How do you check that a number is NaN in JavaScript?

Try this code:

isNaN(parseFloat("geoff"))

For checking whether any value is NaN, instead of just numbers, see here: How do you test for NaN in Javascript?

I just came across this technique in the book Effective JavaScript that is pretty simple:

Since NaN is the only JavaScript value that is treated as unequal to itself, you can always test if a value is NaN by checking it for equality to itself:

var a = NaN;
a !== a; // true 

var b = "foo";
b !== b; // false 

var c = undefined; 
c !== c; // false

var d = {};
d !== d; // false

var e = { valueOf: "foo" }; 
e !== e; // false

Didn't realize this until @allsyed commented, but this is in the ECMA spec: https://tc39.github.io/ecma262/#sec-isnan-number