UMD, or Universal Module Definition, is a module definition format that aims to be compatible with both CommonJS and AMD. It also allows you to export the module as a global variable that you can include in your application through a simple <script> tag.

It does this by wrapping the modules in a boilerplate that checks the environment to detect how the module is used, and produces the correct exported object.

For example, the preceding greeter example would look like this with UMD:

// greeter.js
(function (root, factory) {
  // Requirements are defined here
}(this, function () {
  function helloWorld(name) {
    process.stdout.write(`hello ${name}!\n`
  };
  // Whatever you return is exposed
  return {
    helloWorld: helloWorld
  }
}));

// main.js
(function (root, factory) {
  if (typeof define === 'function' && define.amd) {
    // AMD
    define(['./greeter.js'], factory);
  } else if (typeof exports === 'object') {
    // Node, CommonJS-like
    module.exports = factory(require('./greeter.js'));
  } else {
    // Browser globals (root is window)
    root.returnExports = factory(root.greeter);
  }
}(this, function (greeter) {
  greeter.sayHello("Daniel");
}));