Node.js implementation of transforming a <style> tag to inline style. Useful to tranform Google Documents into Email-compatible format

cssToInlineStyle.js

const cssParser = require('css')
const { JSDOM } = require('jsdom')
  
function cssToInlineStyle (inputHtml) {
  let style = inputHtml.match(/(?:<style[^>]*>)((?:[\r\n]|.)*?)(?:<\/style>)/)
  const html = inputHtml.replace(/<style[^>]*>(([\r\n]|.)*?)<\/style>/, '')

  if (!style) {
    throw new Error('error while parsing')
  } 

  style = style[1]

  const { stylesheet } = cssParser.parse(style)
  const { document } = (new JSDOM(html)).window

  function setStyle (rule, element) {
    rule.declarations.forEach(decl => {
      element.style[decl.property] = decl.value
    })
  } 

  stylesheet.rules.forEach((rule, i) => {
    rule.selectors.forEach(selector => {
      const elements = document.querySelectorAll(selector)
      if (!elements || !elements.length) {
        return
      } 
      elements.forEach(elem => setStyle(rule, elem))
    })
  })

  return document.documentElement.outerHTML
}