gistfile1.js

function Something(overwrite) {
    if (overwrite)
        this.foo = 'bar' // foo will be `bar' only for this object
}

// On the prototype you define properties that should be shared by everything.
Something.prototype = { bar: function(){ return this.foo }
                      , foo: 'baz' }

var x = new Something
x.bar() // 'baz'

var y = new Something(true)
y.bar() // 'bar'

y.foo = 'foo'
x.bar() // 'baz'
y.bar() // 'foo'