Typing of `deriveEntryForLocale` does not allow to return `undefined`, but it is explicitly mentioned in the README and covered in the source code

[TH-VF]

When using the migration.deriveLinkedEntries function TypeScript complains about undefined return values in the function deriveEntryForLocale.

The README mentions that deriveEntryForLocale may return undefined.

If it returns undefined, the new entry will have no values for the current locale.

There is also an example showing to return early in deriveEntryForLocale.

The source code also actively covers this scenario:

https://github.com/contentful/contentful-migration/blob/main/src/lib/action/entry-derive.ts#L73

But the TypeScript typing (index.d.ts) forbids this.

[elylucasctfl]

Hi @TH-VF,

I'm not sure I quite follow here on what the issue is. Would you be able to provide some more context, steps to reproduce the issue and what the expected outcome should be?

Thanks

[TH-VF]

Hi @elylucasctfl,

I am working with the 'contentful-migration' package in TypeScript and I use the .d.ts declaration files provided by the package.

The issue is in the typing of the method deriveEntryForLocale in the interface IDeriveLinkedEntriesConfig.

See line 607 here: https://www.npmjs.com/package/contentful-migration?activeTab=code

The current method signature looks like following:

deriveEntryForLocale: (inputFields: ContentFields, locale: string) => { [field: string]: any }

This signature does not reflect that deriveEntryForLocale is allowed to return undefined (see README or implementation).

I need to return undefined, because I do not want to create a new entry for specific field value combinations. This is possible according to the README and implementation. But the TypeScript typing does not reflect that.

Expected signature:

deriveEntryForLocale: (inputFields: ContentFields, locale: string) => { [field: string]: any } | undefined

Because the TypeScript declarations do not allow to return undefined at the moment, we need to use workarounds.
It would be nice if the TypeScript declaration files could be fixed.

Is that clearer?

[elylucasctfl]

yes, thanks, will look into it