Typescript / generics: dependent type variables

I’ve spent some time looking for solution so will write it here for the future when I will need to find it again =)

Imagine you have a generic component with 2 type variables:

const Component = <TObjectPreview, TObject>({
}: {
  preview: TObjectPreview
  onLoaded: (obj: TObject) => void
}) {
  ...
}

Pretty legitimate huh?

Now what if we are working with unions:

type SupportedPreview = PhotoPreview | VideoPreview
type SupportedObject = Photo | Video

and we want for consistency have always TObject = Photo when TObjectPreview = PhotoPreview and TObject = Video when TObjectPreview = VideoPreview?

Turns out we can do this with ternary operator in type definition + never usage like this:

type ObjectForPreview<T> = T extends PhotoPreview
  ? Photo
  : T extends VideoPreview
  ? Video
  : never

const Component = <
  TObjectPreview extends SupportedPreview,
  TObject = ObjectForPreview<TObjectPreview>
>({
}: {
  preview: TObjectPreview
  onLoaded: (obj: TObject) => void
}) {
  ...
}

// Bonus: in the end user does not need to specify 2nd type variable
// Actually 1st one is also not needed - typescript does its magic based
// on supplied `preview` type.

Looks a bit hardcoded tbh but it solved my issue.
Ideally would be to use something more dynamic like Record<T1,T2> but I could not understand how to do that.

If you know how to do this better — please let me know!