NaN (Not a Number)

NaN is a special value in JavaScript that stands for "Not a Number".

What Causes NaN?

NaN occurs when you attempt to convert non-numeric variables (such as strings) to numbers:

Copyconst result = parseInt("hello")

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

Even though NaN means "Not a Number", its type is actually "number".

Checking for NaN

You might think you can check for NaN like this, but it doesn't work:

Copyconst a = parseInt("hello")

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

Why doesn't this work? NaN is never equal to anything, including itself!

Copyconsole.log(NaN == NaN) // false

The Correct Way: isNaN()

To check if a value is NaN, use the built-in isNaN() function:

Copyconst a = parseInt("hello") // NaN
const b = 5 // number

console.log(isNaN(a)) // true (a is NaN)
console.log(isNaN(b)) // false (b is a valid number)