isNaN(123) //false isNaN(-1.23) //false isNaN(5-2) //false isNaN(0) //false isNaN('123') //false isNaN('Hello') //true isNaN('2005/12/12') //true isNaN('') //false isNaN(true) //false isNaN(undefined) //true isNaN('NaN') //true isNaN(NaN) //true isNaN(0 / 0) //true
function isNaN(x) { return x !== x; }; isNaN(NaN);//true