Redirecting from server side in NextJS

This is now possible on Next.js 10+ without having to do any sort of response header manipulation directly. Use getServerSideProps or getStaticProps and return a redirect key as the only value from it if your conditions are met:

export async function getServerSideProps(context) {
  if(context.req.body.username == "user" && context.req.body.password == "123"){
    return {
      redirect: {
        permanent: false,
        destination: "/"
      }
    }
  }
}