Defaultdict but make it JavaScript

defaultdict.js

const TARGET = Symbol("proxy target");

function defaultdict(missing) {
  var dict = {};
  var instantiate = missing;
  if (missing instanceof Function) {
    if (missing.constructor) {
      instantiate = () => new missing();
    }
  } else if (missing instanceof Object) {
    instantiate = () => structuredClone(missing);
  } else {
    instantiate = () => missing;
  }
  var proxy = new Proxy(dict, {
    get(target, key) {
      if (key == TARGET) return target;
      return target[key] ??= instantiate();
    }
  });
  return proxy;
}

var counter = defaultdict(0);
"abccdddddeedfab".split("").forEach(l => counter[l]++);
console.log(counter[TARGET]);

var countunique = defaultdict(Set);
countunique["AZ"].add(2022);
countunique["TX"].add(2023);
countunique["AZ"].add(2023);
console.log(countunique[TARGET]);