mixin-example.js

function mixin(obj, mix) {
  for (var n in mix) {
    // only mixin behavior, not data
    var f = mix[n];
    if (typeof f === 'function') {
      obj[n] = f;
    }
  }
}

function Person (name, age) {
  if (!(this instanceof Person)) {
    return new Person(name, age);
  }

  this.name = name;
  this.age = age;
}

var thingThatCanFly = {};

thingThatCanFly.fly = function () {
  var flyingMessage = "";
  if (typeof this.name !== "undefined") {
    flyingMessage += "My name is " + this.name + " and ";
  }
  flyingMessage += "I'm flying!";
  return flyingMessage;
};

var wingedPerson = new Person("Jennifer", 18);

mixin(wingedPerson, thingThatCanFly);
console.log(wingedPerson.fly());
console.log(wingedPerson.age);
//=> My name is Jennifer and I'm flying!
console.log("wingedPerson.hasOwnProperty('fly') " + wingedPerson.hasOwnProperty('fly'));
//=> true

var unnamedThing = {};
mixin(unnamedThing, thingThatCanFly);
console.log(unnamedThing.fly());
//=> I'm flying!
console.log("unnamedThing.hasOwnProperty('fly') " + unnamedThing.hasOwnProperty('fly'));
//=> true

// Now make Person.prototype capable of flight, which makes all newly
// created instances of Person capable of flight, but no longer do they
// haveOwnProperty('fly') because it's now inherited from the prototype.
mixin(Person.prototype, thingThatCanFly);
console.log("Person.prototype.hasOwnProperty('fly') " + Person.prototype.hasOwnProperty('fly'));

// new agnostic!
var sally = Person('Sally');
console.log(sally.fly()); 
//=> My name is Sally and I'm flying!

console.log("sally.hasOwnProperty('fly') " + sally.hasOwnProperty('fly'));
//=> false

// Note, however, that the original wingedPerson still hasOwnProperty('fly')
console.log("wingedPerson.hasOwnProperty('fly') " + wingedPerson.hasOwnProperty('fly'));
//=> true