为什么a未定义而b在var a = b = 3时为3？ [英] Why a is undefined while b is 3 in var a=b=3?

问题描述

在以下代码中，我预计 a 和 b 将 3 。但是， a 是 undefined 而 b 是 3 。为什么？

In the following code, I expected both a and b to be 3. However, a is undefined and b is 3. Why?

(function(){
    var a = b = 3;
})();

console.log(typeof a);//"undefined"
console.log(b);//3

推荐答案

这里的问题是大多数开发人员都理解语句 var a = b = 3; 是简写：

The issue here is that most developers understand the statement var a = b = 3; to be shorthand for:

var b = 3;
var a = b;

但事实上，var a = b = 3;实际上是简写：

But in fact, var a = b = 3; is actually shorthand for:

b = 3;
var a = b;

因此，b最终成为一个全局变量（因为它前面没有var关键字）和即使在封闭函数之外，它仍然在范围内。

Therefore, b ends up being a global variable (since it is not preceded by the var keyword) and is still in scope even outside of the enclosing function.

a未定义的原因是a是该自执行匿名函数的局部变量

The reason a is undefined is that a is a local variable to that self-executing anonymous function

(function(){
    var a = b = 3;
})();

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