in javascript all things are passed by value, however in the case of an object the value that is passed is a refference to that underlying objects. So once we change location it changes all references to that location: Example:

var obj1={
	num:10,
	name: "luka"
}
obj2 = obj1;

If we change:

 obj1.num = 11;

then obj2.num also will be 11... because we changed obj1.num and it's the location of the reference obj2.num also will be changed...