Why is NaN === NaN false? [duplicate]

问题:

This question already has answers here:

What is the rationale for all comparisons returning false for IEEE754 NaN values? (13 answers)

Closed 2 years ago.

Why does NaN === NaN return false in Javascript?

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

On the documentation page I see this:

Testing against NaN

Equality operator (== and ===) cannot be used to test a value against NaN. Use isNaN instead.

Is there any reference that answers to the question? It would be welcome.

回答1:

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.

回答2:

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?

回答3:

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