The concept of NaN being a number is not exclusive to JavaScript. In fact, it is a widely used numeric data type value in computing which is meant to represent undefined or unrepresentable mathematical values (such as 0/0, etc.):
0/0; // NaN Math.sqrt(-1); // NaN Math.log(-1); // NaN // ...
NaN is actually, rarely used in a program. It is typically returned from a function or operation to indicate a failed operation on numbers. It is used to suggest that the value is not a legal number (where you may be expecting the return value to be a legal number).
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