Function onStart

packages/next/src/client/index.tsx:654–692

()

  // This function has a return type to ensure it doesn't start returning a
  // Promise. It should remain synchronous.
  function onStart(): boolean {
    if (
      !styleSheets ||
      // We use `style-loader` in development, so we don't need to do anything
      // unless we're in production:
      process.env.NODE_ENV !== 'production'
    ) {
      return false
    }

    const currentStyleTags: HTMLStyleElement[] = looseToArray<HTMLStyleElement>(
      document.querySelectorAll('style[data-n-href]')
    )
    const currentHrefs: Set<string | null> = new Set(
      currentStyleTags.map((tag) => tag.getAttribute('data-n-href'))
    )

    const noscript: Element | null = document.querySelector(
      'noscript[data-n-css]'
    )
    const nonce: string | null | undefined =
      noscript?.getAttribute('data-n-css')

    styleSheets.forEach(({ href, text }: { href: string; text: any }) => {
      if (!currentHrefs.has(href)) {
        const styleTag = document.createElement('style')
        styleTag.setAttribute('data-n-href', href)
        styleTag.setAttribute('media', 'x')

        if (nonce) {
          styleTag.setAttribute('nonce', nonce)
        }

        document.head.appendChild(styleTag)
        styleTag.appendChild(document.createTextNode(text))
      }
    })
    return true
  }

  function onHeadCommit(): void {
    if (