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.