get-products.js

const PACKAGEPRODUCTS = [
    { packagename: "Package 1" },
    {
          productlist: [
                  { productname: "product1" },
                  { productname: "product2" },
                  { productname: "product3" }
                ]
        },
    { packagename: "Package 2" },
    { productlist: [{ productname: "product3" }, { productname: "product6" }] },
    { packagename: "Package 3" },
    { productlist: [{ productname: "product6" }, { productname: "product9" }] },
    { packagename: "Package 4" },
    {
          productlist: [
                  { productname: "product2" },
                  { productname: "product3" },
                  { productname: "product5" }
                ]
        }
];

console.log(PACKAGEPRODUCTS.reduce((productNames, productObj) => {
  if (productObj.productlist) {
    // if the item in the array has the productlist attribute, we
    // add it to the array
    productNames.push(...productObj.productlist);
  }
  return productNames;
}, []));