JS: NaN

NaN is a primitive value that means “Not a Number”.

NaN is typically returned when converting a value to number that doesn't make sense.

// convert string to number
// if arg does not make sense, result is NaN
console.log ( Number ("xyz")  ); // NaN

[see JS: Number Constructor]

Type of NaN is "number".

[see JS: Value Types]

console.log ( typeof NaN === "number" ); // true

NaN === NaN return false.

console.log ( NaN === NaN ); // false

To test if a value is NaN, use Number.isNaN or the global function isNaN.

[see JS: Number.isNaN]

[see JS: isNaN]

Reference