js code accessing var in objects of different level

object_level.js

var a = "one";
var obj = {
	a:"two",
	b: {
		a:"three",
		fn: function(){
			return this.a;
		}
	}
}

var t1 = obj.b.fn;
console.log(t1()); // the function is being invoked in global scope, hence, 'this' will refer to window object, output 'one' 

var t2 = obj.b.fn(); // the function is being invoked and assigned to t2 var
console.log(t2); // output 'three'

// what if we want to refer to 'a' var in the first level obj
console.log(t1.call(obj)); // output 'two'