web:javascript:es:nan

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The IEEE floating-point standard requires that NaN must be treated as unequal to itself.

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

console.log(isNaN(NaN));    // true

console.log(isNaN("foo"));              // true
console.log(isNaN(undefined));          // true
console.log(isNaN({}));                 // true

you check then a NaN value not with the isNaN function but with the fact that this is the only value not equal to itself

var foo = NaN;
console.log(foo !== foo);    // true; This is NaN
var bar = "bar";
console.log(bar !== bar);    // false; This is not a NaN

function isReallyNaN(x) {
    return x !== x;
}

console.log(isReallyNaN(NaN));
console.log(isReallyNaN("bar"));