JavaScript Number isNaN() Method

What You’ll Learn

Number.isNaN() is a static method on Number (same API as MDN Number.isNaN). Call Number.isNaN(value) — not on a number instance. It returns true only for the number value NaN. This tutorial covers syntax, why === fails, the difference from global isNaN(), five examples, and try-it labs.

Introduction

Failed math and bad parses often produce NaN (“Not a Number”). You cannot detect it with x === NaN, because NaN is not equal to itself.

Number.isNaN() is the reliable, modern check. Unlike the older global isNaN(), it does not coerce strings first — so Number.isNaN("NaN") is false.

This page is part of JavaScript Number Methods. Related static checks include Number.isFinite() and Number.isInteger().

Understanding the isNaN() Method

Pass any value to Number.isNaN(value). You get true only when both of these are true:

• The type is "number".
• The value is NaN (including Number.NaN and results like 0 / 0).

Everything else — strings, objects, undefined, normal numbers — returns false. That strictness is why it is safer than global isNaN().

📝 Syntax

General form of the static method Number.isNaN:

Number.isNaN(value)

Parameters

• value (any) — the value to test for being the number NaN.

Return value

A boolean:

Exceptions

• None for normal calls — invalid types simply return false.

Common patterns

Number.isNaN(NaN);           // true
Number.isNaN(0 / 0);         // true
Number.isNaN(37);            // false
Number.isNaN("NaN");         // false
isNaN("NaN");                // true (global — coerces!)
NaN === NaN;                 // false (never use ===)

Examples Gallery

Examples follow MDN patterns. Use View Output or Try It Yourself for each case.

Number.isNaN(NaN);  // true

true

Number.isNaN(0 / 0);  // true

true

Number.isNaN(37);  // false

false

Number.isNaN("NaN");  // false
// isNaN("NaN");      // true (global coerces)

false

Number.isNaN(Number.NaN);  // true

true

🚀 Common Use Cases

• Guard after parse — Number.isNaN(Number(raw)) before using form input.
• Failed math — catch 0 / 0 and other indeterminate results.
• API validation — reject NaN before JSON or database writes.
• Replace === NaN — the only reliable equality-style check for NaN.
• Safer than global isNaN — avoid treating strings/objects as NaN by accident.
• Pair with isFinite — finite + not NaN is already covered by Number.isFinite.

📝 Notes

• This is a static method — use Number.isNaN(x) only.
• Does not coerce strings, objects, undefined, or booleans.
• NaN === NaN is always false — never use === to find NaN.
• Global isNaN is older and coercing — prefer the Number static method in new code.
• Number.isNaN(Infinity) is false — use Number.isFinite() for infinity checks.
• Among all JS values, only NaN satisfies x !== x.

💡 Best Practices