什么是`无效0`是什么意思？ [英] What does `void 0` mean?

问题描述

可能重复：结果
  什么＆ldquo;的javascript：无效（0）rdquo;的是什么意思？

通过Backbone.js的源$ C ​​$ C阅读：

Reading through the Backbone.js source code:

validObj[attr] = void 0;

什么是 0无效？这到底是怎么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

<一个href=\"https://developer.mozilla.org/en/JavaScript/Reference/Operators/Special_Operators/void_Operator\"><$c$c>void^[MDN]是它有一个参数，并始终是preFIX关键字返回未定义。

例子

void 0
void (0)
void "hello"
void (new Date())
//all will return undefined

什么的地步？

这似乎pretty没用，不是吗？如果它总是返回未定义，这有什么错只使用未定义本身？

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?

在一个完美的世界里，我们将能够安全地只使用未定义：它比无效0 。但如果你以前从来没有注意到，的这不是一个完美的世界的，特别是当它涉及到的JavaScript。

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.

使用问题未定义是未定义不是保留字（的it其实是全局对象的属性 ^[wtfjs] ）。也就是说，未定义是允许的变量名，所以你可以在你自己的任性分配一个新的值了。

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"

请注意：这不再是在任何环境下，支持的ECMAScript 5还是有问题的更新（即在实践中无处不在，但IE 8），它定义了在未定义属性作为全局对象只读（因此只可能阴影自己的局部范围内的变量）。但是，此信息仍然是有用的向后兼容的目的。

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"

无效，在另一方面，不能overidden。 0无效将总是的回报未定义。 未定义，在另一方面，可以不管的Javascript先生决定，他希望它是。

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 0无效？有什么特别之处 0 ？我们能不能​​很容易地使用 1 或 42 或百万或你好，世界！？

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!"?

和答案是，是的，我们可以，而且它会很好的工作。路过的 0 唯一的好处，而不是其他一些说法是， 0 短地道。

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.

虽然未定义通常可以在现代JavaScript环境是可信的，有一个无效0 有细微的优势：它是短。所不同的是没有足够的理由担心写code时，但它可以在大code碱基，大部分code minifiers替换未定义有足够的加起来 0无效来减少发送到浏览器的字节数。

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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