JS-035
Educational JavaScript lesson graphic for NaN: Understand what NaN means and when it appears
Beginner4 min~4 min read#nan#number#type-conversion

NaN

Not a Number

Goal

Understand what NaN means and when it appears.

Explanation

  • NaN means Not a Number.
  • It appears when JavaScript cannot produce a valid number.
  • For example, Number("Pizza") returns NaN.
  • NaN has the Number type, even though it actually represents an invalid numeric result.
  • Use Number.isNaN() to check for NaN.

Analogy

Imagine giving a calculator the task of calculating the word “Pizza.” It does not know what to do, so it returns the special value NaN.

Code

console.log(Number("Pizza"));
console.log(Number("123"));

Result: NaN 123

Common mistake

Beginners may think NaN is a string or an error. It is actually a special numeric value.

Remember

  • ✦ NaN means Not a Number.
  • ✦ NaN appears after a failed numeric conversion.
  • ✦ Number.isNaN() checks for NaN.
  • ✦ NaN has the Number type.

Quick Quiz

What does Number("Pizza") return?

Resources