Why is isNaN(null) == false in JS?

Question

This code in JS gives me a popup saying \"i think null is a number\", which I find slightly disturbing. What am I missing?

Answer 1

This is indeed disturbing. Here is an array of values that I tested:

var x = [undefined, NaN, 'blah', 0/0, null, 0, '0', 1, 1/0, -1/0, Number(5)]

It evaluates (in the Firebug console) to:

,NaN,blah,NaN,,0,0,1,Infinity,-Infinity,5

When I call x.map(isNaN) (to call isNaN on each value), I get:

true,true,true,true,false,false,false,false,false,false,false

In conclusion, isNaN looks pretty useless! (Edit: Except it turns out isNaN is only defined over Number, in which case it works just fine -- just with a misleading name.)

Incidentally, here are the types of those values:

x.map(function(n){return typeof n})
-> undefined,number,string,number,object,number,string,number,number,number,number