Описание тега nan
NaN is an abbreviation for "Not a Number". NaN is sometimes not equal to itself.
NaN (Not a Number) is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations. Systematic use of NaNs was introduced by the IEEE 754 floating-point standard in 1985, along with the representation of other non-finite quantities such as infinity.
A common issue of NaN in many programming languages is that NaN is not equal to itself.