NaN

The global NaN property is a value representing Not-A-Number.  

Property attributes of NaN
Writable no
Enumerable no
Configurable no

Description

NaN is a property of the global object. In other words, it is a variable in global scope.

The initial value of NaN is Not-A-Number ‚ÄĒ the same as the value of Number.NaN. In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it. It is rather rare to use NaN in a program.

There are five different types of operations that return NaN:

  • Number cannot be parsed (e.g.¬†parseInt("blabla") or Number(undefined))
  • Math operation where the result is not a real number¬†(e.g.¬†Math.sqrt(-1))
  • Operand of an argument is NaN (e.g.¬†7 ** NaN)
  • Indeterminate form (e.g. 0 * Infinity)
  • Any operation that involves a string and is not an addition operation (e.g. "foo" / 3)

Examples

Testing against NaN

NaN compares unequal (via ==, !=, ===, and !==) to any other value -- including to another NaN value. Use Number.isNaN() or isNaN() to most clearly determine whether a value is NaN. Or perform a self-comparison: NaN, and only NaN, will compare unequal to itself.

NaN === NaN;        // false
Number.NaN === NaN; // false
isNaN(NaN);         // true
isNaN(Number.NaN);  // true
Number.isNaN(NaN);  // true

function valueIsNaN(v) { return v !== v; }
valueIsNaN(1);          // false
valueIsNaN(NaN);        // true
valueIsNaN(Number.NaN); // true

However, do note the difference between isNaN() and Number.isNaN(): the former will return true if the value is currently NaN, or if it is going to be NaN after it is coerced to a number, while the latter will return true only if the value is currently NaN:

isNaN('hello world');        // true
Number.isNaN('hello world'); // false

Additionally, some array methods cannot find NaN, while others can.

let arr = [2, 4, NaN, 12];
arr.indexOf(NaN);                      // -1 (false)
arr.includes(NaN);                     // true
arr.findIndex(n => Number.isNaN(n));   // 2

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'NaN' in that specification.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
NaNChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 4Opera Full support 4Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

Legend

Full support  
Full support

See also