dc09.ru/remark-post-meta.mjs

// Adds metadata to a Markdown page:
// description -- text extracted from the first paragraph
// readingTime -- reading time of the whole text, in float (not integer)

import { select } from 'unist-util-select'
import { visit } from 'unist-util-visit'
import getReadingTime from 'reading-time'

export default function remarkPostMeta() {
  return (tree, { data }) => {
    const firstPara = select(':not(heading) paragraph:first-of-type', tree)

    const firstParaText = mdToString(firstPara)
    const articleText = mdToString(tree)
    const readingTime = getReadingTime(articleText)

    data.astro.frontmatter.readingTime = readingTime.minutes
    data.astro.frontmatter.description = firstParaText
  }
}

function mdToString(tree) {
  let str = ''
  visit(tree, 'text', node => {
    str += node.value
    str += ' '
  })
  return str.trimEnd()
}