Also, NaN is the only value in JS that isn't === to itself, so if for some reason you want to test for strict value identity with the value NaN of type number,

Also, NaN is the only value in JS that isn't === to itself, so if for some reason you want to test for strict value identity with the value NaN of type number, that's one way to do it: if(x !== x) ... // x is NaN