What is the NaN property in JavaScript?

MediumEasyJavascript

By FrontendAtlas Team · Updated Jan 30, 2026

Quick Answer

NaN stands for Not-a-Number and is the only value in JavaScript that is not equal to itself. Use Number.isNaN to check for it reliably, because global isNaN coerces values. This is important for numeric validation and edge cases. Edge cases include empty strings and non-numeric input; test with Number.isNaN.

Answer

The Core Idea

NaN stands for Not-a-Number. It is a special numeric value that represents the result of an invalid mathematical operation — something that cannot produce a meaningful number.

JAVASCRIPT

console.log(0 / 0);         // NaN
console.log(parseInt('Hi'));  // NaN
console.log(Math.sqrt(-1));   // NaN

Property	Description	Example
Type	The type of `NaN` is actually `number`.	`typeof NaN // 'number'`
Equality	`NaN` is the only value in JavaScript that is not equal to itself.	`NaN === NaN // false`
Check	Use `Number.isNaN()` to check if a value is truly NaN.	`Number.isNaN(NaN) // true`

Key characteristics of NaN in JavaScript.

Why not just 'isNaN'?

The global isNaN() function tries to coerce values before checking, which can give confusing results. Always prefer Number.isNaN() because it doesn’t perform type coercion.

JAVASCRIPT

isNaN('Hello');         // true ❌ (string coerced)
Number.isNaN('Hello');   // false ✅

Practical scenario
A numeric input can return '' or 'foo'; you need to detect NaN before calculations.

Common pitfalls

Using global isNaN, which coerces values.
Assuming NaN === NaN is true.
Mixing parseInt and Number without checks.

Trade-off or test tip
Prefer Number.isNaN for accuracy. Test empty strings, null, and non-numeric inputs.

Still so complicated?

Think of NaN like a 'broken calculator' output — the operation ran, but the result doesn’t make mathematical sense, so JavaScript gives you NaN instead of crashing.

Summary

NaN represents 'Not-a-Number' but its type is actually number.
It results from invalid numeric operations.
It’s the only value in JS that is not equal to itself.
Use Number.isNaN() instead of isNaN() to check safely.