Project: TypeScript Utility Library

Introduction

This project packages a tiny typed utility library for npm: generic helpers, emitted .d.ts declarations, and a README with usage examples. You practice library tsconfig, Rollup or tsc emit, and versioning—skills for shared internal packages and open source.

Prerequisites

Project Goals

Export groupBy, uniqueBy, and assertNever
Emit ESM + CJS (or ESM-only) with .d.ts
Document generic usage in README

Setup

bash

mkdir ts-mini-utils
cd ts-mini-utils
npm init -y
npm install --save-dev typescript rollup @rollup/plugin-typescript

text

ts-mini-utils/
├── package.json
├── tsconfig.json
├── rollup.config.mjs
├── src/
│   └── index.ts
└── README.md

tsconfig.json:

json

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",
    "declaration": true,
    "declarationMap": true,
    "outDir": "dist",
    "rootDir": "src",
    "strict": true
  },
  "include": ["src"]
}

Library Code

src/index.ts:

typescript

Code explanation:

groupBy preserves key type K in the return record
assertNever supports exhaustive switch in consumer apps

Rollup Config

rollup.config.mjs:

javascript

import typescript from "@rollup/plugin-typescript";
 
export default {
  input: "src/index.ts",
  output: [
    { file: "dist/index.js", format: "esm", sourcemap: true },
    { file: "dist/index.cjs", format: "cjs", sourcemap: true },
  ],
  plugins: [typescript({ tsconfig: "./tsconfig.json" })],
};

package.json Exports

json

README Usage Section

Document in README.md that consumers import named exports:

typescript

import { groupBy, uniqueBy } from "ts-mini-utils";
 
const users = [
  { id: "1", role: "admin", name: "Ada" },
  { id: "2", role: "member", name: "Lin" },
];
 
const byRole = groupBy(users, (u) => u.role);
const uniqueNames = uniqueBy(users, (u) => u.name);

Build and Local Test

bash

npm run build
npm pack

In another project:

bash

npm install ../ts-mini-utils/ts-mini-utils-0.1.0.tgz

Semantic Versioning

Patch — bug fixes, no API change
Minor — backward-compatible features
Major — breaking type or runtime changes

Document breaking changes in CHANGELOG.

Extensions

Add Vitest unit tests
Publish to npm with npm publish --access public
Add exports subpaths for tree-shaking modules

FAQ

tsc only without Rollup?

Run tsc with declaration: true—fine for simple libraries; Rollup bundles multiple formats.

Type tests?

tsd or expect-type verify inferred types in tests.

Peer dependency on TypeScript?

Libraries usually do not require TS at runtime—types ship in .d.ts only.