Primitive Value:  NaN

In ECMAScript, NaN is classified as a primitive value.
NaN means "Not-a-Number". It is used to signify that a value is not a legal number.
Note that the primitive value, Infinity, is used to signify that a number, 1.5x10321 for example, has exceeded the defined range of legal values for a floating point number in JavaScript.
You can use the isNan function to test a value to see if it is a NaN.

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