mt = {}
Object {  }

mt.x = 3
3
mt.y = 4;
4
mt
Object { x: 3, y: 4 }

mt.foo = function(){return this.x + this.y};
function foo()

mt
Object { x: 3, y: 4, foo: foo() }

mt.foo()
7
mt.goo = function(){this.x*this.y}
function goo()

mt.goo()
undefined
mt.goo = function(){return this.x*this.y}
function goo()

mt.goo()
12