Universal Loader.js

function universalLoader(req, res) {
  const filePath = path.resolve(__dirname, '..', 'build', 'index.html')

  fs.readFile(filePath, 'utf8', (err, htmlData)=>{
    if (err) {
      console.error('err', err)
      return res.status(404).end()
    }
    match({ routes, location: req.url }, (err, redirect, renderProps) => {
      if(err) {
        console.error('huh err', err)
        return res.status(404).end()
      } else if(redirect) {
        res.redirect(302, redirect.pathname + redirect.search)
      } else if(renderProps) {
        let store = configureStore()
        const ReactApp = renderToString(
          createElement(Provider, {store},
            createElement(RouterContext, renderProps)
          )
        )
        const RenderedApp = htmlData.replace('{{SSR}}', ReactApp)
        res.send(RenderedApp)
      } else {
        return res.status(404).end()
      }
    })
  })
}