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I am wondering why undefined == undefined but NaN != NaN.
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Javascript null represents the intentional absence of any object value. The undefined property indicates that the variable has not been assigned a value or not declared at all. The NaN property represents a “Not-a-Number” value. The NaN property indicates that a value is not a legitimate number.
When NaN is one of the operands of any relational comparison ( > , < , >= , <= ), the result is always false . NaN compares unequal (via == , != , === , and !== ) to any other value — including to another NaN value.
undefined isn't converted into any number, so using it in maths calculations returns NaN. NaN (Not-A-Number ) represents something which is not a number, even though it's actually a number.
NaN (Not a Number) is a numeric data type that means an undefined value or value that cannot be represented, especially results of floating-point calculations.
Because that's how it is defined in both the Abstract Equality Comparison Algorithm, and the Strict Equality Comparison Algorithm.
If either operand to == or === is NaN, it returns false.
Abstract
- If Type(x) is Number, then
- If x is NaN, return false.
- If y is NaN, return false.
- If x is the same Number value as y, return true.
- If x is +0 and y is −0, return true.
- If x is −0 and y is +0, return true.
- Return false.
EDIT: The motivation for the unequal comparison as noted by @CMS is compliance with the IEEE 754 standard.
From the Wikipedia link provided in the comment below:
...The normal comparison operations however treat NaNs as unordered and compare −0 and +0 as equal. The totalOrder predicate will order these cases, and it also distinguishes between different representations of NaNs and between the same decimal floating point number encoded in different ways.
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