override web files depending on site path

app.js

var fs = require('fs');
var express = require('express');
var app = express();
var path = require('path');


app.get('/:site/:file', function(req, res, next){
  var filePath = path.join(__dirname + '/sites' , req.params.site, req.params.file);
  if (fs.existsSync(filePath)){
    res.sendfile(filePath);
  }else {
    next();
  }
});

//remove site
app.use(function(req, res, next) {
  if (req.url.indexOf('/', 1) !== -1) {
    req.url = req.url.slice(req.url.indexOf('/', 1));
  }
  console.error('url', req.url);
  next();
});
app.use(express.static(__dirname + '/public'));
app.listen(8080);

tree

├── app.js
├── public
│   ├── index.html
│   ├── logo.png
│   ├── main.css
│   └── main.js
└── sites
    └── asite
        ├── logo.png
        ├── main_site.css
        └── main_site.js