使用 typescript 接口/类型限制访问 toString

Question

I want to have an interface like this:

export interface Point {
  readonly x: number;
  readonly y: number;
  readonly toString: never;
}

I thought it will work like this:

const p: Point = {x: 4, y: 5}; // OK
p.toString(); // triggers typescript error

However I get also this error on the first line:

TS2322: Type '{ x: number; y: number; }' is not assignable to type 'Point'.
Types of property 'toString' are incompatible.
Type '() => string' is not assignable to type 'never'.

Is there an option to restrict usage of toString on some interface, without the need to write type assertions like const p: Point = {x: 4, y: 5} as Point; everywhere?

My use case: I am currently rewriting what used to be

class Point {
    x: number;
    y: number;
    toString() {
        return `${x} ${y}`;
    }
}

to interfaced object with companion functions:

interface Point {
    x: number;
    y: number;
}
function pointToString({x, y}: Point) {
    return `${x} ${y}`;
}

and I want to make calls to point.toString() in old codebase trigger an error, because they currently do not since every object in JS has a toString() method.

Answer 1

I was able to get this working with the unknown type:

type NullPrototype = Record<keyof Object, unknown>;

interface Point extends NullPrototype {
  readonly x: number;
  readonly y: number;
}

const p: Point = {x: 4, y: 5}; // OK
p.toString(); // TS error: Object is of type 'unknown'.

I called it NullPrototype because it's essentially the type-level version of this pattern in vanilla JS:

const p = Object.create(null); // TS infers type `any`
p.x = 4;
p.y = 5;
p.toString(); // Fails at runtime, but passes type check???

Here 'sa related issue on the TypeScript GitHub, though I don't think anyone's come up with this specific solution over there