In JavaScript, `NaN`

is a special numeric value:

typeof NaN; // 'number'

Please note that `NaN`

(property of the global object) and `Number.NaN`

(property of the `Number`

object) are equivalent.

It represents "Not-a-Number" (as defined in the IEEE-754 standard). `NaN`

never equals to any other number, or even to itself:

NaN === NaN; // false

Therefore, to check if a number is `NaN`

or not, you would use the `Number.isNaN()`

method (or the slightly different `isNaN()`

global function). However, do not confuse these as a means to check whether a non-numeric value is a number or not. For that, you should instead use:

typeof value === 'number'; // true/false

`NaN`

is actually, rarely used in a program. Typically, it is returned from a function or operation to indicate a failed operation on numbers, suggesting that the value is not a legal number (where you may be expecting the return value to be a legal number). Such is the case, for example, when:

- Parsing numbers;
- Using
`NaN`

as an operand; - Using
`undefined`

as an operand; - Using indeterminate form;
- Math operation does not result in a real number;
- Performing non-addition operation on a string.

## Parsing Numbers

`NaN`

is returned when a value cannot be parsed into a number. For example:

parseInt('foo'); // NaN Number('foo'); // NaN parseInt(undefined); // NaN Number(undefined); // NaN // ...

## Using `NaN`

as an Operand

When an expression has `NaN`

, the resulting value is `NaN`

. Consider, for example, the following:

3 + NaN; // NaN 3 * NaN; // NaN 3 / NaN; // NaN // ...

## Using `undefined`

as an Operand

When `undefined`

is used as an operand, the resulting value is `NaN`

. Consider, for example, the following:

3 + undefined; // NaN 3 * undefined; // NaN 3 / undefined; // NaN // ...

## Using Indeterminate Form

When an operation is in indeterminate form, it returns `NaN`

. For example:

undefined + Infinity; // NaN undefined + undefined; // NaN 0 * Infinity; // NaN 0 * undefined; // NaN // ...

## Math Operation That Does Not Result in a Real Number

A math operation that does not yield a real number, such as when a mathematical function receives an invalid argument, `NaN`

is returned. For example:

Math.sqrt(-1); // NaN // ...

## Performing Non-Addition Operation on a String

Using a operations other than addition on a string yields `NaN`

. For example:

'foo' + 3; // 'foo3' 'foo' - 3; // NaN 'foo' / 3; // NaN 'foo' * 3; // NaN // ...

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