README.md

npm install
git push heroku master

index.js

var express = require('express')
var app = express();
var proxy = require('proxy-middleware');
var url = require('url');

app.set('port', (process.env.PORT || 5000))

app.get('/', function(request, response) {
   response.send('Hello World!')
})


app.all('/*', function(req, res, next) {
   res.header("Access-Control-Allow-Origin", "http://assets.domain.com");
   res.header("Access-Control-Allow-Credentials", "true");
   res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
   res.header("Access-Control-Allow-Headers", req.headers["access-control-request-headers"]);

   // intercept OPTIONS method
   if ('OPTIONS' == req.method) {
      res.send(200);
   }
   else {
      req.headers['origin'] = 'https://back.domain.com';
      req.headers['referer'] = 'https://back.domain.com';
      next();
   }
});


app.use('/api', proxy(url.parse('https://back.domain.com/api')));

app.listen(app.get('port'), function() {
  console.log("Node app is running at localhost:" + app.get('port'))
})

package.json

{
  "name": "proxy-server",
  "version": "0.0.0",
  "description": "",
  "main": "index.js",
  "dependencies": {
    "connect": "^3.1.1",
    "express": "~3.4.x",
    "proxy-middleware": "^0.5.1"
  },
  "devDependencies": {},
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC"
}

Procfile

web: node index.js