isNaN gotcha in JavaScript

Writing this in the JavaScript console:

var num42 = "42";
var hello = "hello";
var empty = "";
var nothing = null;
console.log(parseInt(num42) + ", " + isNaN(num42));
console.log(parseInt(hello) + ", " + isNaN(hello));
console.log(parseInt(empty) + ", " + isNaN(empty));
console.log(parseInt(nothing) + ", " + isNaN(nothing));

gives you:

42, false
NaN, true
NaN, false
NaN, false

You would think isNaN's behavior should be consistent with parseInt returning NaN...

Comments

[DavidLG]

This is easily explained. isNaN is meant to be called on a number, so "" and null get coerced to 0 before the test for NaN is even done (the same as when you do (+x) or Number(x) on one of these values). parseInt, on the other hand, takes a string, and does not successfully parse "" or "null" as integers.