从 mongodb 获取数据时，节点 js javascript 中的 Map.get() 总是返回 undefined

Question

I am using a map here in which I stored id:quantity pair I want to add this quantity to each item of products array (containing objects) and I am getting this quantity through map.get(id) method, despite the type of id and the id passed into get function are of same value and data type,I am getting undefined, I tried to verify the type and value of id in map and id passed, both are same.still the problem exists

this is the faulty code, actually I am returning a promise through this function

var cartMap = new Map();
for (let i = 0; i < this.cart.items.length; i++) {
    const e = this.cart.items[i];
    cartMap.set(mongodb.ObjectId(e._id), e.quantity);
}
db.collection("products").find({ _id: { $in: [...productIds] } }).toArray().then((products) => {
    console.log(cartMap);
    let newProd = products.map(prod => {
        console.log(cartMap.get(prod._id)); // returns undefined
        prod.quantity = cartMap.get(prod._id);
        // cartMap.set(prod._id, -1);
        return prod;
    })
    console.log(newProd);
    if (productIds.length > products.length) {
        let garbageCart = cartMap.keys().filter(prod => {
            if (cartMap.get(prod) != -1) {
                return prod;
            }
        })
        garbageCart.sort();
        this.cart.items.sort();
        let i = 0;
        let updatedCart = this.cart.items.filter(prod => {
            if (i >= garbageCart.length) {

            }
            else if (prod._id == garbageCart[i]) {
                i++;
            }
            else {
                return prod;
            }
        })
        db.collection("users").updateOne({ _id: this.id }, {
            $set: { cart: updatedCart }
        })
    }
    resolve(newProd);

console messages

Map(2) { 608c49bca4d1ed0bfc8c41fb => 1, 608c4a41a4d1ed0bfc8c41ff => 1 }
undefined
undefined
[
  {
    _id: 608c49bca4d1ed0bfc8c41fb,
    title: 'chhole bhature',
    description: 'Lorem ipsum dolor sit amet consectetur adipisicing elit. Qui dolor ratione officia quam magni sed, consequuntur maiores illo labore quas molestiae suscipit asperiores beatae voluptatem saepe commodi explicabo perspiciatis non sequi in praesentium illum, nesciunt iste.',   
    price: '1000',
    imageurl: 'imageurl',
    userId: null,
    quantity: undefined
  },
  {
    _id: 608c4a41a4d1ed0bfc8c41ff,
    title: 'hat',
    description: 'Lorem ipsum dolor sit amet consectetur adipisicing elit. Qui dolor ratione officia quam magni sed, consequuntur maiores illo labore quas molestiae suscipit asperiores beatae voluptatem saepe commodi explicabo perspiciatis non sequi in praesentium illum, nesciunt iste.',   
    price: '478',
    imageurl: 'imageurl',
    userId: null,
    quantity: undefined
  }
]

Answer 1

This is because mongodb.ObjectId(e._id) on one hand, and prod._id on the other hand, are references to two different locations in memory, and as such they will not compare as equal to each other - even though I understand the objects available at these locations represent the same ObjectId .

To further illustrate this point in the context of the map() usage:

var map = new Map()
// object1 and object2 represent the same object, but point to different locations in memory
var object1 = createObject()
var object2 = createObject()

map.set(object1, "some value")

console.log(map.get(object1))   // output: "some value"
console.log(map.get(object2))   // output: undefined

function createObject() {
    return {
        key: "value"
    }
}

You could instead use those objects string representations as the keys for the map - by converting them using their ObjectId.toString() method. Since they are value - as opposed to reference - types, strings representing the same sequence of characters will compare as equal to each other.