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Branded types — nominal typing in TypeScript
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branded typesnominal typingphantom typeopaque typenewtype
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Problem
TypeScript uses structural typing, so UserId and PostId are both 'string' and interchangeable. This allows passing a user ID where a post ID is expected with no compile error.
Solution
Use branded types (phantom types) to create nominally-typed primitives that are incompatible at compile time but have zero runtime cost.
Code Snippets
Branded types for type-safe IDs
// Brand utility
type Brand<T, B extends string> = T & { readonly __brand: B };
// Define branded types
type UserId = Brand<string, 'UserId'>;
type PostId = Brand<string, 'PostId'>;
// Constructor functions
const UserId = (id: string) => id as UserId;
const PostId = (id: string) => id as PostId;
// Now TypeScript enforces correct usage
function getUser(id: UserId) { /* ... */ }
function getPost(id: PostId) { /* ... */ }
const uid = UserId('usr_123');
const pid = PostId('post_456');
getUser(uid); // OK
getUser(pid); // Compile error! PostId is not UserIdRevisions (0)
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