Branded Types

A user id and an order id are both strings, so TypeScript happily lets you pass one where the other belongs. A branded type adds an invisible tag, string & { readonly __brand: "UserId" }, so the two no longer mix. Branded values are created only through a small function that validates and casts, which makes that function the single gate every value must pass through.

How it reads

• loadUser(order) would be a compile error: an OrderId is not a UserId
• userId checks the format before branding
• At run time a branded id is just a string

Cloud tip: Brand units too: meters and feet, cents and dollars. Mixed units have crashed real spacecraft.