JavaScript and TypeScript Conventions

Apply these conventions in JavaScript and TypeScript projects. These rules are written for any coding agent, including Claude Code, Codex, Cursor, and Copilot.

Package Manager

• Use pnpm, not npm or yarn.

Format and Lint Toolchain

A project has exactly one authoritative formatter/linter setup. Detect it before writing or reformatting code, and never run a second one over the same files.

Detection order — first match wins:

1. Biome — biome.json, biome.jsonc, or @biomejs/biome in devDependencies. See Biome (when present).
2. XO — xo.config.ts, xo.config.js, an xo field in package.json, or xo in devDependencies. See XO (when present).
3. ESLint — eslint.config.{js,mjs,ts} (flat config) or a legacy .eslintrc*.
4. Nothing configured — for a new project prefer Biome: one binary covers formatting, linting, and import sorting, so no Prettier is needed. Ask before adding it to an existing project.