Understanding non-serializable JavaScript functions

function createCounter() {
  let count = 0;
  return function() {
    return ++count;
  };
}

function factorial(n) {
  return n <= 1 ? 1 : n * factorial(n - 1);
}

function processConfig({
  user: { name = "Anonymous", settings: { theme = "light" } = {} } = {},
  options = { verbose: false }
}) {
  return { userName: name, theme, verbose: options.verbose };
}

function selfReturn() {
  return selfReturn;
}

function createMultiplier(factor) {
  return function(x) {
    return x * factor;
  };
}

const target = { message: "hello" };
const handler = {
  get(obj, prop) {
    return prop in obj ? obj[prop].toUpperCase() : undefined;
  }
};
const proxy = new Proxy(target, handler);
console.log(proxy.message);

const parent = { name: "parent", child: null };
const child = { name: "child", parent: parent };
parent.child = child;