IMPROVISING MODULE SYSTEMS

In order to offer the encapsulation required by the module pattern, pre-ES6 modules sometimes used function scope and immediately invoked function expressions (IIFEs) to wrap the module definition in an anonymous closure. The module definition is executed immediately, as shown here:

(function() {
 // private Foo module code
 console.log('bar');
})();

// 'bar'

When the return value of this module was assigned to a variable, this effectively created a namespace for the module:

var Foo = (function() {
 console.log('bar');
})();

'bar'

In order to expose a public API, the module IIFE would return an object whose properties would be the public members inside the module namespace:

var Foo = (function() {
 return {
  bar: 'baz',
  baz: function() {
   console.log(this.bar);
  }
 };
})();

console.log(Foo.bar); // 'baz'
Foo.baz();    // 'baz'

A similar pattern to the previous one, termed the...