constructor(initialCapacity) {
    this._table = new Array(initialCapacity || 64);
    this._items = 0;

    Object.defineProperty(this, 'capacity', {
      get: function() {
        return this._table.length;
      }
    });

    Object.defineProperty(this, 'size', {
      get: function() {
        return this._items;
      }
    });
  }
HashTable constructor
@param Number? initially allocated size