赋值返回的值 [英] Value returned by the assignment

问题描述

为什么常规赋值语句(例如，x = 5)返回分配的值(在本例中为5)，而赋值与变量声明(var x = 5) 返回 undefined?

Why does the regular assignment statement (say, x = 5) return the value assigned (5 in this case), while the assignment combined with a variable declaration (var x = 5) returns undefined?

我通过在 Chrome 浏览器的 Javascript 控制台中执行这些语句获得了返回值:

I got the return values by executing these statements in the Chrome browser's Javascript console:

> var x = 5;
undefined
> y = 5;
5

推荐答案

这就是语言的设计方式.它与大多数语言一致.

That's the way the language was designed. It is consistent with most languages.

让变量声明返回除 undefined 之外的任何内容是没有意义的，因为您永远不能在表达式上下文中使用 var 关键字.

Having a variable declaration return anything other than undefined is meaningless, because you can't ever use the var keyword in an expression context.

赋值是一个 expression 而不是 statement 当你想一次将多个变量设置为相同的值时很有用:

Having assignment be an expression not a statement is useful when you want to set many variable to the same value at once:

x = y = z = 2;

也可以这样使用:

x = 2*(y = z); // Set y = z, and x = 2*z

然而，这不是最易读的代码，最好写成:

However that is not the most readable code and it would probably be better written as:

y = z;
x = 2*z;

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