wm.js

class WeakMap {

  constructor() {
    this.s = Symbol();
  }

  set(key, val) {
    key[this.s] = val;
  }

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

  has(key) {
    return key.hasOwnProperty(this.s);
  }

  delete(key) {
    delete key[this.s];
  }
}