Check for illegal number with isNaN() in JavaScript

In JavaScript, isNaN() function checks if a value is "Not a Number" (NaN). It's commonly used to validate numeric operations and detect illegal numbers in calculations.

What is NaN?

NaN stands for "Not a Number" and is returned when a mathematical operation fails or produces an undefined result, such as multiplying a string with a number.

Syntax

isNaN(value)

Parameters

value: The value to be tested. If not a number, JavaScript attempts to convert it before testing.

Return Value

Returns true if the value is NaN, false otherwise.

Example: Detecting Illegal Numbers in Calculations

function multiplication(firstValue, secondValue, callback) {
    var res = firstValue * secondValue;
    var err = isNaN(res) ? 'Something is wrong in input parameter' : undefined;
    callback(res, err);
}

multiplication(10, 50, function (result, error) {
    console.log("The multiplication result=" + result);
    if (error) {
        console.log(error);
    }
});

multiplication('Sam', 5, function (result, error) {
    console.log("The multiplication result=" + result);
    if (error) {
        console.log(error);
    }
});

The multiplication result=500
The multiplication result=NaN
Something is wrong in input parameter

Basic isNaN() Examples

console.log(isNaN(123));        // false - valid number
console.log(isNaN('123'));      // false - string converts to number
console.log(isNaN('Hello'));    // true - cannot convert to number
console.log(isNaN(NaN));        // true - explicitly NaN
console.log(isNaN(undefined));  // true - undefined becomes NaN

false
false
true
true
true

Comparison: isNaN() vs Number.isNaN()

console.log(isNaN('Hello'));         // true (converts, then checks)
console.log(Number.isNaN('Hello'));  // false (no conversion)
console.log(Number.isNaN(NaN));      // true (exact match)

true
false
true

Common Use Cases

Form Validation: Check if user input is a valid number before processing.

Mathematical Operations: Validate results of calculations to prevent errors in further computations.

Conclusion

Use isNaN() to detect invalid numeric results in calculations. For strict NaN checking without type conversion, prefer Number.isNaN().