NaN in JavaScript: What It Is and How to Check for It

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

Example:

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

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.