The value NaN is a special quantity defined by IEEE 754. It stands for not a number, even though:

typeof NaN === 'number'    // true

The value can be produced by attempting to convert a string to a number when the string is not in the form of a number. For example:

+ '0'       // 0
+ 'oops'    // NaN

If NaN is an operand in an arithmetic operation, then NaN will be the result. So, if you have a chain of formulas that produce NaN as a result, at least one of the inputs was NaN, or NaN was generated somewhere.

You can test for NaN. As we have seen, typeof does not distinguish between numbers and NaN, and it turns out that NaN is not equal to itself. So, surprisingly:

NaN === NaN    // false
NaN !== NaN    // true

JavaScript provides an isNaN function that can distinguish between numbers and NaN:

isNaN(NaN)       // true
isNaN(0)         // false
isNaN('oops')    // true
isNaN('0')       // false

The isFinite function is the best way of determining whether a value can be used as a number because it rejects NaN and Infinity. Unfortunately, isFinite will attempt to convert its operand to a number, so it is not a good test if a value is not actually a number. You may want to define your own isNumber function:

var isNumber = function isNumber(value) {
            return typeof value === 'number' && isFinite(value);
};