javascript赋值问题

	var a = {n:1}
	var b = a;
	a.x = a = {n:2}
	console.log(a.x);//undefined
	console.log(b.x);//{n:1,x:{n:2}

a.x和b.x的结果为什么会这样？
问题出在a.x = a = {n:2},

	a.x = a = {n:2}
	//等同于
	a.x = {n:2}
	a = {n:2}
	
	//并非等同于
	a = {n:2}
	a.x = a