รวมสองอาร์เรย์หลังจากใช้วิธีการกรอง

const productDetails = [{ productID: 1, productTitle: 'Product Title 1', productPrice: 2000 }, { productID: 2, productTitle: 'Product Title 2', productPrice: 5000 }, { productID: 3, productTitle: 'Product Title 3', productPrice: 1000 }, { productID: 4, productTitle: 'Product Title 4', productPrice: 10000 }];
const cartProducts = [{ productID: 1, quantity: 5 }, { productID: 3, quantity: 2 }]; 

function cartItemDetails() {
  return productDetails
    .filter(el => cartProducts.some(f => f.productID === el.productID))
    .map(item => ({
      ...item,
      "quantity": cartProducts.find(f => f.productID === item.productID).quantity
    }));
}

console.log(cartItemDetails());

const productDetails = [{ productID: 1, productTitle: 'Product Title 1', productPrice: 2000 }, { productID: 2, productTitle: 'Product Title 2', productPrice: 5000 }, { productID: 3, productTitle: 'Product Title 3', productPrice: 1000 }, { productID: 4, productTitle: 'Product Title 4', productPrice: 10000 }];
const cartProducts = [{ productID: 1, quantity: 5 }, { productID: 3, quantity: 2 }]; 

function cartItemDetails() {
  return productDetails.reduce((acc, curr) => {
    let item = cartProducts.find(f => f.productID === curr.productID);

    if (item) {
      acc.push({ ...curr,
        "quantity": item.quantity
      });
    }

    return acc;
  }, []);
}

console.log(cartItemDetails());

const productDetails = [{ productID: 1, productTitle: 'Product Title 1', productPrice: 2000 }, { productID: 2, productTitle: 'Product Title 2', productPrice: 5000 }, { productID: 3, productTitle: 'Product Title 3', productPrice: 1000 }, { productID: 4, productTitle: 'Product Title 4', productPrice: 10000 }];
const cartProducts = [{ productID: 1, quantity: 5 }, { productID: 3, quantity: 2 }]; 

function cartItemDetails() {
  return cartProducts.map(item => ({
    ...productDetails.find(f => f.productID === item.productID),
    ...item
  }));
}

console.log(cartItemDetails());

const productDetails = [{"productID":1,"productTitle":"Product Title 1","productPrice":2000},{"productID":2,"productTitle":"Product Title 2","productPrice":5000},{"productID":3,"productTitle":"Product Title 3","productPrice":1000},{"productID":4,"productTitle":"Product Title 4","productPrice":10000}]
const cartProducts = [{"productID":1,"quantity":5},{"productID":3,"quantity":2}]

const productDetailsMap = new Map(productDetails.map(o => [o.productID, o]))

const result = cartProducts.map(o => ({
  ...productDetailsMap.get(o.productID),
  ...o
}))

console.log(result)

let productDetails = [
            {
              productID: 1,
              productTitle: 'Product Title 1',
              productPrice: 2000
            },
            {
              productID: 2,
              productTitle: 'Product Title 2',
              productPrice: 5000
            },
            {
              productID: 3,
              productTitle: 'Product Title 3',
              productPrice: 1000
            },
            {
              productID: 4,
              productTitle: 'Product Title 4',
              productPrice: 10000
            }
          ];

let cartProducts = [
            {
              productID: 1,
              quantity: 5,
            },
            {
              productID: 3,
              quantity: 2,
            }
          ];
cartProducts.map(cart => Object.assign(cart, productDetails.find(product => product.productID === cart.productID))  )
console.log(cartProducts)