class Rack{
    constructor(noOfShelf) {
        this.noOfShelf = noOfShelf;
    }

    addShelf(){
        this.noOfShelf = this.noOfShelf + 1;
    }

    isEmpty(){
        if (this.noOfShelf === 0){
            return true;
        }else {
            return false;
        }
    }
}