에뮬레이션 구현 ES6의 set 클래스

function Set() {
    var items = {};
    // this.has = function(value){
    //     return value in items;
    // }
    this.has = function(value){
        return items.hasOwnProperty(value);
    },
    this.add = function(value){
        if (!this.has(value)) {
            items[value] = value;
            return true;
        }
        return false;
    },
    this.remove = function(value){
        if (this.has(value)) {
            delete value;
            return true;
        }
        return false;
    },
    this.clear = function(){
        items={};
    },
    this.size = function(){
        var count = 0;
        for(var prop in items){
            if (items.hasOwnProperty(prop)) {
                ++count;
            }
        }
        return count;
    },
    this.values= function(){
        var values = [];
        for(var value in items){
            if (items.hasOwnProperty(value)) {
                values.push(value);
            }
        }
        return values;
    },
    // 
    this.union = function(otherSet){
        var unionSet = new Set();
        var values = this.values();
        for (var i = 0; i < values.length; i++) {
            unionSet.add(values[i]);
        }

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

        return unionSet;
    },
    // 
    this.intersection = function(otherSet){
        var intersection = new Set();
        var values = this.values();
        for (var i = 0; i < values.length; i++) {
            if (otherSet.has(values[i])) {
                intersection.add(values[i]);
            }
        }
        return intersection;
    },
    // 
    this.difference = function(otherSet){
        var difference = new Set();
        var values = this.values();
        for (var i = 0; i < values.length; i++) {
            if (!otherSet.has(values[i])) {
                difference.add(values[i]);
            }
        }
        return difference;
    },
    // 
    this.subset = function(otherSet){
        var values = this.values();
        if (this.size() > otherSet.size()) {
            return false;
        }
        else{
            for (var i = 0; i < values.length; i++) {
                if (!otherSet.has(values[i])) {
                    return false;
                }
            }
            return true;
        }
    }
}