Parsing HTML or Markdown and processing both as markdown then outputting to HTML

index.js

const unified = require('unified')
const parseMarkdown = require('remark-parse')
const parseHtml = require('rehype-parse')
const remark2rehype = require('remark-rehype')
const rehype2remark = require('rehype-remark')
const rehypeStringify = require('rehype-stringify')
const visit = require('unist-util-visit')

async function go() {
  const inputString = `
<div><p><h3>Hello</h3> <strong>world</strong></p></div>
  `
  const isHTMLInput = inputString.trim().startsWith('<')

  const {contents} = await unified()
    .use(isHTMLInput ? parseHtml : parseMarkdown)
    .use(isHTMLInput ? rehype2remark : () => {})
    .use(function logHeadings() {
      // this is where I'll put my plugin stuff... just logging for now
      return function transformer(root, file) {
        visit(root, 'heading', node => {
          console.log(node.depth, node.children[0].value)
        })
      }
    })
    .use(remark2rehype)
    .use(rehypeStringify)
    .process(inputString)

  console.log(contents.toString())
}

go()