js.js

function Foo(options) {
  var privateInstanceVar = options.thing;
  var publicInstanceVar = options.otherThing;
  
  var publicInterface = {
    publicFunction: publicFunction,
    publicInstanceVar: publicInstanceVar
  };
  
  return publicInterface;
  
  function publicFunction() {
    /* privateInstanceVar
    is accessible within the closuer */
    if (privateInstanceVar) {
      privateFunction();
    }
  }
  
  function privateFunction() {
    /* ... */
  }
}

var instance = Foo({ thing: "foo", otherThing: "bar" });
var otherInstance = new Foo({ thing: "foo", otherThing: "bar" });