How to make an object extendable with any keys

Extendable.ts

// Extendable regular object
type Extends<T extends object> = T & Record<string, unknown>; 


// Extendable object with extendable nested objects
type Extendable<T> = Extends<{ [Key in keyof T]: T[Key] extends object ? Extends<T[Key]> : T[Key] }>;

type Employee = {
    name: string,
    age: number,
    salary: {
        value: number,
        currency: 'USD' | 'EUR',
    }
}
   
const passenger: Extendable<Employee> = {
    name: 'John Doe',
    age: 30, 
    salary: {
        value: 3500,
        currency: 'USD',
        whatever: 'Alien', // ✅
    },
    whatever: 'Alien: Resurrection', // ✅
};