How to use variable in promise? NodeJS - node.js

Question

I'm writing a shop-ecomerce website. I want to add product to my cart, so there are two circumstances
1. The product with that id is in the cart => add more quantity (SOLVED)
2. The product is not exist => create new one
So how to check if a product (with specific id) exist in cart?
I use another variable (let exist) to check but it seems doesn't work (due to promise, I think)
// Add product to cart
router.post('/add', checkToken, (req, res) => {
let _idProduct = req.body._idProduct;
let quantity = req.body.quantity;
let exist = false;
Cart
.findOne({ user: req.decoded.userId })
.exec()
.then(cart => {
cart.items.map(item => {
// Product exist => add more quantity
if (item.product == _idProduct) {
item.quantity += quantity;
}
})
// How to check if no product with that id in Cart ??
cart.save(err => console.log(err));
res.json({
cart: cart
})
})
.catch(err => { console.log(err)});
})
Cart model
var Cart = new mongoose.Schema({
user: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
items: [
{
product: { type: mongoose.Schema.Types.ObjectId, ref: 'Product' },
quantity: { type: Number }
}
],
})

Answer 1

You can use a boolean/flag here to see if the item is found.
cart.items.map(item => {
let found = false; // boolean to see if we found the item
// Product exist => add more quantity
if (item.product == _idProduct) {
item.quantity += quantity;
found = true; // if we find an existing item, set the flag to true
}
if (!found){ // if the item isn't found, we can add it to the cart.
// add item to cart
}
})

Answer 2

This structure might make a bit more sense - instead of using map, you can filter your cart.items array to find the matching product. If it's there, increment the quantity. If not, add it to the array or whatever it is that you need to do.
.then(cart => {
const existingItem = cart.items.filter(item => item.product == _idProduct)[0]
if(existingItem) existingItem.quantity += quantity
else {
//item does not exist, do what you need to do
}
cart.save(err => console.log(err));
res.json({
cart: cart
})
})