Nginx location not work on digital-ocean

I created two node.js instance on DigitalOcean's Ubuntu machine. Both listen to localhost, with different ports. Then test by curl http:\localhost:${port}. Both worked.

Then I added them to /etc/nginx/site-available/default

location / {
            # First attempt to serve request as file, then
            # as directory, then fall back to displaying a 404.
            #try_files $uri $uri/ =404;
            proxy_pass http://localhost:8837;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
}

location /app {
            proxy_pass http://localhost:9837;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
}

Only http://${ip_address}:${port}/ worked.

http://${ip_address}:${port}/app return "Cannot GET /app"

I used express in node.js is it the problem come from?

via Wu Baiquan