Cannot set headers after they are sent to the client sentry

Question

I get an error from sentry.io on this code, i's tell me that I can't set headers after they are sent to the client. It's a next js app

export async function getServerSideProps(ctx) {
  const { res } = ctx

  res.setHeader('Content-Type', 'text/plain')
  res.write('User-agent: *\n')
  res.write('Disallow: \n')
  res.write('Sitemap: https://kalla.com/sitemap.xml\n')
  res.end()

  return {}
}

const Robots = () => null

export default Robots

Answer 1

Response header is only sent once with a request, and if you try to set it and send it again you will get an error. writeHead() can be merged with setHeader(), but the writeHead() has precedence over the latter. WriteHead() acepts these args :writeHead(statusCode, statusMessage, headers), however accepts setHeader(key, value).

 export async function getServerSideProps(ctx) {
  const { res } = ctx

  res.setHeader('Content-Type', 'text/plain')
  res.setHeader('User-agent','*\n')
  res.setHeader('Disallow', '\n')
  res.setHeader('Sitemap', 'https://kalla.com/sitemap.xml\n')
  res.end()

  return {}
}

 const Robots = () => null

 export default Robots