求解这一小段JS代码

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

请问第三行连续赋值发生了什么~

---

javascript 连等赋值问题