/socket.io/?EIO=3&transport=polling errors

[chengtie]

I have a web application. Its backend is by nodejs, and runs with pm2 and nginx.

When I do sudo pm2 logs in the server, I see the following error keeps rolling out constantly, though the main features of the application don't seem to be disturbed.

0|npm  | index.js router.get *
0|npm  | kpi.js POST /socket.io/?EIO=3&transport=polling&t=OXijmvI
0|npm  | index.js POST /socket.io/?EIO=3&transport=polling&t=OXijmvI
0|npm  | Error: Not Found
0|npm  |     at /opt/funpro/app.js:122:13
0|npm  |     at Layer.handle [as handle_request] (/opt/funpro/node_modules/express/lib/router/layer.js:95:5)
0|npm  |     at trim_prefix (/opt/funpro/node_modules/express/lib/router/index.js:317:13)
0|npm  |     at /opt/funpro/node_modules/express/lib/router/index.js:284:7
0|npm  |     at Function.process_params (/opt/funpro/node_modules/express/lib/router/index.js:335:12)
0|npm  |     at next (/opt/funpro/node_modules/express/lib/router/index.js:275:10)
0|npm  |     at /opt/funpro/node_modules/express/lib/router/index.js:635:15
0|npm  |     at next (/opt/funpro/node_modules/express/lib/router/index.js:260:14)
0|npm  |     at /opt/funpro/routes/index.js:23:2
0|npm  |     at Layer.handle [as handle_request] (/opt/funpro/node_modules/express/lib/router/layer.js:95:5)
0|npm  |     at trim_prefix (/opt/funpro/node_modules/express/lib/router/index.js:317:13)
0|npm  |     at /opt/funpro/node_modules/express/lib/router/index.js:284:7
0|npm  |     at Function.process_params (/opt/funpro/node_modules/express/lib/router/index.js:335:12)
0|npm  |     at next (/opt/funpro/node_modules/express/lib/router/index.js:275:10)
0|npm  |     at Function.handle (/opt/funpro/node_modules/express/lib/router/index.js:174:3)
0|npm  |     at router (/opt/funpro/node_modules/express/lib/router/index.js:47:12)
0|npm  |     at Layer.handle [as handle_request] (/opt/funpro/node_modules/express/lib/router/layer.js:95:5)
0|npm  |     at trim_prefix (/opt/funpro/node_modules/express/lib/router/index.js:317:13)
0|npm  |     at /opt/funpro/node_modules/express/lib/router/index.js:284:7
0|npm  |     at Function.process_params (/opt/funpro/node_modules/express/lib/router/index.js:335:12)
0|npm  |     at next (/opt/funpro/node_modules/express/lib/router/index.js:275:10)
0|npm  |     at /opt/funpro/node_modules/express/lib/router/index.js:635:15

/opt/funpro/app.js:122:13 starts here:

// catch 404 and forward to error handler
app.use(function (req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

If I search socket.io in the code base, I can see some, but I totally forgot why we used it and how to use it.

Does anyone know what the error is about and how to remove it?