const inventory = {
  items: ['apple', 'banana', 'orange'],
  [Symbol.iterator]: function() {
    let index = 0;
    const items = this.items;
    
    return {
      next: function() {
        return index < items.length ?
          { value: items[index++].toUpperCase(), done: false } :
          { done: true };
      }
    };
  }
};

const result = [...inventory].join(' + ');
console.log(result);