Question: The question comes from reddit's r/javascript and is a recurrent one: Why is decimal-math so funky?

Additional question: Why is NaN != NaN?

The answer is actually pretty simple and it's mostly unrelated to JavaScript.

The thing is that JavaScript has only one number type and that is the standard IEEE-754 for floating-point arithmetic. Now, once you're limited to doing floating point arithmetic in binary, a number of limitations appear. These have already been described nicely enough elsewhere, so I won't do it here. Additional excellent link

We could wonder why we only have one number type and why that one, but I guess, once again, it all boils down to trying to simplify the language, both for programmers and implementers.

Note that this also answers why NaN != NaN, as the standard also defines de behaviour of NaN. At least in this respect. The underlying reason why NaN != NaN is simply logic: That X is not something and Y is also not something cannot tell you anything about X and Y being equal or different. We could make NaN == NaN throw some sort of exception or return undefined, but:

1. allowing that a simple comparison might throw an exception seems hardly practical in any situation
2. allowing a comparison to return anything other than true or false would rather hilariously complicate boolean logic
3. undefined will, in a boolean context, be evaluated as false. Returning false is also less risky than returning true.