(CAPITAL_LETTERS as ReadonlyArray<string>).includes(str); // ############## OR ############## // global augmentation needed if your code is in a module // if your code is not in a module, get rid of "declare global": declare global { interface ReadonlyArray<T> { includes<U>(x: U & ((T & U) extends never ? never : unknown)): boolean; } interface Array<T> { includes<U>(x: U & ((T & U) extends never ? never : unknown)): boolean; } }