[JavaScript] Implement Set data type

set.js

/* The Set object lets you store unique values of any type, whether primitive values or object references.
   Resource: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
   Set has 6 methods;
   new Set
   add(item)
   delete(item)
   has(item)
   forEach(callbackFunction)
   values() // Returns array of items.
   Set has 1 property which is,
   size: Returns number of elements in a Set.
*/

// Old school way to create class.
// Usage: new Set([1, 2, 3]);
function Set (items = []) {
  // O(n) | Reducer creates hashmap from that array.
  this.items = items.reduce((previousValue, currentValue) => (previousValue = {...previousValue, [currentValue]: 1}),{});
  // O(n) | Read whole object keys,
  this.size = Object.keys(this.items).length;
};

Set.prototype.has = function (item) {
  return item in this.items;
};

Set.prototype.add = function (item) {
  if (this.has(item)) return;
  this.items[item] = 1;
  this.size++;
  return this;
};

Set.prototype.delete = function (item) {
  if (!this.has(item)) return;
  delete this.items[item];
  this.size--;
  return this;
};

Set.prototype.forEach = function (callbackFunction) {
  Object.keys(this.items).forEach(callbackFunction);
};

Set.prototype.values = function () {
  return Object.keys(this.items);
};