Keys of types that differ from a base type

Difference types.ts

interface IBase {
  nånProp: string;
}

interface IFirst extends IBase {
  fulProp: string;
}

interface ISecond extends IBase {
  FulProp: string;
}

type TypeDiff<T extends string, U extends string> = ({ [P in T]: P } &
  { [P in U]: never } & { [x: string]: never })[T];

type TypeOmit<T, K extends keyof T> = Pick<T, TypeDiff<keyof T, K>>;

type propName = keyof TypeOmit<IFirst & ISecond, keyof IBase>;

var p: propName = "fulProp";

const värde: IFirst | ISecond = {
  nånProp: "katt",
  [p]: "hund"
};