combine_flatten.js

const fields = [{ name: "email", value: "" }];
const hiddenFields = [
  { name: "lists", value: "waitlist" },
  { name: "source", value: "twitter" },
  { name: "medium", value: "social" },
  { name: "campaign", value: "header" },
  { name: "campaign_type", value: "website" },
];

const urlParams = new URLSearchParams(window.location.search);

console.log(
  fields.concat(hiddenFields).reduce(
    (obj, field) =>
      Object.assign({}, obj, {
        [field.name]: urlParams.has(field.name)
          ? urlParams.get(field.name)
          : field.value || "",
      }),
    {}
  )
);