Book: The TypeScript Type System — From Generics to DSL-Level Types

Also by me: The TypeScript Library — the 5-book collection

My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools

Me: xgabriel.com | GitHub

A function takes a userId: string and an orderId: string .

During a refactor, two arguments cross. The new call reads

refund(orderId, userId) instead of refund(userId, orderId) .

The checker shrugs. Both are strings. Both pass. The unit test for

refund ran against a happy-path fixture where the IDs happened

to be different lengths, so the SQL still returned a row. Just

the wrong one. A customer reads someone else's order history

before the on-call notices.

This bug is older than TypeScript. Languages with nominal type

systems can declare a UserId distinct from every other string,

and the compiler refuses to confuse them. TypeScript is

structural. A string is a string .…