Overriding primitive type instance methods

[tepid kiln]

Hey :)

I want to simply override the type that's returned by my custom bigints' .toString() method, so it:
a) does not allow a radix
b) returns my custom string type

Setup:

type MarkedBigInt<M extends symbol> = bigint & { readonly m: M } & { toString(): MarkedString<M>; };
type MarkedString<M extends symbol> = string & { readonly m: M };

Usage:

declare const brand: unique symbol;

type Assert<T extends true> = T
type IsMarkedString<V> = V extends MarkedString<typeof brand> ? true : false;

const string0 = "1234" as MarkedString<typeof brand>;
type assert_working = Assert<IsMarkedString<typeof string0>>;

const id = 1234n as MarkedBigInt<typeof brand>;

const string1 = id.toString();
type test = Assert<IsMarkedString<typeof string1>>;

[stiff moat]

daenamnu#0

Playground Link

Preview:```ts
type MarkedBigInt<M extends symbol> = bigint & {
readonly m: M
} & {toString(radix?: number): MarkedString<M>}
type MarkedString<M extends symbol> = string & {
readonly m: M
}

declare const brand: unique symbol

type Assert<T extends true> = T
type IsMarkedString<V> = V extends MarkedString<
typeof brand

? true
: false
...```

You can choose specific lines to embed by selecting them before copying the link.

[prime dome]

i think you're just adding an overload of toString when you do that. if you hover over id.toString you can see that

you could Omit the toString from bigint first, but then your type is no longer a subtype of bigint:

[stiff moat]

mkantor#0

Playground Link

Preview:```ts
type MarkedBigInt<M extends symbol> = Omit<
bigint,
"toString"

& {readonly m: M} & {
toString(radix?: number): MarkedString<M>
}
type MarkedString<M extends symbol> = string & {
readonly m: M
}
...```

[prime dome]

hah okay i have a better option actually

since overload signatures have an order and the first match is chosen at call sites, you can just rearrange your intersections:

[stiff moat]

mkantor#0

Playground Link

Preview:ts type MarkedBigInt<M extends symbol> = { toString(radix?: number): MarkedString<M> } & bigint & {readonly m: M} type MarkedString<M extends symbol> = string & { readonly m: M } ...

[prime dome]

it's stupid, but that works

[tepid kiln]

Wait what. That's actually brilliant. I tried excluding the method from BigInt and it drove me crazy. tysm :p