为什么 NaN = = = NaN 假？

为什么 NaN === NaN在 Javascript 中返回 false？

> undefined === undefined
true
> NaN === NaN
false
> a = NaN
NaN
> a === a
false

在美国广播公司的节目中，我看到了这样的画面:

对 NaN 的测试

相等运算符(==和 ===)不能用于针对 NaN测试值。请改为使用 isNaN。

这个问题有什么参考答案吗? 欢迎提供。

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最佳答案

Strict answer: Because the JS spec says so:

If Type(x) is Number, then

If x is NaN, return false.

If y is NaN, return false.

Useful answer: The IEEE 754 spec for floating-point numbers (which is used by all languages for floating-point) says that NaNs are never equal.

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This behaviour is specified by the IEEE-754 standard (which the JavaScript spec follows in this respect).

For an extended discussion, see What is the rationale for all comparisons returning false for IEEE754 NaN values?

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Although either side of NaN===NaN contains the same value and their type is Number but they are not same. According to ECMA-262, either side of == or === contains NaN then it will result false value.

you may find a details rules in here-

http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3