NodeJS – CORS middleware `origin` undefined

If you do not want to block REST tools or server-to-server requests, add a !origin check in the origin function like so:

var corsOptions = {
  origin: function (origin, callback) {
    if (!origin || whitelist.indexOf(origin) !== -1) {
      callback(null, true)
    } else {
      callback(new Error('Not allowed by CORS'))
    }
  }
}