express.mobile.js

// Render view helper
// if the request is coming from mobile and if you have a view
// with users.mobile.jade, then that view will be rendered
app.use(function (req, res, next) {
  res._render = res.render

  res.render = function (template, locals, cb) {
    var ua = req.header('user-agent')
    var fs = require('fs')

    var view = app.get('views') + '/' + template + '.mobile.jade'

    if (/mobile/i.test(ua) && fs.existsSync(view)) {
      res._render(template + '.mobile', locals, cb)
    } else {
      res._render(template, locals, cb)
    }
  }

  next()
})