NaN stands for “Not a Number” in JavaScript. It is a special value of the Number data type that represents an unrepresentable or undefined value. NaN is often returned as the result of invalid or undefined mathematical operations, such as dividing zero by zero or trying to convert non-numeric values to numbers.

## NaN Data Type

The NaN data type is unique in JavaScript because it is considered a number. However, it is not equal to any other number, including itself. For example:

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

In the above code snippet, we are comparing two NaN values using the strict equality operator (===). Surprisingly, the comparison returns false because NaN is not equal to itself.

To check if a value is NaN, we can use the isNaN() function:

console.log(isNaN(NaN));// Output: trueconsole.log(isNaN(10));// Output: false

The isNaN() function returns true if the passed value cannot be converted to a valid number; otherwise, it returns false.

## NaN and Arithmetic Operations

When performing arithmetic operations involving NaN, the result will always be NaN:

console.log(NaN + 5);// Output: NaNconsole.log(NaN * 10);// Output: NaNconsole.log(Math.sqrt(NaN));// Output: NaN

In the above examples, adding 5 to NaN or multiplying it by 10 both result in NaN. Even taking the square root of NaN using Math.sqrt() returns NaN.

## Detecting NaN

To determine if a value is NaN without using the isNaN() function, we can use the Number.isNaN() method introduced in ECMAScript 2015:

console.log(Number.isNaN(NaN));// Output: trueconsole.isNaN(10));// Output: false

The Number.isNaN() method returns true if the passed value is NaN; otherwise, it returns false. Unlike the global isNaN() function, Number.isNaN() does not try to convert non-number values to numbers before checking for NaN.

## Conclusion

NaN is a special value in JavaScript that represents an unrepresentable or undefined number. It is considered a number data type but behaves uniquely compared to other numbers.

NaN is not equal to any other value, including itself. When performing arithmetic operations with NaN, the result will always be NaN. We can use the isNaN() function or Number.isNaN() method to check if a value is NaN.