purifycss.js

const fs = require('fs')
const _sass = require('node-sass')
const cssnano = require('cssnano')
const purifycss = require('purify-css')

const promisify = (ctx, func = ctx) => (...args) => {
  return new Promise((resolve, reject) => {
    func.apply(ctx, [...args, (err, result) => err ? reject(err) : resolve(result)])
  })
}

const writeFile = promisify(fs.writeFile)
const sass = promisify(_sass.render)

const css = () => sass({ file: `src/app/styles/entry.scss` })
  .then(({ css }) => purifycss(['src/app/components/**/*.js'], css.toString()))
  .then((purified) => cssnano.process(purified, { autoprefixer: { add: true } }))
  .then(({ css }) => writeFile(`build/public/bundle.css`, css))

css()