nonSPArouter.js

var express    = require('express');  
var app        = express();
...
//in your server.js (or routes file), include the following above your api routes and the main route that hands users your index.html file
nonSPArouter = express.Router();
//in a general route, you check all incoming traffic for your preferred crawlers through the user-agent
app.use(function(req,res,next) { 
  var ua = req.headers['user-agent'];
  if (/^(facebookexternalhit|twitterbot)/gi.test(ua)) {
     nonSPArouter(req,res,next);
  } else {
    next();
  } 
});
...
//this should be your very last route - it will hand all non-crawler users your index.html file (that includes the links to your app files - the vendor.js, app.js etc. files deployed on cloud distribution system such as Amazon S3 + cloudfront)
// MAIN CATCHALL ROUTE --------------- 
app.get('*', function(req, res) {
  res.sendFile(path.join(__dirname + '/dist/index.html'));
});