function C() { this.x = 0; }
C.prototype.foo = function() { return this.x; }

var c = new C();
var x: string = c.foo();

function foo() { return this.y; }
function bar() { return this.foo(); }
var o = { y: "", foo: foo, bar: bar };
var o2 = { y: 0, foo: foo, bar: bar };

o.bar();
var y: number = o2.bar();