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In this case I found NaN==NaN is false. I don't understand why this is happen because of both type is number and value is same. So how it possible.

Sami Rahman
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    I wonder which cases you found when this was true??? – Bergi Jan 27 '15 at 06:58
  • You will probably find `isNaN(num)` useful. See [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN) for details. – jfriend00 Jan 27 '15 at 06:58
  • try it.... alert(NaN == NaN) will always be false.. Not in some cases, in all... – Nico Jan 27 '15 at 06:59
  • 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 – Md Mehedi Hasan Jan 27 '15 at 07:03

4 Answers4

1

Treat this as NaN is UNKNOWN so,

UNKNOWN can never be same as another UNKNOWN....UNKNOWN(NaN)==UNKNOWN(NaN)//false

another example:

Lets say x='a', y='b' both are not numbers so x=NaN, y=NaN but x!=y

http://es5.github.io/#x11.9.6.

http://en.wikipedia.org/wiki/IEEE_floating_point

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

Vijay
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1

NaN's are never equal to each other by design.

Math.log(-1) is NaN

Math.log(-2) is NaN

does that mean that Math.log(-1) == Math.log(-2)?

Amir Popovich
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0

NaN's are unusual case. They are not equal to anything (to itself too).

So, it returns false when you test NaN==NaN.

To test the NaN you can use as @Bergi commented:

var x = NaN;
if(x !== x){
  console.log('It is NaN');
}
Bhojendra Rauniyar
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  • I don't think it's just Javascript. I think this is pretty common, and may even be required by IEEE-754. – Barmar Jan 27 '15 at 06:59
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Nan - JavaScript what a beautiful case.

Testing against NaN

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

  NaN === NaN;        // false 
  Number.NaN === NaN; // false 
  isNaN(NaN);         // true 
  isNaN(Number.NaN);  // true
Barmar
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user12121234
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