Not a Number (NaN) Cheatsheet

A Cheatsheet about Not a Number (NaN) in JavaScript.

Properties & Functions

JavaScript has the global NaN property and two functions to check for NaN values.

NaN

Represents Not-A-Number
Is the result of expressions that should produce a number but failed (e.g. parsing the string "hello" as a number)
Is of type number
(typeof NaN === 'number')
Fails equality comparison
(NaN === NaN is false)

isNaN() 👎

Checks if the value is NaN after parsing it to a number
Returns true if the value is NaN
Returns true if the value can't be parsed to a number (like "hello")
Returns false if the value can be parsed to a number (like "42")
Returns false if the value is a valid number (like 42)

Number.isNaN() 👍

Checks if the value is NaN (without parsing the value to a number)
Returns true if the value is NaN
Returns false for all other values

Producing NaN

There are several ways to produce a NaN value.

Convert to number

Converting a value to a number that can't be converted to a number results in NaN.

parseInt("hello")

Not a real number

Doing a math operation where the result is not a real number results in NaN.

Math.sqrt(-1)

Operation with NaN

Doing a math operation with NaN results in NaN.

42 + NaN

Indeterminate form

Operations that are in indeterminate forms produce NaN.

undefined + undefined

Operation with string

Doing an operation with a string that can't be parsed to a number produces NaN, except for addition operations (which join the values).

"hello" * 2

Overview

The table below shows for each value the result of the operators and functions.

`const x =`	`typeof x`	`parseInt(x)`	`Number(x)`	`isNaN(x)`	`Number.isNaN(x)`	`x === x`
`NaN`	`number`	`NaN`	`NaN`	`true`	`true`	`false`
`undefined`	`undefined`	`NaN`	`NaN`	`true`	`false`	`true`
`null`	`object`	`NaN`	`0`	`false`	`false`	`true`
`""`	`string`	`NaN`	`0`	`false`	`false`	`true`
`"hello"`	`string`	`NaN`	`NaN`	`true`	`false`	`true`
`"42"`	`string`	`42`	`42`	`false`	`false`	`true`
`"42hello"`	`string`	`42`	`NaN`	`true`	`false`	`true`
`"hello42"`	`string`	`NaN`	`NaN`	`true`	`false`	`true`
`42`	`number`	`42`	`42`	`false`	`false`	`true`

The isNaN() function returns true whenever Number() returns NaN. The parseInt() function works different than Number() but this doesn't affect the result of isNaN().