Is NaN equal to NaN?

Solution 1:

Update 2

New to ECMAScript 6 is the Object.is() function. This is designed to be a further enhancement of the === check. One of the benefits of this new function is that Object.is(NaN, NaN) will now return true. If you're able to utilize ECMAScript 6, then this would be the most readable and consistent solution for you.

Original

The proper way to check this would be:

isNaN(parseInt(variable))

If whatever you're checking is a NaN, that function will return true. This method is built into the JavaScript spec.

Using jQuery

jQuery built in their own isNaN function originally to help counter some discrepancies between browsers, and add some additional checks so their version can be used instead of the one in VanillaJS.

Update for jQuery

After jQuery 1.7, they changed this function to $.isNumeric().

Documentation of the switch

If you take a look at this Stack Overflow question, you'll find plenty of times where isNaN() returns what would intuitively be considered an "incorrect" answer, but is correct by the spec.

One of the big reasons to avoid the vanilla isNaN() is that null will return false, making you think it is a number. However, the jQuery function covers a much larger range of intuitive results.

From their documentation:

As of jQuery 3.0 $.isNumeric() returns true only if the argument is of type number, or if it's of type string and it can be coerced into finite numbers. In all other cases, it returns false.

Solution 2:

When a JavaScript function returns NaN, this is not a literal string but an object property in the global space. You cannot compare it to the string "NaN".

See https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/NaN

Solution 3:

It's a special case, NaN is the only thing in Javascript not equal to itself.

Although the other answers about strings vs the NaN object are right too.

Solution 4:

NaN is one of the few examples of an object which is not equal to itself. In fact, this very property is used to implement the common bool IsNaN(number) method:

function isNaN(x)
{ 
    return x != x; 
}