module.js

defs = {};
modules = {};
function define(name, fn) {
  defs[name] = fn;
}
function require(name) {
  console.log("Loading " + name);
  if (modules.hasOwnProperty(name)) return modules[name];
  if (defs.hasOwnProperty(name)) {
    var fn = defs[name];
    defs[name] = function () { throw new Error("Circular Dependency"); }
    return modules[name] = fn();
  }
  throw new Error("Module not found: " + name);
}

sampleusage.js

define('foo', function () {
  return 42;
});
define('mymod', function () {
  return {
    stuff: "goes here",
    "and here": require('foo')
  };
});

var MyMod = require('mymod');