The JavaScript Number.isNaN() method

Q: How do I check for NaN in JavaScript?

Use Number.isNaN(value). Don't use value === NaN, which is always false.

Q: What is the difference between Number.isNaN() and the global isNaN()?

The global isNaN() coerces its argument to a number first, so isNaN("abc") is true. Number.isNaN() does no coercion and returns true only for the actual NaN value.

Q: Why is NaN === NaN false?

NaN is defined as not equal to anything, including itself. That's exactly why Number.isNaN() exists.

Q: When does a value become NaN?

When a numeric operation fails — parseInt("x"), 0/0, Number(undefined) and similar.

Method JavaScript All modern browsers Updated June 23, 2026

Quick answer

The Number.isNaN() method returns true only when a value is the special NaN value. Number.isNaN(NaN) is true; Number.isNaN("abc") is false. Use it instead of the older global isNaN(), which coerces its argument first and gives misleading results.

Overview

Number.isNaN() exists to solve a genuinely awkward problem: NaN is the only value in JavaScript that isn't equal to itself, so NaN === NaN is false and you can't test for it with a normal comparison. Number.isNaN(value) is the reliable check, returning true only when the value really is NaN.

It replaces the older global isNaN(), which has a notorious flaw: it coerces its argument to a number first. So isNaN("abc") is true (because "abc" converts to NaN) even though "abc" isn't the NaN value — that coercion produces confusing results. Number.isNaN() does no coercion: it returns true only for an actual NaN, so Number.isNaN("abc") is correctly false.

You hit NaN whenever math goes wrong — parseInt("x"), 0/0, Number(undefined). After parsing user input or doing arithmetic that might fail, Number.isNaN() is how you detect the failure and respond. It sits alongside Number.isInteger() and Number.isFinite() as the modern, coercion-free numeric checks.

Syntax

Number.isNaN(value)

Number.isNaN(NaN)        // true
Number.isNaN(0 / 0)      // true
Number.isNaN("abc")      // false (no coercion)
isNaN("abc")             // true  (global, coerces - avoid)

Parameters

The Number.isNaN() method accepts the following parameters.

Parameter	Description
`value`	The value to test. Returns `true` only if it is exactly the `NaN` value.

Example

Live example

<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const result = parseInt('not a number', 10);

  document.getElementById('out').textContent =
    'value: ' + result + '\n' +
    'is NaN? ' + Number.isNaN(result); // value: NaN / is NaN? true
</script>

Best practices

Use Number.isNaN(), not the global isNaN(), to avoid the coercion bug.
Check for NaN after parsing input or doing math that could fail.
Never test with value === NaN — it's always false because NaN isn't equal to itself.
Pair it with Number.isInteger() and Number.isFinite() for thorough validation.

Frequently asked questions

How do I check for NaN in JavaScript?

Use Number.isNaN(value). Don't use value === NaN, which is always false.

What is the difference between Number.isNaN() and the global isNaN()?

The global isNaN() coerces its argument to a number first, so isNaN("abc") is true. Number.isNaN() does no coercion and returns true only for the actual NaN value.

Why is NaN === NaN false?

NaN is defined as not equal to anything, including itself. That's exactly why Number.isNaN() exists.

When does a value become NaN?

When a numeric operation fails — parseInt("x"), 0/0, Number(undefined) and similar.

Number.isInteger() number.toFixed()

References