Javascript Data Structure Set via function

/**/*from w  w  w.  ja  v  a  2  s .c  om*/
 * ECMSCRIPT 6 already have a Set class implementation:
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
 * We will try to copy the same functionalities
 * @constructor
 */
function Set() {

    let items = {};

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

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

    this.has = function(value){
        return items.hasOwnProperty(value);
        //return value in items;
    };

    this.clear = function(){
        items = {};
    };

    /**
     * Modern browsers function
     * IE9+, FF4+, Chrome5+, Opera12+, Safari5+
     * @returns {Number}
     */
    this.size = function(){
        return Object.keys(items).length;
    };

    /**
     * cross browser compatibility - legacy browsers
     * for modern browsers use size function
     * @returns {number}
     */
    this.sizeLegacy = function(){
        let count = 0;
        for(let key in items) {
            if(items.hasOwnProperty(key))
                ++count;
        }
        return count;
    };

    /**
     * Modern browsers function
     * IE9+, FF4+, Chrome5+, Opera12+, Safari5+
     * @returns {Array}
     */
    this.values = function(){
        let values = [];
        for (let i=0, keys=Object.keys(items); i<keys.length; i++) {
            values.push(items[keys[i]]);
        }
        return values;
    };

    this.valuesLegacy = function(){
        let values = [];
        for(let key in items) {
            if(items.hasOwnProperty(key)) {
                values.push(items[key]);
            }
        }
        return values;
    };

    this.getItems = function(){
      return items;
    };

    this.union = function(otherSet){
        let unionSet = new Set(); //{1}

        let values = this.values(); //{2}
        for (let i=0; i<values.length; i++){
            unionSet.add(values[i]);
        }

        values = otherSet.values(); //{3}
        for (let i=0; i<values.length; i++){
            unionSet.add(values[i]);
        }

        return unionSet;
    };

    this.intersection = function(otherSet){
        let intersectionSet = new Set(); //{1}

        let values = this.values();
        for (let i=0; i<values.length; i++){ //{2}
            if (otherSet.has(values[i])){    //{3}
                intersectionSet.add(values[i]); //{4}
            }
        }

        return intersectionSet;
    };

    this.difference = function(otherSet){
        let differenceSet = new Set(); //{1}

        let values = this.values();
        for (let i=0; i<values.length; i++){ //{2}
            if (!otherSet.has(values[i])){    //{3}
                differenceSet.add(values[i]); //{4}
            }
        }

        return differenceSet;
    };

    this.subset = function(otherSet){

        if (this.size() > otherSet.size()){ //{1}
            return false;
        } else {
            let values = this.values();
            for (let i=0; i<values.length; i++){ //{2}
                if (!otherSet.has(values[i])){    //{3}
                    return false; //{4}
                }
            }
            return true;
        }
    };
}
let set = new Set();

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
console.log(set.sizeLegacy());   //3

set.delete(1);
console.log(set.values()); //outputs [2]

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