Just to add to what Jashan said, heres how you actually declare a class in js:

class YourClassName
{
}

or

class YourClassName extends OtherClass
{
}

In Unity 3, you'll need to use implements instead of extends if you're implementing interfaces

In fact, you are creating JavaScript classes whenever you are creating a script in Unity which uses JavaScript / UnityScript as implementation language. Those "scripts" are classes derived of MonoBehaviour (that's something that UnityScript/JavaScript hides from you - in C#, these things are much clearer ... you see that you create a class, and you also see which class the class is derived from).

Of course, in JavaScript, you could also create non-MonoBehaviour classes which could be used e.g. for plain data containers that should not be attached to any game objects.