🦺 A minimal `Result` type system for TypeScript

example.ts

import { Err, Ok, PromiseResult } from 'path/to/module';

interface User {
  // ...
}

enum UserError {
  USER_NOT_FOUND = 'USER_NOT_FOUND', 
}

async function getUser(id: string): PromiseResult<User, { type: UserError }> {
  const user = await db.query(`SELECT * FROM users WHERE id = ?`, [id]);
  if (!user) return Err({ type: UserError.USER_NOT_FOUND });

  return Ok(user);
}

const result = await getUser("123");
if (result.ok) {
  console.log(result.value);
  //                   ^? User
}

result.ts

export type Result<T, E> =
  | {
      ok: true;
      value: T;
    }
  | {
      ok: false;
      error: E;
    };

export type PromiseResult<T, E> = Promise<Result<T, E>>;

export const Ok = <T, E>(value: T): Result<T, E> => ({
  ok: true,
  value,
});

export const Err = <T, E>(error: E): Result<T, E> => ({
  ok: false,
  error,
});