Преобразования строк на уровне типов

type Feature = "user" | "payment" | "order";

type SuccessEvent<T extends string> = `${Uppercase<T>}_SUCCESS`;

type events = SuccessEvent<Feature>;
// "USER_SUCCESS" | "PAYMENT_SUCCESS" | "ORDER_SUCCESS"

type EventHandler<T extends string> = `on${Capitalize<T>}Success`;

type event_handler = EventHandler<Feature>;
// "onUserSuccess" | "onPaymentSuccess" | "onOrderSuccess"

@WebDev_Plus