class Card {
  static async add(product) {
    const card = await Card.fetch();
    
    const ind = card.products.findIndex(c => c.id === product.id);
    const candidate = card.products[ind]
    
    if (candidate) {
      candidate.count++;
      card.products[ind] = candidate;
    } else {
      product.count = 1
      card.products.push(product);
    }

    card.price += +product.price;

    return new Promise((resolve, reject) => {
      fs.writeFile(p, JSON.stringify(card), err => {
        if(err) {
          reject(err);
        } else {
          console.log(resolve())
          resolve();
        }
      })
    })
  }