javascript - 为什么对象的属性在赋值给对象时会是未定义的？

Question

这个问题已经有答案了:

Multiple left-hand assignment with JavaScript, really right associative? [duplicate] (3 个回答)

已关闭 7 年前。

var foo = {n: 1};
var bar = foo;
foo.x = foo = {n: 2};
console.log(foo.x) // undefined

我意识到变量将对象存储为引用。 foo.x 不就是另一个变量吗？为什么它不能将引用指向对象？

Answer 1

扩展你所做的事情:

var foo = {n: 1}; // foo = ref#1
var bar = foo; // bar = ref#1
foo.x = foo = {n: 2}; // (ref#1) foo.x = foo (ref#2); foo = ref#2;
console.log(foo.x) // ref#2.x ... which is undefined (from ref#2)
console.log(bar.x)  // ref#2 ... which is ref#1

关键是 foo.x 首先被评估，这会导致对 ref#1 的引用...然后分配ref#2。