NaN

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

Value

The same number value as Number.NaN.

Description

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

In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.

There are five different types of operations that return NaN:

• Failed number conversion (e.g. explicit ones like parseInt("blabla"), Number(undefined), or implicit ones like Math.abs(undefined))
• Math operation where the result is not a real number (e.g. Math.sqrt(-1))
• Indeterminate form (e.g. 0 * Infinity, 1 ** Infinity, Infinity / Infinity, Infinity - Infinity)
• A method or expression whose operand is or gets coerced to NaN (e.g. 7 ** NaN, 7 * "blabla") — this means NaN is contagious
• Other cases where an invalid value is to be represented as a number (e.g. an invalid new Date("blabla").getTime(), "".charCodeAt(1))

NaN and its behaviors are not invented by JavaScript. Its semantics in floating point arithmetic (including that NaN !== NaN) are specified by IEEE 754. NaN's behaviors include:

• If NaN is involved in a mathematical operation (but not bitwise operations), the result is usually also NaN. (See counter-example below.)
• When NaN is one of the operands of any relational comparison (>, <, >=, <=), the result is always false.
• NaN compares unequal (via ==, !=, ===, and !==) to any other value — including to another NaN value.

NaN is also one of the falsy values in JavaScript.