Javascript Data Structure Set via class

let Set2 = (function () {

    const items = new WeakMap();/* w  ww . j a v  a2s.  co  m*/

    class Set2 {

        constructor () {
            items.set(this, {});
        }

        add(value){
            if (!this.has(value)){
                let items_ = items.get(this);
                items_[value] = value;
                return true;
            }
            return false;
        }

        delete(value){
            if (this.has(value)){
                let items_ = items.get(this);
                delete items_[value];
                return true;
            }
            return false;
        }

        has(value){
            let items_ = items.get(this);
            return items_.hasOwnProperty(value);
        }

        clear(){
            items.set(this, {});
        }

        size(){
            let items_ = items.get(this);
            return Object.keys(items_).length;
        }


        values(){
            let values = [];
            let items_ = items.get(this);
            for (let i=0, keys=Object.keys(items_); i<keys.length; i++) {
                values.push(items_[keys[i]]);
            }
            return values;
        }

        getItems(){
            return items.get(this);
        }

        union(otherSet){
            let unionSet = new Set();

            let values = this.values();
            for (let i=0; i<values.length; i++){
                unionSet.add(values[i]);
            }

            values = otherSet.values();
            for (let i=0; i<values.length; i++){
                unionSet.add(values[i]);
            }

            return unionSet;
        }

        intersection(otherSet){
            let intersectionSet = new Set();

            let values = this.values();
            for (let i=0; i<values.length; i++){
                if (otherSet.has(values[i])){
                    intersectionSet.add(values[i]);
                }
            }

            return intersectionSet;
        }

        difference(otherSet){
            let differenceSet = new Set();

            let values = this.values();
            for (let i=0; i<values.length; i++){
                if (!otherSet.has(values[i])){
                    differenceSet.add(values[i]);
                }
            }

            return differenceSet;
        };

        subset(otherSet){

            if (this.size() > otherSet.size()){
                return false;
            } else {
                let values = this.values();
                for (let i=0; i<values.length; i++){
                    if (!otherSet.has(values[i])){
                        return false;
                    }
                }
                return true;
            }
        };
    }
    return Set2;
})();
let set = new Set2();

set.add(1);
console.log(set.values()); //outputs [1]
console.log(set.has(1));   //outputs true
console.log(set.size());   //outputs 1

set.add(2);
console.log(set.values()); //outputs [1, 2]
console.log(set.has(2));   //true
console.log(set.size());   //2
set.delete(1);
console.log(set.values()); //outputs [2]

set.delete(2);
console.log(set.values()); //outputs []