ReadonlyKeys<Type> constructs a union type by picking all readonly properties of object type Type
interface UserThatCannotChangeEmailAndName {
id: number;
readonly email: string;
readonly name: string;
}
type PropertiesThatUserCannotChange = ReadonlyKeys<UserThatCannotChangeEmailAndName>;
// ^? 'email' | 'name'It means properties which values cannot be reassigned
declare const user: UserThatCannotChangeEmailAndName;
// error: Cannot assign to 'email' because it is a read-only property
user.email = "[email protected]";
// ^^^^^
// error: Cannot assign to 'name' because it is a read-only property
user.name = "Alex";
// ^^^^TS Playground – https://tsplay.dev/wRBbxw