Implement foldRight on Array.prototype in javascript

I want to implement a foldRight method with multiple reducer functions

Array.prototype.foldRight = function(f, acc){
    if(this.length == 0){
        return null;
    }
    else {
        const head = this[0];
        const tail = this.slice(1);
        return foldRight(f(acc, head), tail);
    }
};

But im still new to javascript, am I using the Array.prototype right to add an own method to the Array class? And why doesn’t it work when im testing it with exampleArray.foldRight((a,b)=>b+a) etc?