Typescript function returning two different objects

Question

I'm fairly new to TypeScript and I'm trying to work out how to get this thing working. I've done some extensive Googling but have finally resorted to Stack Overflow.

I have a function that returns a User Object or an Error Object, based upon some condition. When I call this function, I can't seem to handle the different cases without getting the follow error.

Property 'error' does not exist on type 'User | Error'.
  Property 'error' does not exist on type 'User'.ts(2339)

Here is some code to demonstate my goal, the error occurs at the >> in the main() funciton.

interface User {
    email: string;
    username: string;
}

interface Error {
    error: string;
    code: number;
}

async function get_user_info(user_id: number): Promise<User | Error> {
    const result = await db.get_user(user_id); // let's say this returns false if no user found..
    if (result) {
        const user: User = {
            email: result.email,
            username: result.username
        }
        return user;
    } else {
        const err: Error = {
            error: "Failed to fetch user.",
            code: 583
        }
        return err;
    }
}

async function main() {
    const res = await get_user_info(42);
>>  if (res.error) {
        // handle this...
    } else {
        // continue onwards...
    }
}

Am I missing anything, or is this sort of thing just not possible?

Answer 1

Apply a type guard that tests if it is an User or an Error object by checking if the error property exists:

async function main() {

    const res = await get_user_info(42);

    if ('error' in res) {
         if (res.error) {
             // handle this...
         }
    } else {
        // continue onwards...
    }
}