spreadAssignmentOperatorPolyfill.js

const spreadAssignmentOperatorPolyfill = (assignable, extending) => {
  if (assignable === null || assignable === undefined) {
    throw new TypeError(`${assignable} is not iterable`);
  } else if (typeof assignable === "object" && !Array.isArray(assignable)) {
    return { ...assignable, ...extending };
  }
  const assignableIterator = assignable[Symbol.iterator];
  if (!assignableIterator) {
    throw new TypeError(`${assignable} is not iterable`);
  }

  return [...assignable, ...extending];
};