NaN

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

Property attributes of NaN
Writable	no
Enumerable	no
Configurable	no

Syntax

NaN

Description

NaN is a property of the global object.

The initial value of NaN is Not-A-Number — 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. It is the returned value when Math functions fail (Math.sqrt(-1)) or when a function trying to parse a number fails (parseInt("blabla")).

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

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

Specifications

Specification	Status	Comment
ECMAScript 1st Edition (ECMA-262)	Standard	Initial definition. Implemented in JavaScript 1.3
ECMAScript 5.1 (ECMA-262) The definition of 'NaN' in that specification.	Standard
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'NaN' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'NaN' in that specification.	Draft

Browser compatibility

See also

• Number.NaN
• Number.isNaN()
• isNaN()