The Quiet Power of TypeScript: Branded Types for Safer APIs

You've shipped this bug. Everyone has. A function takes a string for the user id, and one day someone passes the email instead. TypeScript doesn't blink — both are strings.

Branded types fix this without runtime cost.

type UserId = string & { readonly _brand: "UserId" };
type Email  = string & { readonly _brand: "Email" };

function getUser(id: UserId) { /* ... */ }

const e: Email = "anna@x.com" as Email;
getUser(e); // ❌ Type error

Use it for IDs, monetary amounts, durations, anything that "looks like a string or number but means something specific". The type system catches the mistake before the test does.