javascript - Is NaN equal to NaN?

Question

parseFloat("NaN")

returns "NaN", but

parseFloat("NaN") == "NaN"

returns false. Now, that's probably a good thing that it does return false, but I don't understand how this is so. Did the JavaScript creators just make this a special case? Because otherwise I can't understand how this returns false.

Answer 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.