Illustrated side effect of modifying children of object properties on prototypes. IMO: Object.create() should have a deep copy option.

objectCreate.js

var Person = function () {};

Person.prototype.name = 'Person';

Person.prototype.config = {
    arms: 2
};

Person.prototype.metadata = {
    type: 'Person'
};

var Pirate = function () {};

Pirate.prototype = Object.create(Person.prototype);

Pirate.prototype.name = 'Pirate'; // This doesn't affect instances of Person
Pirate.prototype.metadata = { type: 'Pirate' }; // This doesn't affect instances of Person
Pirate.prototype.config.arms = 1; // This *affects* instances of Person (Person's prototype is modified)

var pirate = new Pirate();

console.log('pirate.name : ' + pirate.name);
console.log('pirate.config.arms : ' + pirate.config.arms);
console.log('pirate.metadata.type : ' + pirate.metadata.type);

var person = new Person();

console.log('person.name : ' + person.name);
console.log('person.config.arms : ' + person.config.arms);
console.log('person.metadata.type : ' + person.metadata.type);

console.log('Person.prototype.config.arms: ' + Person.prototype.config.arms);

FWIW: The same behavior is also seen using the "new" operator on line 15.

Pirate.prototype = new Person();

JSFiddle
http://jsfiddle.net/XC9Em/

Similar Issue:

var Person = {
    name: undefined,
    bodyParts: {
        arms: 2,
        legs: 2,
        head: 1
    }
};

var bob = Object.create(Person);

bob.bodyParts.arms = 1;  // This modifies Person!

console.log(Person.bodyParts.arms);