@example_register.js

// This returns a Javascript string
require.registerExtension('.coffee', function(async, source, callback) {
  return compileCoffee(source);
});

// True
require('coffee').test === 123;

// This returns a object
require.registerExtension('.mu', function(async, source, callback) {
  return {
    html: compileTemplate(source),
    test: 321
  };
});

// True
require('template').test === 321;

// this one returns the module function
require.registerExtension(".foo", function (async, source, callback) {
  return function (exports, require, module, __filename, __dirname) {
    exports.hello = "world"
  }
});

require("foofile").hello === "world"

coffee.coffee

exports.test: 123