双重分配对象属性会导致未定义的属性

问题描述

任何人都可以说出输出是如何变成undefined的吗?

Can anyone tell how the output became undefined?

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

推荐答案

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

foo.x将属性x引用到foo引用的对象.但是，foo = {n:2}为foo分配了一个全新的对象. x确实已分配给一个对象，但是该对象立即被另一个对象替换.具有x属性的对象已不再被任何引用.

The foo.x refers to the property x to the object refered to by foo. However, foo = {n:2} assigns a completely new object to foo. x is indeed assigned to an object, but that object is immediately replaced by another object. The object with the x property isn’t referenced by anything anymore.

您可以将该行读为

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

---

图形说明

var foo = {n:2};

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

console.log(foo.x);

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