app.js

var fs = require('fs');
var vm = require('vm');
var _ = require('underscore');

var src = fs.readFileSync('test.js');
var code = (_.template("'use strict';(<%= src %>)(req,res)"))({src: src});

var app = require('express')();
app.get('/',function(req,res){
  var cxt = {
    req: req, res: res,
    console: global.console
  };
  var proc = vm.createScript(code);
  proc.runInNewContext(cxt);
});
app.listen(3000);

test.js

function(req,res){
  console.log(req);
  var ret = {success: true, text: 'this is from test.js', time: new Date()};
  res.setHeader('Content-Type', 'application/json');
  res.send(ret);
}