Javascript代码技巧：foo.x的价值是什么 [英] Javascript code trick :What's the value of foo.x

问题描述

我在Github前端采访问题集中遇到了这个问题：

I met this problem in Github front-end interview questions collection:

var foo = {n: 1};
var bar = foo;
foo.x = foo = {n: 2};

问题：foo.x的价值是多少？

Question: What is the value of foo.x?

答案是 undefined 。

我做过一些研究和我理解的问题是（如果我错了，请纠正我）：

I've done some research and what I understand this problem is (correct me if I'm wrong):

• var foo = {n：1}; 声明一个对象 foo ，其属性 n 等于1 。


• var bar = foo; 声明一个对象 bar 哪个引用与 foo 相同的对象。


• foo.x = foo = {n：2}; 我相信它等于 foo.x =（foo = {n：2}）;


• 然后我得到 foo.x 等于 undefined 。但是， bar.x 的值是对象 {n：2} 。

• var foo = {n: 1}; declares an object foo which has property n equal to 1.
• var bar = foo; declared an object bar which refer to same object as foo.
• foo.x = foo = {n: 2}; which I believe is equal to foo.x = (foo = {n: 2});
• And then I got foo.x equals to undefined. However, the value of bar.xis the object {n:2}.

如果 bar 和 foo 引用同一个对象，为什么 bar.x 获得价值，而 foo.x 是未定义？ foo.x = foo = {n：2}; ？

If bar and foo refer to same object, why bar.x got value while foo.x is undefined? What is really happening in foo.x = foo = {n: 2};?

推荐答案

foo.x = foo = {n: 2};

确定 foo.x 是指a属性 x {n：1} 对象，指定 {n：2} 到 foo ，并指定新值 foo - { n：2} - 到 {n：1} 对象的属性 x 。

determines that foo.x refers to a property x of the {n: 1} object, assigns {n: 2} to foo, and assigns the new value of foo – {n: 2} – to the property x of the {n: 1} object.

重要的是 foo foo.x 指的是在 foo 更改之前确定的。

The important thing is that the foo that foo.x refers to is determined before foo changes.

参见 ES5规范第11.13.1节：

1. 让 lref 成为评估 LeftHandSideExpression 的结果。

让 rref 成为评估 AssignmentExpression 的结果。

赋值运算符从右到左关联，因此得到：

The assignment operator associates right to left, so you get:

foo.x = (foo = {n: 2})

左手边在右手边评估。

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