handled correctly.

Code style

Fixing common issues

pnpm check:fix                              # fix everything auto-fixable
npx biome check src/pages/index.astro       # check a single file
npx biome format --write src/app.css        # format a single file