How do you check that a number is NaN in JavaScript?

NaN is a JavaScript property representing "Not-a-Number" value. It indicates that a value is not a legal number. JavaScript provides two methods to check for NaN values: Number.isNaN() and the global isNaN() function.

Syntax

Number.NaN
Number.isNaN(value)
isNaN(value)

Using Number.isNaN() (Recommended)

Number.isNaN() is the most reliable method as it only returns true for actual NaN values without type coercion:

    Check with Number.isNaN()
    

    

Using isNaN() (Global Function)

The global isNaN() function converts values to numbers before checking, which can lead to unexpected results:

    Check with isNaN()
    

    

Comparison

Common Use Cases

// Checking calculation results
let result = Math.sqrt(-1);
console.log(Number.isNaN(result)); // true

// Validating parsed numbers
let userInput = parseInt("abc");
console.log(Number.isNaN(userInput)); // true

// Safe division check
let division = 0 / 0;
console.log(Number.isNaN(division)); // true

true
true
true

Conclusion

Use Number.isNaN() for accurate NaN detection as it doesn't perform type coercion. The global isNaN() function can produce unexpected results due to automatic type conversion.