Use a branded type so a validated ID is not interchangeable with an arbitrary string: ```ts export type UserId = string & { readonly __brand: "UserId" }; export function parseUserId(value: string): UserId { if (!/^usr_[a-zA-Z0-9]+$/.test(value)) { throw new Error("Invalid user ID"); } return value as UserId; } export function loadUser(id: UserId): Promise<User> { return repository.findById(id); } ``` Only the parser (or a schema validator at the input boundary) should create the brand. The `l...
Scanned 9/5/2026
Install to Claude Code
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Use a branded type so a validated ID is not interchangeable with an arbitrary string:
```ts
export type UserId = string & { readonly __brand: "UserId" };
export function parseUserId(value: string): UserId {
if (!/^usr_[a-zA-Z0-9]+$/.test(value)) {
throw new Error("Invalid user ID");
}
return value as UserId;
}
export function loadUser(id: UserId): Promise<User> {
return repository.findById(id);
}
```
Only the parser (or a schema validator at the input boundary) should create the brand. The `loadUser` signature now rejects a plain `string`; avoid a non-null assertion and keep the runtime validation separate from the compile-time brand.
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