只是出于好奇。

typeof NaN是number似乎不太符合逻辑。就像NaN === NaN或NaN == NaN返回false。这是JavaScript的特性之一吗,或者有什么原因吗?

编辑:谢谢你的回答。然而,让人明白这不是一件容易的事情。阅读答案和维基我明白了更多,但仍然,一句话像

与NaN的比较总是返回无序结果,即使是与NaN本身比较也是如此。比较谓词是信号或非信号,信号版本表示此类比较的无效异常。等式和不等式谓词是非信号,因此x = x返回false可用于测试x是否为静态NaN。

让我头晕目眩。如果有人能把它翻译成人类(而不是数学家)可读的语言,我会很感激。


当前回答

Javascript uses NaN to represent anything it encounters that can't be represented any other way by its specifications. It does not mean it is not a number. It's just the easiest way to describe the encounter. NaN means that it or an object that refers to it could not be represented in any other way by javascript. For all practical purposes, it is 'unknown'. Being 'unknown' it cannot tell you what it is nor even if it is itself. It is not even the object it is assigned to. It can only tell you what it is not, and not-ness or nothingness can only be described mathematically in a programming language. Since mathematics is about numbers, javascript represents nothingness as NaN. That doesn't mean it's not a number. It means we can't read it any other way that makes sense. That's why it can't even equal itself. Because it doesn't.

其他回答

Javascript只有一种数字数据类型,即标准的64位双精度浮点数。所有东西都是双的。NaN是double的特殊值,但它仍然是double。

所有parseInt所做的是将字符串“转换”为数值数据类型,因此结果总是“number”;只有当原始字符串不可解析时,它的值才会是NaN。

好吧,这似乎有点奇怪,所谓的“不是一个数字”被认为是一个数字,但NaN仍然是一个数字类型,尽管事实如此:-)

NaN仅仅意味着特定的值不能在数字类型的限制范围内表示(尽管这可以说是所有必须四舍五入以适应的数字,但NaN是一个特殊情况)。

一个特定的NaN不等同于另一个NaN,因为它们可能是不同的值。然而,NaN仍然是一种数字类型,就像2718或31415一样。


至于你更新的问题,用外行的话解释一下:

与NaN的比较总是返回无序结果,即使是与NaN本身比较也是如此。比较谓词是信号或非信号,信号版本表示此类比较的无效异常。等式和不等式谓词是无信号的,因此x = x返回false可用于测试x是否为静态NaN。

所有这一切的意思是(分解为几个部分):

与NaN的比较总是返回无序结果,即使是与NaN本身比较也是如此。

基本上,一个NaN不等于任何其他数字,包括另一个NaN,甚至包括它自己。

比较谓词是信号或非信号,信号版本表示此类比较的无效异常。

试图在一个NaN和另一个数字之间进行比较(小于、大于等)操作,可能导致抛出异常(发出信号),也可能导致结果为false(非发出信号或为quiet)。

等式和不等式谓词是无信号的,因此x = x返回false可用于测试x是否为静态NaN。

相等(等于,不等于)的测试永远不会发出信号,因此使用它们不会导致异常。如果有一个正则数x,那么x == x总是成立的。如果x是一个NaN,那么x == x将永远是假的。它为您提供了一种轻松(悄悄)检测NaN的方法。

typeof NaN返回'number',因为:

ECMAScript spec says the Number type includes NaN: 4.3.20 Number type set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity So typeof returns accordingly: 11.4.3 The typeof Operator The production UnaryExpression : typeof UnaryExpression is evaluated as follows: Let val be the result of evaluating UnaryExpression. If Type(val) is Reference, then If IsUnresolvableReference(val) is true, return "undefined". Let val be GetValue(val). Return a String determined by Type(val) according to Table 20. ​ Table 20 — typeof Operator Results ================================================================== | Type of val | Result | ================================================================== | Undefined | "undefined" | |----------------------------------------------------------------| | Null | "object" | |----------------------------------------------------------------| | Boolean | "boolean" | |----------------------------------------------------------------| | Number | "number" | |----------------------------------------------------------------| | String | "string" | |----------------------------------------------------------------| | Object (native and does | "object" | | not implement [[Call]]) | | |----------------------------------------------------------------| | Object (native or host and | "function" | | does implement [[Call]]) | | |----------------------------------------------------------------| | Object (host and does not | Implementation-defined except may | | implement [[Call]]) | not be "undefined", "boolean", | | | "number", or "string". | ------------------------------------------------------------------

此行为符合IEEE浮点运算标准(IEEE 754):

4.3.19 Number value primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value 4.3.23 NaN number value that is a IEEE 754 “Not-a-Number” value 8.5 The Number Type The Number type has exactly 18437736874454810627 (that is, 253−264+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (that is, 253−2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.)

NaN代表Not a Number。它是数值型数据类型(通常是浮点型,但不总是)的值,表示无效操作(如除零)的结果。

尽管它的名称表明它不是数字,但用于保存它的数据类型是数字类型。因此在JavaScript中,请求NaN的数据类型将返回number(正如alert(typeof(NaN))清楚地演示的那样)。

你必须热爱Javascript。它有一些有趣的小怪癖。

http://wtfjs.com/page/13

如果你停下来从逻辑上算出这些怪癖,或者如果你知道一点数论,大多数怪癖都可以解释清楚,但如果你不知道它们,它们仍然会让你出错。

顺便说一下,我建议你阅读http://wtfjs.com/的其余部分——还有很多比这个更有趣的怪癖!