Javascript class structure for game component

I am here today with a more design/conceptual doubt on how to approach a certain class scenario I'm planning to code.

Here's the deal, I am to implement a class Monster, which will hold most of the data related to kinds of monster existent in my game, but it will not hold effective data, in fact, what I want to store here are range of possible values, so when I actually create a monster object, it will have it's attributes within the range for the race/kind of the monster encountered.

Exemplifying: I will have different types of monster, Rat, Spider, Snake, Wolf. Each of this species will roll attributes within certain ranges:

Rat: Attack [5,9], Defense [3,5]
Spider: Attack [6,11], Defense [4,5]
...

I would like to have these data on a JSON object in my js file, from which I would then load its data and prepare to create the monster object on encounter taking this values into consideration.

What is troubling me is the design I should make for this to happen. I would say that a good way to make reference to the types of monsters would be something like, Monster.Rat, Monster.Spider, and call their constructor when the encounter happens: Monster.Spider() -> Would give me a monster object with random attributes within a spider range.

Also some point in time I would like to create specific areas for finding specific types of monsters, and I fail to see clearly how I would do that in a simple way, without making the code hard to read.

I did something like this a couple months ago, using Swift, and the approach I made was having a class method on monster for every type of monster I wanted, but this was hard to maintain, since if I wanted to change the way monsters were created I would have to change all the methods, also add a new method for every new type of monster desired.

What do you think would be the best way to handle the design of a game like that? I know I don't want to create new classes extending Monster for every new type of monster I have, as that feels terribly wrong and bad design to me.

Thanks for your time, I'll keep this post updated with whatever interesting piece of information I find.