How to change intercepted requests?

[kocv59]

This plugin intercepts /incoming and returns /try/incoming instead at the original URL.

The part I can't make work is when it loads /try/incoming it makes an API call. Depending on the result it should fail the original request with a 500 code, or return /try/incomingIfExists if that exists otherwise /try/incomingFallback.

configureServer(server) {
  server.middlewares.use(async (req, res, next) => {
    if (req.url === "/incoming") {
      req.url === "/try/incoming"
      await next()
    }
    if (req.url === "/try/incoming") {
     const apistatus = (await fetch(api)).status
      if (apistatus === 400) {
        res.statusCode = 500
      } else {
        req.url = "/try/incomingIfExists"
        await next()
      }
    }
    if (req.url === "/try/incomingIfExists") {
      // How to do it only if /try/incomingIfExists doesn't exist?
      req.url = "/try/incomingFallback"
    }
  })
}