@rm30/biome-config

The shared RM30 Biome configuration — lint and format, in one tool, replacing ESLint and Prettier.

Install

pnpm add -D @biomejs/biome @rm30/biome-config

Then, at the repo root:

{
  "$schema": "https://biomejs.dev/schemas/2.5.12/schema.json",
  "extends": ["@rm30/biome-config"]
}

Replace lint, lint:fix, format and format:check with:

{
  "check": "biome check .",
  "check:fix": "biome check --write ."
}

What it sets

Formatter — 140 columns, 2 spaces, single quotes, no semicolons, ES5 trailing commas, always-parenthesised arrow params, LF line endings.

LF is deliberate. Prettier's endOfLine: "auto" preserved whatever a file already had; Biome's auto forces the platform's, which would rewrite every file to CRLF on a Windows checkout and fight CI on the next pass. Git stores LF regardless. JSON formatting is off: those files are maintained by hand.

Linter — Biome's recommended set, plus:

| Rule | Level | Why | |---|---|---| | noUnusedVariables, noUnusedImports, noUnusedFunctionParameters | error | Identifiers prefixed with _ are ignored, as before. | | noExplicitAny | warn | | | noEmptyInterface, noBannedTypes | warn | | | noConsole (allowing warn, error) | warn | Use a logger. |

Import sorting is Biome's own organizer, via the assist actions.

Domains are left to Biome's detection: it enables the next, react, reactNative, drizzle, tailwind, turborepo and test rule sets per package, from the dependencies that package declares. In a monorepo this resolves per package.json, so a Next app and an Expo app in one repo each get the right rules without configuration.

The one known gap

Tailwind class sorting. Biome's useSortedClasses is still nursery and only understands the default Tailwind config, while every RM30 project runs v4 with its own custom scales. Standard utilities still sort; custom tokens will not order optimally. This is cosmetic, it is the only gap, and it is accepted — keeping Prettier alive for it alone would forfeit the point of the migration.

Solidity is not covered; solhint stays separate, as it already was.

Why Biome and not ESLint

typescript-eslint supports >=4.8.4 <6.1.0. TypeScript 7 ships without a stable programmatic API, and every type-aware typescript-eslint rule is built against that API — so staying on ESLint means never adopting TypeScript 7. Biome parses and infers in Rust and never loads the TS API.

Biome is the precondition for TypeScript 7, not a parallel nice-to-have.

Overriding

Project-specific rules stay in the project, visible:

{
  "extends": ["@rm30/biome-config"],
  "linter": {
    "rules": {
      "style": {
        "noRestrictedImports": {
          "level": "error",
          "options": { "patterns": [{ "group": ["../features/*"], "message": "Cross-feature imports go through the feature's index." }] }
        }
      }
    }
  }
}