ts-library

name	ts-library
description	Use when authoring TypeScript libraries or npm packages - covers project setup, package.json exports, Rollup-first build tooling, API design patterns, type inference tricks, testing, and publishing to npm. Use when bundling, preserving published package contracts, configuring dual CJS/ESM output, or setting up release workflows.

TypeScript Library Development

Patterns for authoring high-quality TypeScript libraries, extracted from studying unocss, shiki, unplugin, vite, vitest, vueuse, zod, trpc, drizzle-orm, and more.

When to Use

• Starting a new TypeScript library (single or monorepo)
• Setting up package.json exports for dual CJS/ESM
• Configuring tsconfig for library development
• Choosing build tools, especially Rollup for stable published packages
• Designing type-safe APIs (builder, factory, plugin patterns)
• Writing advanced TypeScript types
• Setting up vitest for library testing
• Configuring release workflow and CI

For Nuxt module development: use nuxt-modules skill

Quick Reference

Working on...	Load file
New project setup	.junie/skills/ts-library/references/project-setup.md
Package exports	.junie/skills/ts-library/references/package-exports.md
tsconfig options	.junie/skills/ts-library/references/typescript-config.md
Build configuration	.junie/skills/ts-library/references/build-tooling.md
ESLint config	.junie/skills/ts-library/references/eslint-config.md
API design patterns	.junie/skills/ts-library/references/api-design.md
Type inference tricks	.junie/skills/ts-library/references/type-patterns.md
Testing setup	.junie/skills/ts-library/references/testing.md
Release workflow	.junie/skills/ts-library/references/release.md
CI/CD setup	.junie/skills/ts-library/references/ci-workflows.md

Loading Files

Consider loading these reference files based on your task:

• .junie/skills/ts-library/references/project-setup.md - if starting a new TypeScript library project
• .junie/skills/ts-library/references/package-exports.md - if configuring package.json exports or dual CJS/ESM
• .junie/skills/ts-library/references/typescript-config.md - if setting up or modifying tsconfig.json
• .junie/skills/ts-library/references/build-tooling.md - if configuring Rollup, tsdown, unbuild, or build scripts
• .junie/skills/ts-library/references/eslint-config.md - if setting up ESLint for library development
• .junie/skills/ts-library/references/api-design.md - if designing public APIs, builder patterns, or plugin systems
• .junie/skills/ts-library/references/type-patterns.md - if working with advanced TypeScript types or type inference
• .junie/skills/ts-library/references/testing.md - if setting up vitest or writing tests for library code
• .junie/skills/ts-library/references/release.md - if configuring release workflow or versioning
• .junie/skills/ts-library/references/ci-workflows.md - if setting up GitHub Actions or CI/CD pipelines

DO NOT load all files at once. Load only what's relevant to your current task.

Published Package Rule

• For an existing published library, inspect the current package.json exports, npm pack output, and all documented subpaths before changing the builder.
• If exact filenames, flat subpaths, or legacy typings must remain stable, prefer Rollup over convention-driven builders.
• Treat changes to entry filenames, export-map ordering, or declaration layout as package-contract changes that must be verified from a packed consumer install.

New Library Workflow

1. Create project structure → load .junie/skills/ts-library/references/project-setup.md
2. For published packages, inspect the current export map and packed tarball before changing builders
3. Configure package.json exports → load .junie/skills/ts-library/references/package-exports.md
4. Set up build with Rollup by default → load .junie/skills/ts-library/references/build-tooling.md
5. Verify build: pnpm build && npm pack --json --ignore-scripts — check output includes the exact expected .esm.js, .cjs.js, and .d.ts public entry points
6. Add package-contract tests and clean-consumer checks → load .junie/skills/ts-library/references/testing.md
7. Configure release → load .junie/skills/ts-library/references/release.md

Quick Start

// package.json (minimal)
{
  "name": "my-lib",
  "main": "./dist/index.cjs.js",
  "module": "./dist/index.esm.js",
  "types": "./dist/index.d.ts",
  "exports": {
    ".": {
      "types": "./dist/index.d.ts",
      "import": "./dist/index.esm.js",
      "require": "./dist/index.cjs.js"
    }
  },
  "files": ["dist"]
}

// rollup.config.js
import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import typescript from 'rollup-plugin-typescript2';
import dts from 'rollup-plugin-dts';

export default [
  {
    input: 'src/index.ts',
    output: [
      { file: 'dist/index.cjs.js', format: 'cjs', exports: 'named' },
      { file: 'dist/index.esm.js', format: 'es' },
    ],
    plugins: [
      resolve({ mainFields: ['module', 'main'] }),
      commonjs(),
      typescript({
        tsconfig: './tsconfig.build.json',
        useTsconfigDeclarationDir: true,
      }),
    ],
  },
  {
    input: 'dist/types/index.d.ts',
    output: { file: 'dist/index.d.ts', format: 'es' },
    plugins: [dts()],
  },
];

Key Principles

• Preserve existing published paths and typings unless the user explicitly wants a package-contract migration
• Dual format: always support both CJS and ESM consumers
• Prefer Rollup when exact filenames, flat subpaths, or multiple public entry points must stay stable
• Use a dedicated build tsconfig when declaration emit needs different compiler settings than local typechecking
• Smart defaults: detect environment, don't force config
• Verify from a packed tarball and a clean consumer install, not just from repo-local imports
• Tree-shakeable: lazy getters, proper sideEffects: false

Token efficiency: Main skill ~300 tokens, each reference ~800-1200 tokens