WeakMap

new WeakMap([iterable])

var wm1 = new WeakMap(),
    wm2 = new WeakMap(),
    wm3 = new WeakMap(
var o1 = {},
    o2 = function() {},
    o3 = window;

wm1.set(o1, 37
wm1.set(o2, 'azerty'
wm2.set(o1, o2 // a value can be anything, including an object or a function
wm2.set(o3, undefined
wm2.set(wm1, wm2 // keys and values can be any objects. Even WeakMaps!

wm1.get(o2 // "azerty"
wm2.get(o2 // undefined, because there is no key for o2 on wm2
wm2.get(o3 // undefined, because that is the set value

wm1.has(o2 // true
wm2.has(o2 // false
wm2.has(o3 // true (even if the value itself is 'undefined')

wm3.set(o1, 37
wm3.get(o1 // 37

wm1.has(o1 // true
wm1.delete(o1
wm1.has(o1 // false

class ClearableWeakMap {
  constructor(init) {
    this._wm = new WeakMap(init)
  }
  clear() {
    this._wm = new WeakMap()
  }
  delete(k) {
    return this._wm.delete(k)
  }
  get(k) {
    return this._wm.get(k)
  }
  has(k) {
    return this._wm.has(k)
  }
  set(k, v) {
    this._wm.set(k, v)
    return this
  }
}