maybeCallback(), slightly modified, from node.js source, for adding default callbacks if they aren't provided, with optional options

app.js

var util = require('util');

function maybeCallback(cb) {
  return util.isFunction(cb) ? cb : function(err) { if (err) throw err; };
}

function iShouldHaveACallback(opts, callback) {
  callback = maybeCallback(arguments[arguments.length-1]);
  if (!opts || util.isFunction(opts)) {
    opts = {};
  }

  // get down to business...
}