TypeScript helpers for nominal typing
- 😀 Nominal types are pretty useful.
- 😞 TypeScript doesn't support nominal types (yet?).
Great! This is easy to fix right? The simple trick below solves the problem.
declare const __brand: unique symbol;
declare const __type: unique symbol;
interface __Brand<T, S extends string> {
[__brand]: S;
[__type]: T;
}
export type Brand<T, S extends string> = T & __Brand<T, S>;But then we encounter some more happy little problems.
type PlayerId = Brand<number, 'PlayerId'>;
type PlayerScores = Record<PlayerId, number>;
const myId = 1337 as PlayerId;
const scores: PlayerScores = { [myId]: 9001 };
// Element implicitly has an 'any' type because expression
// of type 'Brand<number, "PlayerId">' can't be used to
// index type 'Record<Brand<number, "PlayerId">, number>'.
// ts(7053)
const myScore = scores[myId];nom-ts aims to address these problems.
- 🦅 no dependencies
- 🦥 all branded types in nom-ts are assignable to their underlying type without any additional syntax
interface __Flavor<T, S extends string> {
[__brand]?: S;
/**
* Intersection is distributive over union, but we don't want to "lose" information about T.
* `__type?: T` will be useful for Unbrand.
*/
[__type]?: T;
}
export type Flavor<T, S extends string> = T & __Flavor<T, S>;
export type Unbrand<T> = T extends Flavor<infer X, any> ? X : T;
export const Unbrand = <T extends any>(x: T) => x as Unbrand<T>;
export type Dict<K extends PropertyKey, V> = Record<Unbrand<K>, V>;- microsoft/TypeScript#202
- https://basarat.gitbooks.io/typescript/docs/tips/nominalTyping.html
- "Brand" name is taken from https://michalzalecki.com/nominal-typing-in-typescript/
- "Flavor" name is taken from https://spin.atomicobject.com/2018/01/15/typescript-flexible-nominal-typing/
- https://github.com/gcanti/newtype-ts
This project was bootstrapped with TSDX.