Answer

TypeScript uses structural typing, also known as "duck typing". For type compatibility, the structure (signature) is important, not the name or origin of the type.

Example:

interface Point2D { x: number; y: number; }
interface Coord2D { x: number; y: number; }
// These types are interchangeable: Point2D and Coord2D, because the structure is the same.
const foo: Point2D = { x: 1, y: 2 };
const bar: Coord2D = foo; // OK!

Nominal typing: for type compatibility, the "name" or "factory" is important, the structure is not.

In TypeScript, nominal typing is not natively supported, but it can be emulated using branded types:

type USD = number & { readonly __brand: unique symbol }
type EUR = number & { readonly __brand: unique symbol }
let priceUSD: USD;
let priceEUR: EUR;
// priceUSD = priceEUR; // Error! Different brands.

Why apply this? For example, to distinguish between structurally identical, but semantically different types — currencies, userID/tokenID, physical quantities, etc.

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Trick Question

Question: Why does the following code compile without errors, even though Address and UserId are logically different types?

interface Address { value: string; }
interface UserId { value: string; }
let id: UserId = { value: "test" };
let addr: Address = id; // OK

Answer: Because in TypeScript, the structure is important, not the type name. Both types are just "an object with value: string".

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Examples of real errors due to ignorance of the topic nuances

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History

Project: A financial system with calculations in USD/EUR. Amounts were passed as numbers. Once, the currencies were mixed up during addition — due to structural typing, TypeScript did not catch it. Later, branded types were introduced to exclude such errors at compile time.

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History

Project: In the development of a REST API, objects were used for IDs of different entities (userId, groupId), both with a value: string field. By mistake, userId was substituted instead of groupId, and only the business logic on the server detected the error.

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History

Project: In a parsing library for DSL, identical structures were used (type value = { kind: 'num'|'str', value: number|string }). Homogeneous structures got mixed between different parts of the code, leading to logical errors. Artificial brand fields were added for separation.