Need help: "Challenge: Rabbit Racer" (I'm confused)

For some reason I am confused about how this specific line or two of code is meant to be written.

var btn1 = new Button({
    x: ,
    y: ,
    width: ,
    height: ,
    color: color(),
    label: ,
    onClick: function() {
        rabbits[].(); 
    }
});

I think I understand I need to call the onClick to the Rabbit.prototype.hop ?
But I don't think I understand how/where.
I tried writing "rabbits[hop].();" but then it says hop is not defined.
I feel like I've forgot the entire first JS course all of a sudden.
I looked back on the arrays class and I still didn't understand. Am I stupid?

Edit: I've never been this confused about JS before since I started learning.
I made sure I understood everything perfectly well before moving on every previous class and right now I'm so lost I'm convinced I lost my brain.

This function format seems 100% new to me, and is appearing for the first time in a Challenge.
I tried googling this to no avail.