**Update:**New section “Detecting NaN”.

This post looks at two special values that can be the result of operations that normally return numbers: NaN and Infinity.

## 1. NaN

The value NaN in JavaScript stands for “not a number”. It mainly indicates that parsing a string has gone wrong:

> Number("xyz") NaN

NaN has some Koan-like qualities. Its name is “not a number”, but it’s also not not a number (triggered by a tweet by Ariya Hidayat):

> NaN !== NaN true

Yet, its type is “number”.

> typeof NaN 'number'

### 1.1 Detecting NaN

NaN is the only JavaScript value that is not equal to itself. Without equality at your disposal, you have to use the global function isNaN() to detect it.> isNaN(NaN) true

Kit Cambridge (via Mathias Bynens) points out a pitfall of isNaN(): It coerces its argument to number and will thus even return true for strings that cannot be converted to numbers:

> Number("xyz") NaN > isNaN("xyz") true

For the same reason, isNaN will also return true for many objects:

> Number({}) NaN > isNaN({}) true > Number(["xzy"]) NaN > isNaN(["xzy"]) true

Consult this previous post for details on the conversion algorithm. It is possible to override valueOf to control the result of the conversion to number:

> var obj = { valueOf: function () { return NaN } }; > Number(obj) NaN > isNaN(obj) true

Cambridge’s suggested work-around is to exploit the fact that NaN is the only value x that is non-reflexive (x !== x):

function myIsNaN(x) { return x !== x; }

A fixed version of isNaN will probably be added to ECMAScript 6 as Number.isNaN(). Crockford’s specification of that function better reveals what one is trying to do than Cambridge’s version. It looks as follows (simplified for explanatory purposes):

Number.isNaN = function (value) { return typeof value === 'number' && isNaN(value); };

## 2. Infinity

Division by 0 gives you another special value:> 3/0 Infinity

You can’t play positive and negative infinity against each other:

> Infinity - Infinity NaN

It also turns out that “beyond infinity” is still infinity:

> Infinity + Infinity Infinity > 5 * Infinity Infinity

Source: http://www.2ality.com/2012/02/nan-infinity.html

Source: http://www.2ality.com/2012/02/nan-infinity.html

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