Confused about Node order rules (synchronous/asynchronous)

My example is a simple HTTP server:

http.createServer((req, res) => {
  if(req.method === `GET`){

    if(req.headers.cookie === undefined){
      let x = 1
      let y = 2
      let z = 30
    }
    else{
      let x = 10
      let y = 20
      let z = 3
    }

    switch(req.url){

      case `/`:

        // >>>> I need the appropriate variables here for the same client <<<<

        break

      case `/page`:

        // >>>> or here <<<<

        break

      default:
        res.statusCode = 404
        res.end(`Error 404`)
    }
  }
}).listen(3000)

When a client connects, the variables are defined by whether or not the client has a cookie. But what if multiple clients connect at almost the same time? How does Node handle that?

Is it possible for the clients to get mixed up variables, because the if/else statement happens before the switch? Should I instead place 2 copies of the switch in each of the if/else parts?

Or is there no difference?