在JavaScript中与null和undefined进行比较的速度 [英] Speed of comparing to null vs undefined in JavaScript

问题描述

我刚刚运行了一个非常简单的JavaScript 性能测试（不要问为什么）。测试声明了一个变量，但没有为其赋值：

I have just run a very simple JavaScript performance test (don't ask why). The test declares a variable, but doesn't assign anything to it:

var x;

然后将比较值变量的速度与 null 和 undefined ，换句话说：

It then compares the speed of comparing the value variable to null, and to undefined, in other words:

var y =（x == null）; 和 var y =（x == undefined）; 。

我期待与 undefined 的比较成为禁食。事实上，它远远不够。与 null 的比较是最快的，大约快80％。

I was expecting the comparison with undefined to be the fasted. In fact it was nowhere near. The comparison with null was far and away the fastest, around 80% faster.

我描述的结果以上来自于在Chrome中运行测试（版本13）。在Firefox中运行它们会产生更接近我预期的结果（与 undefined 的比较比 null 更快，虽然非常轻微）。

The results I've described above come from running the tests in Chrome (version 13). Running them in Firefox produces results far closer to what I would have expected (the comparison with undefined is faster than with null, albeit very marginally).

所以，我的问题是这可能是什么原因？为什么Chrome似乎更喜欢与 null 进行比较呢？

So, my question is what could the cause of this be? Why does Chrome seem to favour the comparison with null so greatly?

为了快速参考，这里有一个截图结果：

For quick reference, here's a screenshot of the results:

推荐答案

null 是一个无法覆盖的保留关键字，所以当你做的时候与null的比较，您所要做的只是一次比较。

null is a reserved keyword which cannot be overriden, so when you are doing a comparison against null, all you have to do is a single comparison.

但是，当您检查 undefined 时，引擎必须进行类型查找然后进行比较，这意味着它实际上要求更高一些。

However, when you are checking against undefined, the engine must do a type lookup and then a comparison, meaning that it is actually slightly more demanding.

如果您需要实际检查某些内容是否未定义，您应该使用

If you need to actually check to see if something is undefined, you should use

if(typeof notSet == "undefined"){ }

---

证明

尝试...并设置为 null 在您的JavaScript控制台中。

---

Proof

Try it... and set something to null in your JavaScript console.

null = "will error";
// Errors with --> ReferenceError: invalid assignment left-hand side

但是，如果你尝试使用undefined，它赢了不对。错误。这并不是说你可以覆盖 undefined ，因为你不能，但是 undefined 是它自己的原语类型。

However, if you try and do it with undefined, it won't error. That is not to say that you can override undefined, because you can't, but that undefined is its own primitive type.

null和undefined之间唯一真正的相似之处在于它们都可以被强制转换为布尔值false。

The only real similarity between null and undefined, is that they can both be coerced into a boolean false.

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