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?

if (isNaN(null)) {   alert('null is not a number'); } else {   alert('i think null is a number'); }

I’m using Firefox 3. Is that a browser bug?

Other tests:

console.log(null == NaN);   // false console.log(isNaN('text')); // true console.log(NaN == 'text'); // false

So, the problem seems not to be an exact comparison with NaN?

Edit: Now the question has been answered, I have cleaned up my post to have a better version for the archive. However, this renders some comments and even some answers a little incomprehensible. Don’t blame their authors. Among the things I changed was:

• Removed a note saying that I had screwed up the headline in the first place by reverting its meaning
• Earlier answers showed that I didn’t state clearly enough why I thought the behaviour was weird, so I added the examples that check a string and do a manual comparison.

Answer 1

I believe the code is trying to ask, ‘is x numeric?’ with the specific case here of x = null. The function isNaN() can be used to answer this question, but semantically it’s referring specifically to the value NaN. From Wikipedia for NaN:

NaN (Not a Number) is a value of the numeric data type representing an undefined or unrepresentable value, especially in floating-point calculations.

In most cases we think the answer to ‘is null numeric?’ should be no. However, isNaN(null) == false is semantically correct, because null is not NaN.

Here’s the algorithmic explanation:

The function isNaN(x) attempts to convert the passed parameter to a number¹ (equivalent to Number(x)) and then tests if the value is NaN. If the parameter can’t be converted to a number, Number(x) will return NaN². Therefore, if the conversion of parameter x to a number results in NaN, it returns true; otherwise, it returns false.

So in the specific case x = null, null is converted to the number 0, (try evaluating Number(null) and see that it returns 0,) and isNaN(0) returns false. A string that is only digits can be converted to a number and isNaN also returns false. A string (e.g. 'abcd') that cannot be converted to a number will cause isNaN('abcd') to return true, specifically because Number('abcd') returns NaN.

In addition to these apparent edge cases are the standard numerical reasons for returning NaN like 0/0.

As for the seemingly inconsistent tests for equality shown in the question, the behavior of NaN is specified such that any comparison x == NaN is false, regardless of the other operand, including NaN itself¹.