JavaScript NaN

Written by Swapnil RajaPublished 24 Aug 2026

NaN means Not a Number, and it is the result of a failed number operation.

Confusingly, typeof NaN is number.

It is a number value that represents an impossible result.

Example

javascript

const result = Number("hello");

console.log(result);
console.log(typeof result);

The output is NaN then number.

What Produces NaN

Converting text that is not a number.

Arithmetic with a value that cannot be converted.

Mathematically undefined results, such as 0 / 0.

PreviousJavaScript null and undefined NextJavaScript Primitives and References

Example

javascript

function double(text) {
  const number = Number(text);
  if (Number.isNaN(number)) {
    return "not a number";
  }
  return number * 2;
}

console.log(double("21"));
console.log(double("hello"));

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript NaN</title>
</head>
<body>

  <h1>NaN</h1>

  <input id="value" value="hello">

  <p id="out"></p>

  <script>
    const entered = document.getElementById("value").value;
    const number = Number(entered);

    document.getElementById("out").innerHTML =
      "Converted: " + number +
      "<br>typeof: " + typeof number +
      "<br>Is it NaN: " + Number.isNaN(number);
  </script>

</body>
</html>

JavaScript NaN

Example

What Produces NaN

Syntax

NaN Is Not Equal to Itself

Example

Checking Properly

Example

The Old Global isNaN

Example

It Spreads Through Arithmetic

Example

Guarding Input

Example

Complete Example

Try It Yourself

Important Points

Conclusion