Consider the example below. It would be intuitive to think that when we change the value of obj.inner, example is affected. However this is not the case.

var obj = { 
  inner: { x: 10 }
};
var example = obj.inner;
obj.inner   = undefined;

What’s happening here:

1. An object is defined with key ‘inner’, which points to another object, { x:10 }
2. var example points to the object that is pointed to by obj.inner Note: this does NOT mean there now exists a relationship/binding between example and obj.inner
3. The value of obj.inner is changed to undefined Note: the previous object that obj.inner pointed to ({ x:10 }) still exists, and example remains pointing to it
4. The value of example is then { x: 10 }, not undefined

Some more code that may be confusing is below. Once we define player to be undefined, we might think back in the global scope player is now also undefined, however, its score value has been mutated, but its object reference remains in place due to scope.

var player = { score : 4 };

var updateScore = function(player) {
    player.score = 2;
    player = undefined;
}

updateScore(player);