JavaScript isNaN() and Number.isNaN()

The global isNaN() function asks whether a value becomes NaN after numeric conversion. For stricter checks, Number.isNaN() tests only whether a value is actually the special numeric value NaN.

console.log(isNaN("25")); // false: "25" converts to 25
console.log(Number.isNaN("25")); // false: the value is a string, not NaN
console.log(Number.isNaN(NaN)); // true

Show Table of Contents ↓

How the global isNaN() function works Top ↑

isNaN(value) first converts value to a number. It returns true when the converted result is NaN. Because conversion happens first, the result can be surprising when you are really trying to check a value's type.

console.log(isNaN(2));        // false
console.log(isNaN(2.4));      // false
console.log(isNaN("2.4"));    // false
console.log(isNaN("Hello"));  // true
console.log(isNaN("5ab"));    // true
console.log(isNaN());         // true: undefined becomes NaN

Use if...else when your program needs different behavior for valid and invalid numeric input.

Number.isNaN() for a strict NaN check Top ↑

Number.isNaN(value) does not coerce its argument. It returns true only for the numeric value NaN.

console.log(Number.isNaN(NaN));       // true
console.log(Number.isNaN("hello"));   // false
console.log(Number.isNaN("25"));      // false

Why type coercion changes isNaN() results Top ↑

console.log(isNaN(""));       // false: "" becomes 0
console.log(isNaN("   "));    // false: whitespace becomes 0
console.log(isNaN(true));     // false: true becomes 1
console.log(isNaN(null));     // false: null becomes 0

Empty strings and whitespace Top ↑

An empty form field should usually be treated as missing input, even though global isNaN("") returns false. Check for empty or whitespace-only input before numeric conversion.

const raw = "   ";
if (raw.trim() === "") {
  console.log("Enter a value");
}

A safer form-input validation pattern Top ↑

For a field that must contain one complete number, trim it, reject an empty value, convert with Number(), and then test the result.

function readNumber(raw) {
  const text = raw.trim();
  if (text === "") return null;

  const value = Number(text);
  return Number.isNaN(value) ? null : value;
}

The older interactive example is still available at isNaN demo.

Using isNaN() with parseInt() and parseFloat() Top ↑

parseInt() and parseFloat() can return NaN when parsing cannot start with a valid number. Test the parsed result with Number.isNaN().

const whole = parseInt("42px", 10);   // 42
const bad = parseFloat("px42");        // NaN
console.log(Number.isNaN(bad));        // true

Common mistakes Top ↑

Do not use value === NaN; NaN is not equal to itself. Also do not assume !isNaN(value) means the original value is a JavaScript number—the global function may have converted a string, Boolean, empty string, or null.

console.log(NaN === NaN);             // false
console.log(Number.isNaN(NaN));        // true

Related number-validation tutorials Top ↑

Continue with NaN in JavaScript, Number.isInteger(), parseInt(), and parseFloat().

← JavaScript Math Reference

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

About the author   Watch on YouTube

Connect: