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Backend question · Object-oriented programming › Abstract classes, interfaces & structural typing

What does TypeScript (strict: true) say about the last line?

class UserId { constructor(public value: string) {} }
class OrderId { constructor(public value: string) {} }

function loadOrder(id: OrderId): void { /* ... */ }

loadOrder(new UserId("u_1"));
No error: it compiles

It compiles. TypeScript uses structural typing: it compares the shape of types, not their names. Both classes have exactly one public value: string, so a UserId fits where an OrderId is expected.

"Error: UserId is not assignable to OrderId" is what Java or C# would say, because they compare names. To make TypeScript tell them apart, add a private member (for example private readonly brand = "OrderId") or use a branded type.