index.js

import markdoc from '@markdoc/markdoc'
import yaml from 'js-yaml'

export default function preprocessMarkdoc(config = {}) {
  return (input) => {
    if (isMarkdoc(input.filename)) {
      return {
        code: render(input.content, config)
      }
    }
  }
}

function isMarkdoc(path) {
  return /\.markdoc$/.test(path)
}

function render(source, config) {
  const ast = markdoc.parse(source)
  const configWithFrontmatter = addFrontmatter(ast, config)
  const content = markdoc.transform(ast, configWithFrontmatter)
  return markdoc.renderers.html(content)
}

function addFrontmatter(ast, config) {
  const frontmatter = ast.attributes.frontmatter ? yaml.load(ast.attributes.frontmatter) : {}
  const markdoc = { ...(config?.variables?.markdoc || {}), frontmatter }
  const variables = { ...(config?.variables || {}), markdoc }
  return { ...config, variables }
}