This type accepts only one property

exactlyOne.ts

type ExactlyOne<T> = {
   [K in keyof T]: Pick<T, K> & Partial<Record<Exclude<keyof T, K>, never>>
}[keyof T]

type User = {
    name: string;
    age: number;
}

function updateExactlyOne(user: User, newUser: ExactlyOne<User>) {
}

const user = {
    name: 'name',
    age: 10
};

updateExactlyOne(user, {
    age: 20,
})