What is NaN in JavaScript?

In JavaScript, NaN stands for "Not-a-Number." It is a special value used to represent something that is not a valid number, even though it is of type number.

What causes NaN?

Math operations that don't make sense result in NaN. For example, dividing 0 by 0 or trying to convert a non-numeric string to a number.

console.log(0 / 0); // NaN
console.log(Number("hello")); // NaN

Type of NaN:

Even though NaN means "Not-a-Number", its type is still number (which is kind of ironic).

console.log(typeof NaN); // "number"

How to check for NaN:

You can't use == or === to check for NaN because NaN is never equal to itself.

console.log(NaN === NaN); // false

Instead, use isNaN() to check if a value is NaN:

console.log(isNaN(NaN)); // true
console.log(isNaN("hello")); // true
console.log(isNaN(123)); // false

ES6+ (New way to check for NaN):

There’s also a better way to check for NaN using Number.isNaN(), which is more accurate:

console.log(Number.isNaN(NaN)); // true
console.log(Number.isNaN("hello")); // false (because "hello" isn't even a number)

Conclusion:

Basically, NaN is a weird value that tells you something went wrong with a number operation. But it still belongs to the number type, just not a valid number!