How to Use Number.isNaN in JavaScript: Syntax and Examples

Use Number.isNaN(value) to check if value is the special JavaScript value NaN. It returns true only if the value is exactly NaN, unlike the global isNaN() which converts the value first.

Syntax

The syntax for Number.isNaN() is simple:

• Number.isNaN(value): Takes one argument value to test.
• Returns true if value is exactly NaN.
• Returns false for all other values, including non-numbers.

javascript

Number.isNaN(value)

Example

This example shows how Number.isNaN() returns true only for NaN and false for other values:

javascript

console.log(Number.isNaN(NaN));        // true
console.log(Number.isNaN('hello'));    // false
console.log(Number.isNaN(undefined));  // false
console.log(Number.isNaN(123));        // false
console.log(Number.isNaN('123'));      // false
console.log(Number.isNaN(0 / 0));      // true

Output

true
false
false
false
false
true

Common Pitfalls

A common mistake is confusing Number.isNaN() with the global isNaN() function. The global isNaN() converts the value to a number first, which can cause unexpected true results for non-numeric strings.

Use Number.isNaN() when you want to check if a value is exactly NaN without type coercion.

javascript

console.log(isNaN('hello'));         // true (because 'hello' converts to NaN)
console.log(Number.isNaN('hello'));  // false (no coercion, 'hello' is not NaN)

Output

true
false

Quick Reference

Function	Behavior	Example	Returns
Number.isNaN(value)	Checks if value is exactly NaN	Number.isNaN(NaN)	true
Number.isNaN(value)	Returns false for all other values	Number.isNaN('123')	false
isNaN(value)	Converts value to number then checks	isNaN('hello')	true
isNaN(value)	Can give unexpected true for non-numbers	isNaN('123')	false

Key Takeaways

Use Number.isNaN() to check if a value is exactly NaN without type conversion.

Number.isNaN() returns false for all values except the special NaN value.

Avoid using the global isNaN() when you want precise NaN checks because it coerces values.

Number.isNaN() is a reliable way to detect NaN in modern JavaScript.

Remember that NaN is the only value in JavaScript that is not equal to itself.