nginx upstream doesn't work

Browser shows nginx index html instead of 'Hello World' by app.js. Website domain is www.testSite.com. Nginx and upstream server are in the same host. I've read releated question.
/etc/nginx/sites-available/yourdomain:

upstream backend_hosts {
    server 127.0.0.1:3000;
    keepalive 8;
}

server {
    listen 0.0.0.0:80;
    server_name testSite.com;

    location / {
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_set_header X-NginX-Proxy true;

      proxy_pass http://backend_hosts/;
      proxy_redirect off;
    }
 }

/var/www/yourdomain/app.js:

var http = require('http');
http.createServer(function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World\n');
}).listen(3000, "127.0.0.1");
console.log('Server running at http://127.0.0.1:3000/');

via user7328234