function __getStoredValue<K extends keyof Grid.Storage>(
    key: K
): Grid.Storage[K]['anonymousUser'] | undefined {
    let user = 'admin';

    let storageForKey = {
        anonymousUser: [1],
        users: {
            admin: [2]
        }
    } as Partial<Grid.Storage>[K];

    if (!storageForKey) {
        return undefined;
    }

    if (!user) {
        return storageForKey.anonymousUser;  // <--
    } else {
        if (storageForKey.users) {  // <--
            return storageForKey.users[user]; // <--
        }
        return undefined;
    }
}
namespace Grid {
    export interface Storage {
        columnWidths: StorageItem<number[]>;
    }
    export interface StorageItem<V> {
        /** value for anon */
        anonymousUser: V;
        /** value for each user */
        users: Record<string, V>;
    }
}
const result = __getStoredValue('columnWidths');
console.log(result);