WeakSetの使いみち...

gistfile1.txt

class RefCounter {
  constructor() {
    this.counter = 0
    this.decrementers = new WeakSet()
  }

  increment() {
    this.counter += 1

    const decrementer = () => {
      if(this.decrementers.has(decrementer)) {
        this.decrementers.delete(decrementer)
        this.counter -= 1
        return true
      }
      return false
    }
    this.decrementers.add(decrementer)
    return decrementer
  }
}