[DecoPerson]

Sharing my TypeScript Result type for anyone who’s crazy like me and wishes they had “if let” and doesn’t want “.value” everywhere.

You can do this:

    const user = await fetchUserDTO(123);
    // user is Result<UserDTO, Error>
    if(!isOk(user)) { something(user.err); return; }
    user.name; // user is narrowed to UserDTO

Source:

    export const FailSym: unique symbol = (globalThis as any).____FailSym ?? Symbol('FailSym');
    (globalThis as any).____FailSym = FailSym;
    export type Result<T, E> = T | {
        [FailSym]: true,
        err: E
    }
    
    export function Ok<T>(val: T): Result<T, never> { return val; }
    export function Fail<E>(err: E): Result<never, E> { return {[FailSym]: true, err}; }
    
    export function isOk<T, E>(val: Result<T, E>): val is T { return !(typeof val == 'object' && val != null && FailSym in val); }
    export function isFail<T, E>(val: Result<T, E>): val is Result<never, E> { return typeof val == 'object' && val != null && FailSym in val; }
    
    export function resultify<T, E>(promise: Promise<T>): Promise<Result<T, E>> {
        return promise.then(Ok, err => Fail(err as any));
    }