super() initializes its parent constructor. In this case it calls constructor of Rectangle. So using super(s,s) will initialize w and h.

Rectangle.prototype.area = function() {
    return(this.w*this.h);
};

class Square extends Rectangle {
    constructor(s) {
        super(s,s);
    }
};

However if you do not provide proper argument to super then you have to manually assign the value of w and h. Like:

class Square extends Rectangle {
    constructor(s) {
        super()
        this.w = s;
        this.h = s;
    }
};