In software development it’s crucial to be precise when you’re using terms: proper use of terms is essential for people to communicate efficiently. It’s highly important to be on the same wavelength.

One of the terms that is frequently misused is the `NaN`

.

It has its roots in the IEEE754 standard, which defines it as a special value (actually, as we’ll see later – a set of values), that represents a result of an arbitrary evaluation that cannot or should not be represented as a particular number, or simply is not known. Eg: `arcsine(2)`

or `0/0`

. This also leads us to another discovery: `NaN`

value actually is of a numeric type! But just a special number that is treated and represented in its own very special way.

So, again, `NaN`

as per the IEEE754 standard is not any arbitrary value of a non-numeric type, but actually is a number! What it means is that one better not say a string `"foo"`

is a `NaN`

, since it’s not.

It gets even more interesting if you check how the `NaN`

values are represented. For double precision (64 bit long) floating point numbers (that are used to represent a `Number`

data type in javascript and few other languages) a special range with the exponent equal to `0x7FF`

is reserved to represent `NaN`

values. Which means that the other 53 bits of a number can be of any value (unless when a fraction equals to `0x0`

that is reserved for `+Infinity`

and `-Infinity`

). So having a double precision floating point number we can construct a `NaN`

value in `9007199254740990`

(2 power 53 minus 2) distinct ways. That’s a lot isn’t it.

To summarise: if you use `NaN`

as a substitution for simply anything that is not a number (a string, a null value, an object, etc) – please stop. If you don’t – please share the knowledge and help other people be as precise as you.

Useful links:

- http://www.binaryconvert.com/ – a calculator that can both convert to and from binary of various number representation formats
- https://en.wikipedia.org/wiki/IEEE_floating_point, https://en.wikipedia.org/wiki/Single-precision_floating-point_format and https://en.wikipedia.org/wiki/Double-precision_floating-point_format – wikipedia articles on subject