WeakMap implemented via Chrome's internal PrivateSymbol

weakmap.js

class WeakMap {
  #WeakMapData = %CreatePrivateSymbol('[[WeakMapData]]');

  delete(key) {
    const sym = this.#WeakMapData;
    if (!%HasProperty(key, sym)) {
      return false;
    }
    delete key[sym];
    return true;
  }

  get(key) {
    return key[this.#WeakMapData];
  }

  has(key) {
    return %HasProperty(key, this.#WeakMapData);
  }

  set(key, value) {
    const sym = this.#WeakMapData;
    if (typeof key !== 'object') {
      throw new TypeError('nope');
    }
    key[sym] = value;
    return this;
  }
}