isNaN()

isNaN(value): boolean

Checks if value is NaN.

DEPRECATED

Use Number.isNaN() directly instead.

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Parameters

value: unknown

The value to check.

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Returns: boolean

true if value is NaN, else false.

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See Also

• Number.isNaN() - MDN
• Browser support - Can I Use

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Since

2.0.0

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Also known as

isNaN (Lodash, es-toolkit) · ❌ (Remeda, Radashi, Ramda, Effect, Modern Dash, Antfu)

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Example

// ❌ Deprecated approach
isNaN(NaN);        // => true
isNaN(undefined);  // => false (unlike global isNaN)
isNaN('NaN');      // => false (unlike global isNaN)

// ✅ Recommended approach
Number.isNaN(NaN);        // => true
Number.isNaN(undefined);  // => false
Number.isNaN('NaN');      // => false

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How it works?

Checks if value is NaN.

Difference from Global isNaN

Common Checks

Value	Result
NaN	true
undefined	false
'NaN'	false
null	false
42	false

Deprecated: Use Number.isNaN() directly.

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Use Cases

Check NaN value 📌

Check if value is NaN (Not a Number).

#nan#number#validate#invalid

Number.isNaN(NaN);        // true
Number.isNaN(undefined);  // false
Number.isNaN('hello');    // false

Validate parsed numbers

Check if parsing produced valid number.

#parse#validate#number#input

const parsed = parseFloat(userInput);
if (Number.isNaN(parsed)) {
  console.log('Invalid number');
}