为什么程序的结果未定义？ [英] why the program's result is undefined?

问题描述

 var foo = {};
 foo.c = foo = {};
 console.log(foo.c);

为什么结果未定义？
i认为它应该是'[object Object]'

why the result is undefined? i thought it is supposed to be '[object Object]'

推荐答案

奇怪的事情发生在作业：

foo.c = (foo = {})

foo.c 的github.io/#x8.7\">引用并指向旧 foo object，在评估内部表达式之前， foo 重新分配 {} emtpy对象文字。所以你的代码相当于

The reference to foo.c is resolved first and points to the old foo object, before the inner expression is evaluated where foo is re-assigned with the {} emtpy object literal. So your code is equivalent to

var foo1 = {};
var foo2 = {};
foo1.c = foo2;
console.log(foo2.c) // obviously undefined now

你也可以尝试

var foo = {}, old = foo;
foo.c = foo = {};
console.log(old, foo, old.c===foo); // {c:{}}, {}, true

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