# What is the only value not equal to itself in JavaScript?

`NaN`

(Not-a-Number) is the only JavaScript value not equal to itself when comparing with any of the comparison operators. `NaN`

is often the result of meaningless or invalid math computations, so it doesn't make sense for two `NaN`

values to be considered equal.

const x = Math.sqrt(-1); // NaN const y = 0 / 0; // NaN x === y; // false x === NaN; // false Number.isNaN(x); // true Number.isNaN(y); // true isNaN(x); // true isNan('hello'); // true

You can check for `NaN`

values using the `Number.isNaN()`

function. Note that this is different from the original , global `isNaN()`

. Their difference lies in the fact that `isNaN()`

forcefully converts its argument to a number, whereas `Number.isNaN()`

doesn't. This is why `Number.isNaN()`

is considered more robust and preferable in most cases.