Because opaque.d.ts is not module file, unique symbol tag of the Opaque type is exposed
Also, the typescript compiler does not regard following as an error.
import {Opaque} from 'type-fest'
import {tag} from 'type-fest/source/opaque';
type AccountNumber = Opaque<number, 'AccountNumber'>;
declare const accountNumber: AccountNumber
accountNumber[tag]I suggest adding export {} in opaque.d.ts. (This is added if use typescript compiler.)
// ...
export type Opaque<Type, Token = unknown> = Type & Tagged<Token>;
export {}Then, the typescript compiler complains that the tag is not exported.
import {Opaque} from 'type-fest'
import {tag} from 'type-fest/source/opaque'; // Error: Module '"type-fest/source/opaque"' declares 'tag' locally, but it is not exported.ts(2459)
type AccountNumber = Opaque<number, 'AccountNumber'>;
declare const accountNumber: AccountNumber
accountNumber[tag] // Error: Element implicitly has an 'any' type because expression of type 'any' can't be used to index type 'Number & Tagged<"z">'.ts(7053)
sindresorhus / type-fest
A collection of essential TypeScript types
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