getCategories.js

const getCategories = (posts, baseUrl, simpleLog = console.log) => new Promise(async resolve => {
  const apiURL = `${baseUrl.replace(/\/$/, '')}/wp-json/wp/v2/categories`
  // First reduce posts to an array of category numbers.
  simpleLog('Reducing posts to category numbers')
  const categories = await Promise.all(posts.reduce((all, post) => {
    if(!post.category) return all
    if(all.indexOf(post.category) > -1) return all
    return all.concat([post.category])
  }, [])
  .map(async categoryNumber => {
    simpleLog(`Getting information about categories`)
    const categoryData = await getJSON(`${apiURL}/${categoryNumber}`)
    return {
      categoryNumber,
      name: categoryData.name,
      slug: categoryData.slug,
      description: categoryData.description
    }
  }))
  resolve(categories)
})