Javascript ‘void 0’是什么意思？
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What does `void 0` mean?
提问by Randomblue
Reading through the Backbone.js source code, I saw this:
阅读 Backbone.js 源代码，我看到了这个：
validObj[attr] = void 0;
What is void 0? What is the purpose of using it here?
什么是void 0？在这里使用它的目的是什么？
回答by Peter Olson
What does void 0mean?
什么void 0意思？
void[MDN]is a prefix keyword that takes one argument and always returns undefined.
void[MDN]是一个前缀关键字，它接受一个参数并始终返回undefined.
Examples
例子
void 0
void (0)
void "hello"
void (new Date())
//all will return undefined
What's the point of that?
这样做有什么意义？
It seems pretty useless, doesn't it? If it always returns undefined, what's wrong with just using undefineditself?
看起来很没用，不是吗？如果它总是返回undefined，那么只使用undefined它自己有什么问题？
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.
在一个完美的世界中，我们将能够安全地使用undefined：它比void 0. 但是，如果您以前从未注意到，这并不是一个完美的世界，尤其是在 Javascript 方面。
The problem with using undefinedwas that undefinedis not a reserved word (it is actually a property of the global object [wtfjs]). That is, undefinedis a permissible variable name, so you could assign a new value to it at your own caprice.
using 的问题undefined在于undefined它不是一个保留字（它实际上是全局对象[wtfjs] 的一个属性）。也就是说，undefined是一个允许的变量名，因此您可以随意为它分配一个新值。
alert(undefined); //alerts "undefined"
var undefined = "new value";
alert(undefined) // alerts "new value"
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 undefinedproperty 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.
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undefined全局对象的属性定义为只读（因此只能在您自己的本地范围）。但是，此信息对于向后兼容目的仍然有用。
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, on the other hand, cannot be overidden. void 0will alwaysreturn undefined. undefined, on the other hand, can be whatever Mr. Javascript decides he wants it to be.
void，另一方面，不能被覆盖。void 0将始终返回undefined。undefined，另一方面，可以是 Javascript 先生决定他想要的任何东西。
Why void 0, specifically?
为什么void 0，特别是？
Why should we use void 0? What's so special about 0? Couldn't we just as easily use 1, or 42, or 1000000or "Hello, world!"?
为什么要使用void 0？有什么特别的0？我们不能同样轻松地使用1, or 42, or 1000000or"Hello, world!"吗？
And the answer is, yes, we could, and it would work just as well. The only benefit of passing in 0instead of some other argument is that 0is short and idiomatic.
答案是，是的，我们可以，而且效果也很好。传入0而不是其他一些参数的唯一好处0是简短而惯用。
Why is this still relevant?
为什么这仍然相关？
Although undefinedcan 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 undefinedwith void 0to reduce the number of bytes sent to the browser.
虽然undefined在现代 JavaScript 环境中通常可以信任，但void 0它有一个微不足道的优势：它更短。在编写代码时，这种差异还不够，但它可以在大多数代码压缩器替换的大型代码库上加起来足够多undefined，void 0以减少发送到浏览器的字节数。
回答by epascarello
void 0returns undefined and can not be overwritten while undefinedcan be overwritten.
void 0返回 undefined 并且不能被覆盖，而undefined可以被覆盖。
var undefined = "HAHA";
