typescript-biome

name	typescript-biome
description	Use when formatting code, setting up linting, configuring Biome, or enforcing code style in TypeScript projects. Covers Biome configuration, formatting rules, lint rules, and import organization.

Biome Formatting & Linting

Use Biome for formatting and linting in all TypeScript projects. Do not use ESLint + Prettier.

Setup

Dependencies

npm install -D @biomejs/biome

Configuration (biome.json)

{
  "$schema": "https://biomejs.dev/schemas/2.0.0/schema.json",
  "formatter": {
    "indentStyle": "space",
    "indentWidth": 2,
    "lineWidth": 120
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "semicolons": "asNeeded",
      "arrowParentheses": "always"
    }
  },
  "linter": {
    "rules": {
      "complexity": {
        "noBannedTypes": "error",
        "noUselessThisAlias": "error",
        "noUselessTypeConstraint": "error"
      },
      "correctness": {
        "noPrecisionLoss": "error",
        "noUnusedVariables": "off"
      },
      "style": {
        "noNamespace": "error",
        "useAsConstAssertion": "error",
        "useConsistentArrayType": {
          "level": "warn",
          "options": { "syntax": "shorthand" }
        }
      },
      "suspicious": {
        "noDoubleEquals": "error",
        "noExtraNonNullAssertion": "error",
        "noMisleadingInstantiator": "error",
        "noUnsafeDeclarationMerging": "error"
      }
    }
  },
  "assist": {
    "actions": {
      "source": {
        "organizeImports": {
          "level": "on",
          "options": {
            "groups": [
              [":NODE:", "react"],
              [":PACKAGE:", ":PACKAGE_WITH_PROTOCOL:"],
              ["~/**"]
            ]
          }
        }
      }
    }
  }
}

Code Style Rules

Formatting

Setting	Value	Rationale
Indent	2 spaces	Standard for TypeScript/JavaScript
Line width	120 chars	Balances readability and screen usage
Quotes	Single '	Cleaner, less visual noise
Semicolons	As needed (ASI)	Minimal punctuation
Arrow parens	Always	Consistent, easier to add params
Trailing commas	All	Cleaner diffs

Import Organization

Imports must be grouped in this order, separated by blank lines:

// 1. Node builtins and React
import { readFile } from 'node:fs/promises'
import { join } from 'node:path'

// 2. External packages
import pino from 'pino'
import { Injectable } from '@nestjs/common'

// 3. Internal/local imports
import { UserService } from '~/user/user.service'
import { Config } from '~/config'

Strict Equality

Always use === and !==. Never use == or !=.

// Good
if (value === null) {}
if (typeof x === 'string') {}

// Bad — will error
if (value == null) {}

Array Types

Use shorthand array syntax:

// Good
const items: string[] = []
const matrix: number[][] = []

// Bad
const items: Array<string> = []

Exception: use Array<> for complex union types:

const items: Array<string | number> = []

No Namespaces

Never use TypeScript namespace. Use ES modules instead.

// Bad
namespace Utils {
  export function parse() {}
}

// Good
export function parse() {}

Const Assertions

Use as const for literal types:

// Good
const ROLES = ['admin', 'user', 'viewer'] as const

// Bad
const ROLES = ['admin', 'user', 'viewer']

Package.json Scripts

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

Editor Integration

Add .vscode/settings.json for VS Code:

{
  "editor.defaultFormatter": "biomejs.biome",
  "editor.formatOnSave": true,
  "editor.codeActionsOnSave": {
    "source.organizeImports.biome": "explicit"
  }
}

Ignoring Files

In biome.json:

{
  "files": {
    "ignore": ["dist", "node_modules", "*.gen.ts", "coverage"]
  }
}

Migrating from ESLint + Prettier

npx @biomejs/biome migrate eslint
npx @biomejs/biome migrate prettier

Then remove ESLint and Prettier configs and dependencies.