adminAPI.js

const api = {
  addProduct: async function (req, res, next) {
    const newId = await this.getNewId();
    res.send(newId);
  },

  getNewId: async function (){
    const { collection } = await connectDB('product_detail');
    const result = await promisify(
      collection
        .find()
        .project({ product_id: 1 })
        .sort({ product_id: -1 })
        .limit(1)
        .toArray
    )();
    const newId = Number(result[0].product_id) + 1;
    return newId;
  }
}