api-state.ts

type Loaded<T> = {
    state: 'loaded',
    data: unknown,
};

type ErrorState = {
    state: 'error',
    error: Error,
};

type Loading = {
    state: 'loading',
    since: number,
};

type APIState<T> = Loaded<T> | ErrorState | Loading;

function getResult<T>(): APIState<T> {
    return { state: 'error', error: new Error('') };
};

const a = getResult();

if (a.state === 'loaded') {
    console.log(a.data);
} else if (a.state === 'error') {
    console.log(a.error);
} else {
    console.log(a.since);
}