WithContext<T> should be assignable to T

[AeonFr]

After updating from 0.7 to 0.8, I get errors with this code:

const list: WithContext<ItemList> = {
    "@context": "https://schema.org",
    "@type": "ItemList",
    itemListElement: articleList.map((articleItem, i) =>
      getArticleItemStructuredData({
        articleItem,
        position: i + 1,
      })
    ),
}

The property @type gives the following error: Type '"ItemList"' is not assignable to type '"OfferCatalog"'

I would be able to bypass this error by importing and using ItemListLeaf instead of ItemList, but this interface is not exported. For now my workaround was to recreate the interface (based on the sourcecode, I use Omit<> to deny all members except ItemListLeaf from the intersection).

type ItemListLeaf = Exclude<
  ItemList,
  BreadcrumbList | OfferCatalog | HowToStep | HowToSection
>;

Notice that when I run the output of this code in the Rich Results Test or in the Structured Data Testing Tool, I don't get any errors, that's why I think the problem might not be in my original interface, but in the way the typings are built.

[Eyas]

Thanks for reporting this. I think I'll need to see the exact signature of getArticleItemStructuredData. I can't repro this on my own:

// typescript 4.5.3
// schema-dts 0.8.3
import { WithContext, ItemList, Article } from 'schema-dts';

const articleList: string[] = [];
function getArticleItemStructuredData({
  articleItem: string,
  position: number
}): Article {
  return {
    '@type': 'Article'
  };
}

const list: WithContext<ItemList> = {
  '@context': 'https://schema.org',
  '@type': 'ItemList',
  itemListElement: articleList.map((articleItem, i) =>
    getArticleItemStructuredData({
      articleItem,
      position: i + 1
    })
  )
};

The error, "ItemList"' is not assignable to type '"OfferCatalog", sounds like a red herring. But it might be because your itemListElement type mismatched, and TS wasn't sure which type union item to complain about.

[Eyas]

See https://stackblitz.com/edit/typescript-seupf8?file=index.ts

[AeonFr]

Looks like the error comes from the fact that I'm returning WithContext<Article> instead of Article in the getArticleItemStructuredData function.

Previously this didn't raised any issues, but now it does.

Declaring @context at multiple levels is not considered "invalid" by the google validation tools, so we rolled with it for simplicity. But I understand that it's not the intended way to build schemas.

Thanks for the help. And thanks for the good work maintaining this library

[Eyas]

Great to hear this was resolved.

I see how this would have happened:

• Typescript typically ignores excess properties (unless specified in a literal, to help prevent typos), so assigning a WithContext<T> to T should have worked.

• 0.7.4 added support for a @graph nodes, changing the type of WithContext:

  -   export declare type WithContext<T extends Thing> = T & {
  +   export declare type WithContext<T extends Thing> = Graph | (T & {
          "@context": "https://schema.org";
  +   });
  -   };
  +   export interface Graph {
  +       "@context": "https://schema.org";
  +       "@graph": readonly Thing[];
  +   }

and TypeScript now correctly notes that Graph isn't assignable to Article.

I think including Graph in WithContext<> is a mistake, especially since Graph is exported itself as-is. But now I'm not sure why I did that to begin with.

[Eyas]

Fixed in 0.9.0