Using an IIS Virtual Directory or Nested Application via appSettings

<appSettings>
  <add key="virtualDirPath" value="/foo" />
</appSettings>

console.log(process.env.virtualDirPath); // prints /foo

// Access the virtualDirPath appSettings and give it a default value of '/'
// in the event that it doesn't exist or isn't set
var virtualDirPath = process.env.virtualDirPath || '/';

// We also want to make sure that our virtualDirPath 
// always starts with a forward slash
if (!virtualDirPath.startsWith('/', 0))
  virtualDirPath = '/' + virtualDirPath;

// Setup a route at the index of our app    
server.get(virtualDirPath, (req, res) => {
    return res.status(200).send('Hello World');
});

// Public Directory
server.use(express.static(path.join(virtualDirPath, 'public')));
// Bower
server.use('/bower_components', express.static(path.join(virtualDirPath, 'bower_components')));

const express = require('express');
const server = express();

const port = process.env.PORT || 3000;

// Access the virtualDirPath appSettings and give it a default value of '/'
// in the event that it doesn't exist or isn't set
var virtualDirPath = process.env.virtualDirPath || '/';

// We also want to make sure that our virtualDirPath 
// always starts with a forward slash
if (!virtualDirPath.startsWith('/', 0))
  virtualDirPath = '/' + virtualDirPath;

// Public Directory
server.use(express.static(path.join(virtualDirPath, 'public')));
// Bower
server.use('/bower_components', express.static(path.join(virtualDirPath, 'bower_components')));

// Setup a route at the index of our app    
server.get(virtualDirPath, (req, res) => {
    return res.status(200).send('Hello World');
});

server.listen(port, () => {
    console.log(`Listening on ${port}`);
});