handler.js

class ScriptFoldHandler {

  constructor(nonce) {
    this.nonce = nonce
    //
    this.aboveFoldScripts = []
    this.aboveFoldReady = new Promise((resolve, reject) => {
      this.resolveAboveFoldScripts = resolve
    })
  }

  async element(element) {
    if (element.tagName === 'title') {
      await this.aboveFoldReady
      this.aboveFoldScripts.forEach(script => {
        element.after(`<script defer="defer" nonce="${nonce}" src="${script}"></script>`, { html: true })
      })
    } else if (element.tagName === 'script') {
      const type = element.getAttribute('type')
      if (type && type !== 'text/javascript')
        return
      const src = element.getAttribute('src')
      //if (!src)
      //  return
      this.aboveFoldScripts.push(src)
    }
  }

  end(end) {
    this.resolveAboveFoldScripts()
  }
}

usage.js

const FoldHandler = new ScriptFoldHandler(nonce)
// ...
const rewriter = new HTMLRewriter()
    .on('title', FoldHandler)
    .on('script[src]', FoldHandler)
// ...
    .onDocument(FoldHandler)