gistfile1.js

// Foo is a simple type with a unique ID for each instance.
var idCounter = 0
function Foo() {
  this.id = (++idCounter) // assign a unique ID to this instance
}
Foo.prototype.getID = function() {return this.id}

// We make sure that it's working as intended by creating two instances.
var foo  = new Foo()
  , foo2 = new Foo()
console.log(foo.getID()) // 1
console.log(foo2.getID()) // 2
// `foo.id` is `1`, `foo2.id` is `2`. Looks good.


// Now we want our new type Bar to inherit from Foo.
function Bar() { }
Bar.prototype = new Foo()
// since we're creating a *new instance of Foo* here, this means that
// `Bar.prototype.id` will be `3`. Since `Bar.prototype.id` is 3, this
// value will be propagated to all instances of `Bar`. Probably not
// what we wanted!

var bar  = new Bar()
  , bar2 = new Bar()
console.log(bar.getID()) // 3
console.log(bar2.getID()) // 3
// Uh-oh. Both `bar.id` and `bar2.id` will be `2`, since that's the
// value of the property 'id' in `Bar.prototype`. The "private" id that
// the `Foo` constructor sets is visible to all instances of `Bar`.