The automatic type inference in Typescript is faulty

I am currently working with TypeScript version ^4.1.3 and have developed a REST API that deals with albums and art collections. Before sending the response to the web client, I make sure to remove the userId property from the collections.

Below are my Album and Arts interfaces from DynamoDB tables, along with a utility method to remove the userId property from collection objects.

export interface Album {
    userId: string;
    albumId: string;
    creationDate: string;
    visibility: AlbumVisibility;
    title: string;
    description: string;
    coverUrl: string;
}

export interface Art {
    albumId: string;
    artId: string;
    sequenceNum: number;
    userId: string;
    creationDate: string;
    title: string;
    description: string;
    imgUrl: string;
}

export function rmUserIdFromArr(items: Album[] | Art[]) {
    return items.map((item: Album | Art) => {
        const { userId, ...newItem } = item;
        return newItem;
    });
}

However, when running the code on a collection of art items, TypeScript is throwing an error:

const arts = rmUserIdFromArr(result.items).map((item) => ({
        ...item,
        imgUrl: getDownloadSignedUrl(getFsArtsFolder(item.albumId), item.artId),
    }));

The error message says: 'Property 'artId' does not exist on type '{ albumId: string; creationDate: string; visibility: >AlbumVisibility; title: string; description: string; coverUrl: string; } | { albumId: string; >artId: string; sequenceNum: number; creationDate: string; title: string; description: string; >imgUrl: string; }'. Property 'artId' does not exist on type '{ albumId: string; creationDate: string; visibility: >AlbumVisibility; title: string; description: string; coverUrl: string; }'.ts(2339)

This seems odd to me as the artId property is clearly defined in the Art interface type.

Am I overlooking something? While I can resolve this by setting the item within the map function to item: any, I was hoping for a better solution.

const arts = rmUserIdFromArr(result.items).map((item: any) => ({

Thank you very much, BR.

Answer №1

I suggest revising your code to make it more versatile:

function removeUserId<T extends { userId: string }>(items: T[]): Omit<T, 'userId'>[] {
    return items.map((item) => {
        const { userId, ...newItem } = item;
        return newItem;
    });
}

Check out this playground link for a demonstration.

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