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What is NaN in JavaScript?

NaN stands for Not-a-Number.
It's a special value in JavaScript that means the operation result is not a valid number.

NaN has numeric type (typeof NaN === "number"), but isn't a number in the classic sense.

When NaN Occurs

NaN appears in the following situations:

const result1 = 0 / 0;           // NaN
const result2 = Math.sqrt(-1);   // NaN
const result3 = parseInt("abc"); // NaN
const result4 = "hello" - 5;     // NaN

NaN Peculiarities

  • NaN !== NaN → true This is one of JavaScript's strangest features: NaN never equals even itself.
console.log(NaN === NaN); // false
  • To check if a value is NaN, you need to use special methods.

How to Properly Check for NaN

MethodFeatures
isNaN(value)Converts value to number, then checks
Number.isNaN(value)Strict check without conversion — preferable

Example:

isNaN("hello"); // true — because "hello" → NaN
Number.isNaN("hello"); // false — string is not NaN
Number.isNaN(NaN); // true

Important:

Never compare NaN using ===. Use Number.isNaN() for accurate checking.

Conclusion

  • NaN is a number type value meaning invalid numeric expression.
  • It doesn't equal itself, which makes it special.
  • Use Number.isNaN() for reliable checking.