Update items quantity in shopping cart in MongoDB (MERN Stack) - node.js

Question

I have a function to add items to the shopping cart. Inside the function I have an if-else case. If the cart does not exist for that given user then it is created from scratch, otherwise (else case) if the cart already exists for that given user, then the key quantity must be updated to the specific itemId of that given item with the same Id.
In the function there is already the code block of the if case in case the cart does not exist. What can I do if I need to update the quantity of the Items?
Cart Schema
const mongoose = require('mongoose');
const idValidator = require('mongoose-id-validator');
const Schema = mongoose.Schema;
const cartItemSchema = new Schema ({
quantity: { type: Number, required: true },
itemId: { type: mongoose.Types.ObjectId, required: true, ref: 'Meal' }
});
const cartSchema = new Schema ({
cartItems : [
cartItemSchema
],
customer: { type: mongoose.Types.ObjectId, required: true, unique: true, ref: 'User',}
});
cartSchema.plugin(idValidator);
module.exports = mongoose.model('Cart', cartSchema);
Function
const addToCartByUserId = async (req, res, next) => {
const userId = req.params.uid;
const errors = validationResult(req);
if (!errors.isEmpty()) {
return next(
new HttpError('Invalid inputs passed, check your data.', 422)
);
}
/* Find user in Database */
let user;
try {
user = await User.findById(userId);
} catch(err) {
const error = new HttpError('Creating cart failed, try again later.', 500);
return next(error);
}
if (!user) {
const error = new HttpError('Could not find user for provided id.', 404);
return next(error);
}
/* getCartByUserId */
let cart;
try {
cart = await Cart.find({ customer: userId });
} catch(err) {
const error = new HttpError('Something went wrong, could not find an user by its id.', 500);
return next(error);
}
// if cart doesn't exists. It works
if (!cart || cart.length === 0) {
const { cartItems } = req.body;
const createdCart = new Cart ({
cartItems,
customer: userId
});
try {
const session = await mongoose.startSession();
await session.withTransaction(async () => {
createdCart.save({ session });
});
session.endSession();
} catch(err) {
const error = new HttpError(
'Creating cart failed, please try again.',
500
);
console.log(err);
return next(error);
}
res.status(201).json({ cart: createdCart });
} else { // if cart already exists
let quantityToInsert = cart.cartItems[cartItems.itemId].quantity + cartItems.quantity;
cart.cartItems.push({
quantity: quantityToInsert
});
}
};