Nginx location not work on digital-ocean - node.js

Question

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?

Answer 1

I don't know where is wrong, here is what I found:
If remove express from node.js and listen port like this:
var http = require('http');
http.createServer(function (req, res) {
...
}).listen(8001, 'localhost');
The /app worked, no error happen.
If use express like this:
const app = express();
app.listen(8001, 'localhost', function() {
...
});
/app will return error.
To resolve this problem I open another port that create another configuration by copy /etc/nginx/sites-available/default ,then link it to /etc/nginx/sites-enabled
sudo ln -s /etc/nginx/sites-available/default2 /etc/nginx/sites-enabled/
and change the port in server{}