TypeScript: Allow any key with specific value type on an object that has some keys with different value types

Question

I have 2 kinds of Frontmatter:

• FrontmatterTaxonomy, which is an object with kind: "taxonomy" and a taxnomyName that is a string.
• FrontmatterContent, which is an object with kind: "content" that should also allow any other keys, as long as their values are string[].

I'm not sure how to model this - so far, I tried it rather simple, which does not work because "content" is not of type "TaxonomyTerms".

How could I achieve this?

type TaxonomyTerms = string[]
type TaxonomyData = Record<string, TaxonomyTerms>
// alternative, doesn't work either
// type TaxonomyData = { [taxonomyTerm: string]: string[] }

type FrontmatterContent = TaxonomyData & {
  kind: 'content'
}

interface FrontmatterTaxonomy {
  kind: 'taxonomy'
  taxonomyName: string
}

type Frontmatter = FrontmatterContent | FrontmatterTaxonomy

// Doesn't work.
// Type 'string' is not assignable to type 'TaxonomyTerms'
const fm1: Frontmatter = {
  kind: 'content',
  tags: ['foo', 'bar']
}

// Works.
const fm2: Frontmatter = {
  kind: 'taxonomy',
  taxonomyName: 'categories'
}