rsnext/errors/returning-response-body-in-middleware.md

Returning response body in middleware

Why This Error Occurred

Your middleware function returns a response body, which is not supported.

Letting middleware respond to incoming requests would bypass Next.js routing mechanism, creating an unnecessary escape hatch.

Possible Ways to Fix It

Next.js middleware gives you a great opportunity to run code and adjust to the requesting user.

It is intended for use cases like:

• A/B testing, where you rewrite to a different page based on external data (User agent, user location, a custom header or cookie...)

  export function middleware(req: NextRequest) {
    let res = NextResponse.next()
    // reuses cookie, or builds a new one.
    const cookie = req.cookies.get(COOKIE_NAME) ?? buildABTestingCookie()
  
    // the cookie contains the displayed variant, 0 being default
    const [, variantId] = cookie.split('.')
    if (variantId !== '0') {
      const url = req.nextUrl.clone()
      url.pathname = url.pathname.replace('/', `/${variantId}/`)
      // rewrites the response to display desired variant
      res = NextResponse.rewrite(url)
    }
  
    // don't forget to set cookie if not set yet
    if (!req.cookies.has(COOKIE_NAME)) {
      res.cookies.set(COOKIE_NAME, cookie)
    }
    return res
  }
  

• authentication, where you redirect to your log-in/sign-in page any un-authenticated request

  export function middleware(req: NextRequest) {
    const basicAuth = req.headers.get('authorization')
  
    if (basicAuth) {
      const auth = basicAuth.split(' ')[1]
      const [user, pwd] = atob(auth).split(':')
      if (areCredentialsValid(user, pwd)) {
        return NextResponse.next()
      }
    }
  
    return NextResponse.redirect(
      new URL(`/login?from=${req.nextUrl.pathname}`, req.url)
    )
  }
  

• detecting bots and rewrite response to display to some sink

  export function middleware(req: NextRequest) {
    if (isABotRequest(req)) {
      // Bot detected! rewrite to the sink
      const url = req.nextUrl.clone()
      url.pathname = '/bot-detected'
      return NextResponse.rewrite(url)
    }
    return NextResponse.next()
  }
  

• programmatically adding headers to the response, like cookies.

  export function middleware(req: NextRequest) {
    const res = NextResponse.next(null, {
      // sets a custom response header
      headers: { 'response-greetings': 'Hej!' },
    })
    // configures cookies
    res.cookies.set('hello', 'world')
    return res
  }