Is there a better way of structuring this typescript type definition?

Question

I just started using TypeScript in my Node projects and I was wondering if there is a cleaner, more concise way of implementing this:

import { XOR } from "ts-xor";

type _RemoveNull<T> = {
    [P in keyof T] : string;
}

type UserIdParam = {
    a: string;
}

type BudgetIdParam = UserIdParam & {
    b: string | null;
}

type AccountIdParam = _RemoveNull<BudgetIdParam> & {
    c: string | null;
}

type TransIdParam = _RemoveNull<AccountIdParam> & {
    d: string | null;
}

type IdsParam = XOR<XOR<XOR<UserIdParam, BudgetIdParam>, AccountIdParam>, TransIdParam>;

I wanted a type that would accept any of these sample objects:

const a = {a: "1"};
const b = {a: "1", b: "2"};
const c = {a: "1", b: "2", c: "3"};
const d = {a: "1", b: "2", c: "3", d: "4"};

Also, only the last available property of the object can be null, that's why I had to intersect with the previous type and removed the null from the union. I tried to do a union of the four types UserIdParam, BudgetIdParam, AccountIdParam, and TransIdParam but after I read other questions like this, I decided to use an XOR instead (ts-xor) to accomplish what I needed.

Please let me know your thoughts. Thanks!

--

EDIT: as mentioned by @Thomas in the comments, there is no concept of order for the object's properties, so there is no "last" one.