NaN in JavaScript

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN

The global NaN property is a value representing Not-A-Number.

NaN is a property of the global object.

The initial value of NaN is Not-A-Number — the same as the value of Number.NaN.

In modern browsers, NaN is a non-configurable, non-writable property.

Even when this is not the case, avoid overriding it.

It is rather rare to use NaN in a program.

It is the returned value when Math functions fail (Math.sqrt(-1)) or when a function trying to parse a number fails (parseInt("blabla")).

 

Testing against NaN

NaN compares unequal (via ==, !=, ===, and !==) to any other value -- including to another NaN value.

Use Number.isNaN() or isNaN() to most clearly determine whether a value is NaN.

Or perform a self-comparison: NaN, and only NaN, will compare unequal to itself.

NaN === NaN;        // false Number.NaN === NaN; // false isNaN(NaN); // true isNaN(Number.NaN); // true function valueIsNaN(v) { return v !== v; } valueIsNaN(1); // false valueIsNaN(NaN); // true valueIsNaN(Number.NaN); // true

However, do note the difference between isNaN() and Number.isNaN(): the former will return true if the value is currently NaN, or if it is going to be NaN after it is coerced to a number, while the latter will return true only if the value is currently NaN:

isNaN('hello world'); // returns 'true'. Number.isNaN('hello world'); // returns 'false'.