foo.js

Foo = {
  // ...
}

this.load = 2

main.js

function Importer() {
  // Pseudo-code, not actual implementation.
  this.load = function load() {...};
  this.context = {};
  this.context.__proto__ = this.__proto__;
  this.__proto__ = this.context;
}

var i = Importer();

i.load('foo');

assert(i.Foo);

i.load; // a function
i.context.load; // 2.