A special constant that specifies a value that is Not-A-Number
NaN mainly occurs when parsing goes wrong
parseInt(123); // 123 parseInt("123"); // 123 parseInt('foo'); // NaN
NaN does not equal NaN, use the isNaN Function instead to test if a value is not a number
NaN === NaN; // false isNaN(NaN); //true
The NaN constant is a member of the Global object, and is made available when the scripting engine is initialized.
Microsoft Developer Network: Article