为什么JS中的isNaN（null）== false？ [英] Why is isNaN(null) == false in JS?

问题描述

JS中的这段代码给了我一个弹出窗口我认为null是一个数字，我觉得有点令人不安。我缺少什么？

This code in JS gives me a popup saying "i think null is a number", which I find slightly disturbing. What am I missing?

if (isNaN(null)) {
  alert("null is not a number");
} else {
  alert("i think null is a number");
}

我正在使用Firefox 3.这是浏览器错误吗？

I'm using Firefox 3. Is that a browser bug?

其他测试：

null == NaN; // false
isNaN("text"); // true
NaN == "text" // false

所以，问题似乎不能与NaN精确比较？

So, the problem seems not to be an exact comparison with NaN?

编辑：现在问题已经回答了，我已将我的帖子清理干净了有一个更好的档案版本。然而，这使得一些评论甚至一些答案有点难以理解。不要责怪他们的作者。我更改的内容包括：

Now the question has been answered, I have cleaned up my post to have a better version for the archive. However, this renders some comments and even some answers a little incomprehensible. Don't blame their authors. Among the things I changed was:

• 删除了一条说明我首先通过恢复其含义搞砸了标题的说明


• 早期的答案显示我没有说清楚为什么我认为这种行为很奇怪，所以我添加了检查字符串并进行手动比较的示例。
  

• Removed a note saying that I had screwed up the headline in the first place by reverting its meaning
• Earlier answers showed that I didn't state clearly enough why I thought the behaviour was weird, so I added the examples that check a string and do a manual comparison.

推荐答案

我相信代码试图问， 是 x 数字？这里的具体情况是 x = null 。函数 isNaN（）可以用来回答这个问题，但从语义上讲，它特指的是值 NaN 。来自维基百科的 NaN ：

I believe the code is trying to ask, "is x numeric?" with the specific case here of x = null. The function isNaN() can be used to answer this question, but semantically it's referring specifically to the value NaN. From Wikipedia for NaN:

NaN（ N ot a N umber）是一个表示未定义或不可表示的值的数值数据类型的值，尤其是在浮点计算中。

NaN (Not a Number) is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations.

在大多数情况下，我们认为答案是否为空数字？应该是没有。但是， isNaN（null）== false 在语义上是正确的，因为 null 不是 NaN 。

In most cases we think the answer to "is null numeric?" should be no. However, isNaN(null) == false is semantically correct, because null is not NaN.

以下是算法说明：

函数 isNaN（x）尝试将传递的参数转换为数字 ¹ （相当于 Number（x）），然后测试该值是否为 NaN 。如果参数无法转换为数字， Number（x）将返回 NaN ² 。因此，如果将参数 x 转换为数字会导致 NaN ，则返回true;否则，它返回false。

The function isNaN(x) attempts to convert the passed parameter to a number¹ (equivalent to Number(x)) and then tests if the value is NaN. If the parameter can't be converted to a number, Number(x) will return NaN². Therefore, if the conversion of parameter x to a number results in NaN, it returns true; otherwise, it returns false.

所以在特定情况下 x = null ， null 转换为数字0，（尝试评估 Number（null）并查看它返回0，）和 isNaN（0）返回false。只有数字的字符串可以转换为数字，而isNaN也返回false。无法转换为数字的字符串（例如'abcd'）将导致 isNaN（'abcd'）返回true，特别是因为 Number（'abcd'）返回 NaN 。

So in the specific case x = null, null is converted to the number 0, (try evaluating Number(null) and see that it returns 0,) and isNaN(0) returns false. A string that is only digits can be converted to a number and isNaN also returns false. A string (e.g. 'abcd') that cannot be converted to a number will cause isNaN('abcd') to return true, specifically because Number('abcd') returns NaN.

除了这些明显的边缘情况之外，返回NaN的标准数值原因如0/0。

In addition to these apparent edge cases are the standard numerical reasons for returning NaN like 0/0.

至于看似不一致的相等测试问题是， NaN 的行为被指定为任何比较 x == NaN 都是假的，不管其他什么操作数，包括 NaN 本身 ¹ 。

As for the seemingly inconsistent tests for equality shown in the question, the behavior of NaN is specified such that any comparison x == NaN is false, regardless of the other operand, including NaN itself¹.

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