Simple way to put an error on a domain if one is available, and use the default Express error handler otherwise.

express-domains.js

var domain         = require("domain");
var defaultHandler = require("express").errorHandler();

module.exports = function errorHandler(err, req, res, next) {
  if (domain.active) {
    domain.active.emit("error", err);
  }
  else {
    return defaultHandler(err, req, res, next);
  }
};