JavaScript (ES2026) reference · JSON and Global Functions

JavaScript isNaN() Function

Return true if a value is NaN after converting it to a number, so isNaN("hello") is true. Prefer Number.isNaN().

JavaScript (ES2026) Since ES1

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Syntax

isNaN(value)

Parameter	Description
`value`	The value to test.

true or false.

Converts firstTry it yourself

console.log(isNaN(NaN), isNaN("hello"), isNaN("42"), isNaN(""));

Output

true true false false

Surprises: prefer Number.isNaN()Try it yourself

console.log(isNaN(undefined), Number.isNaN(undefined));

Output

true false

Version	Change
JavaScript ES1	Added

Number.isNaN() · Return true only if a value is the number NaN. Unlike the global isNaN(), it doesn't convert strings first.
isFinite() · Return true if a value is a finite number after converting it to a number. Prefer Number.isFinite() for strict checks.
JavaScript Type Conversion · tutorial