Why JavaScript says that a number is not a number? [duplicate]

I have a piece of JavaScript code which is expected to set an integer value to a variable.

Something is broken, so when I try to do alert(A);, it returns NaN. isNaN(A); returns true. But if I alert(typeof(A));, it says number.

So how can a variable be a number and not a number at the same time? Maybe I misunderstood what NaN really is?

Edit: thanks to the answers, I see that I was wrong, because:

• The type of NaN is Number,
• NaN does mean "Not a number", which is not the same thing as "not of type Number",
• 0/0 is a good example of NaN: it is still a number, but JavaScript (and nobody else) can say what is the real value of zero divided by zero. 1/0 on the other hand returns Infinity, which is not NaN.

Solution 1:

As I understand it, NaN is a sentinel instance of the Number class that represents, well, exactly what it stands for - numeric results that cannot be adequately represented. So 0/0 is not a number, in the sense that it's NaN, but it is a Number in terms of its type.

Perhaps it should have been called NaRN (Not a Representable Number).

Solution 2:

If you have a variable and assign it the result of 0/0, the variable is still of numeric type, but the value is undefined (not a number). There are other conditions under which this can occur, but this illustrates what you are seeing.

Solution 3:

You are confusing the type of your object with the value. NaN is a specific value that a an object of type number can be assigned with, for instance in the case of division of zero by zero or when trying to convert a number from a string that does not represent a number.

Solution 4:

You should check out the Wikipedia article. It has more details.