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Chofoteddy
Asked: 2020-02-23 14:49:52 +0800 CST 2020-02-23 14:49:52 +0800 CST

Why NaN != NaN?

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I had been programming in JavaScript for some time and I had never noticed that peculiarity that it has with the value NaN.

I decided to investigate and the most I found was: NaN - JavaScript | MDN / [English] .

I understood that it NaNis the reference of “Not a Number” (it is not a number) but I am not clear why a NaN is not equal to another NaN using the operator ==, ===.

parseFloat('NaN') == parseFloat('NaN'); // false
parseFloat('NaN') == NaN; // false
NaN == NaN; // false

What happens at the logical level that equality is not fulfilled? Why does it not also apply to the expression undefined == undefined?

javascript

3 Answers

  1. Best Answer

    This is how it has been defined and it makes sense, for example imagine the following case:

    var a = 1/"lunes"; //NaN
var b = Math.sqrt(-1) //NaN

    As much aas bthey result NaN. Now, do you think it is correct that they are the same?

    This is why the function is used isNaN()to verify if we are dealing with one NaNor not instead of trying to compare it against another NaN.

    This does not apply to undefinedbecause this has a precise meaning: An undefined identifier. Instead NaNit is a kind of invalid value for what in other languages ​​would be a run-time error or exception.

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  2. In simple words NaN Not a Number(NaN is not a number) and since it is not a number, it is anything. It is not possible to compare anything on one side with anything on the other side, so there is no guarantee that the two are the same. For example, it is not possible to make this comparison:

    (0.0 / 0.0) == (0.0 / 0.0)  // false

    This comparison is the same as:

    NaN == NaN; // false

    In JavaScript undefinedit is a data type , just like the Number, String, Boolean, Objector the Array. In JavaScript, unlike other strongly typed programming languages, we can have variables of type undefined.

    In summary, in JavaScript, all those variables that have not been defined (therefore do not exist) or that have been defined without assigning them a value, are always of type undefined.

    So logically when comparing two things that have the same data type , in this case undefined, it returns Verdadero.

    Reference: Java Language Specification (JLS)

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  3. Reviewing the NaN documentation

    You will see what he comments

    NaN is never equivalent to any other number, including NaN itself; you cannot check the value of a not-a-number against Number.NaN . Use the isNaN() function for that.

    In other words, you must use the isNaN()to achieve what you propose.

    I want to think that this is so because it is an object, which is compared by reference when using the ==so two references will not be equal.

    The same doesn't happen with undefined because this is a primitive value, it's not an object like NaN, so you can compare it to equal.

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