next.js/examples/with-styletron/styletron.js

const isServer = typeof window === 'undefined'

let styletron

export default function getStyletron () {
  if (isServer && !styletron) {
    const Styletron = require('styletron-engine-atomic').Server
    styletron = new Styletron()
  } else if (!styletron) {
    const Styletron = require('styletron-engine-atomic').Client
    const styleElements = document.getElementsByClassName('_styletron_hydrate_')
    styletron = new Styletron(styleElements)
  }
  return styletron
}

export function flush () {
  const _styletron = styletron
  styletron = null
  return _styletron
}