‘void 0’是什么意思? [英] What does `void 0` mean?
问题描述
阅读 Backbone.js 源代码，我看到了:
Reading through the Backbone.js source code, I saw this:
validObj[attr] = void 0;
什么是void 0?在这里使用它的目的是什么?
What is void 0? What is the purpose of using it here?
推荐答案
void 0 是什么意思?
void[MDN] 是一个前缀关键字，它接受一个参数并且总是返回 undefined.
示例
void 0
void (0)
void "hello"
void (new Date())
//all will return undefined
这有什么意义?
这看起来很没用，不是吗?如果它总是返回 undefined，那么只使用 undefined 本身有什么问题?
What's the point of that?
It seems pretty useless, doesn't it? If it always returns undefined, what's wrong with just using undefined itself?
在一个完美的世界中，我们可以安全地使用 undefined:它比 void 0 更简单易懂.但是，如果您以前从未注意到，这不是一个完美的世界，尤其是在 Javascript 方面.
In a perfect world we would be able to safely just use undefined: it's much simpler and easier to understand than void 0. But in case you've never noticed before, this isn't a perfect world, especially when it comes to Javascript.
使用 undefined 的问题在于 undefined 不是保留字 (其实是全局对象的一个属性[wtfjs]).也就是说，undefined 是一个允许的变量名，因此您可以随意为其分配一个新值.
The problem with using undefined was that undefined is not a reserved word (it is actually a property of the global object [wtfjs]). That is, undefined is a permissible variable name, so you could assign a new value to it at your own caprice.
alert(undefined); //alerts "undefined"
var undefined = "new value";
alert(undefined) // alerts "new value"
注意:这在任何支持 ECMAScript 5 或更新版本的环境中都不再是问题(即实际上除了 IE 8 之外的所有地方)，它将全局对象的 undefined 属性定义为只读(因此只能在您自己的本地范围内隐藏变量).但是，此信息对于向后兼容目的仍然很有用.
Note: This is no longer a problem in any environment that supports ECMAScript 5 or newer (i.e. in practice everywhere but IE 8), which defines the undefined property of the global object as read-only (so it is only possible to shadow the variable in your own local scope). However, this information is still useful for backwards-compatibility purposes.
alert(window.hasOwnProperty('undefined')); // alerts "true"
alert(window.undefined); // alerts "undefined"
alert(undefined === window.undefined); // alerts "true"
var undefined = "new value";
alert(undefined); // alerts "new value"
alert(undefined === window.undefined); // alerts "false"
void 另一方面，不能被覆盖.void 0 将总是返回 undefined.另一方面，undefined 可以是 Javascript 先生决定的任何内容.
void, on the other hand, cannot be overidden. void 0 will always return undefined. undefined, on the other hand, can be whatever Mr. Javascript decides he wants it to be.
为什么要使用void 0?0 有什么特别之处?难道我们不能简单地使用 1，或 42，或 1000000 或 Hello, world!"?
Why should we use void 0? What's so special about 0? Couldn't we just as easily use 1, or 42, or 1000000 or "Hello, world!"?
答案是，是的，我们可以，而且效果也很好.传入 0 而不是其他参数的唯一好处是 0 简短且符合习惯.
And the answer is, yes, we could, and it would work just as well. The only benefit of passing in 0 instead of some other argument is that 0 is short and idiomatic.
尽管在现代 JavaScript 环境中通常可以信任 undefined，但 void 0 有一个微不足道的优势:它更短.在编写代码时，差异还不够，但它可以在大型代码库中累积足够多，以至于大多数代码压缩器将 undefined 替换为 void 0 以减少字节数发送到浏览器.
Although undefined can generally be trusted in modern JavaScript environments, there is one trivial advantage of void 0: it's shorter. The difference is not enough to worry about when writing code but it can add up enough over large code bases that most code minifiers replace undefined with void 0 to reduce the number of bytes sent to the browser.
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