how to only update specific fields and leave others with the previous values in findandupdate with Mongoose

I’ve created an update function using Mongoose. However whenever I update the data using findOneAndUpdate. It updates only the fields entered and making it so that the others are now null or empty. I would like it so that when the user is updating and they dont update a filed it should remain as what it was before instead of being empty now. essentially only update the fields entered.

heres my routes

router.post("/updateStock", (req, res) => {
    const filter = {prodId: req.body.prodID}

    const update = req.body

    Product.findOneAndUpdate(filter, update, {new: true}).then((product) => {
        console.log("success");
        res.send(product);
  }).catch(err => {
       console.log("err", err);
       res.status(500).send(err);
  })

});

here’s an example of req.body. I’ve only updated the title and catergory on the frotnend meaning everything else is blank. When it saves in the DB it ony updates the title and catergory and leaves everything else blank. I would like it to not insert the blank ones into the DB and leave the fields as they are

{
  prodId: 'iPhone 111001200',
  title: 'iPhone 11 Pro',
  manufacturer: '',
  catergory: 'Electronics',
  price: '',
  quantity: ''
}

Here’s my model

const mongoose = require("mongoose");

const Product = mongoose.model(
  "Product",
  new mongoose.Schema({
    title: String,
    manufacturer: String,
    price: String,
    catergory: String,
    quantity: String,
    prodID: String, 
    images: Array
  })
);

module.exports = Product;

Answer

Something like this then:

const update = {};
for (const key of Object.keys(req.body)){
    if (req.body[key] !== '') {
        update[key] = req.body[key];
    }
}
test.findOneAndUpdate(filter, {$set: update}, {new: true}).then((product) => {
        console.log("success");
        res.send(product);
  }).catch(err => {
       console.log("err", err);
       res.status(500).send(err);
  })}

You should use the $set only to update the fields that you want to update. In your case, the fields that are not ''.