具有WeakMap或WeakSet的Javascript Flyweight

Question

I want a Flyweight object so I created an Object and stored it's instances in a Map like this:

const FlyweightNumber = (function(){

    "use strict";

    const instances = new Map();

    class FlyweightNumber{

        constructor(number){

            Object.defineProperty(this, 'number', {
                value: number
            });

            if(!instances.get(number)){
                instances.set(number, this);
            }
            else {
                return instances.get(number);
            }

        }

        toString() {
            return this.number;
        }

        valueOf(){
            return this.number;
        }

        toJSON(){
            return this.number;
        }

    }

    return FlyweightNumber;

})();

module.exports = FlyweightNumber;

The problem is that when I'm not using a FlyweightNumber value anymore it is still in memory, stored in this Map.

Since WeakMap and WeakSet are supposed to let garbage colector clear it if it is not used anymore, how could I write a constructor to return the object in the WeakSet or WeakMap or create a new object if it is not stored anymore?

Answer 1

You're looking for a soft reference to implement your number cache. Unfortunately, JS doesn't have these.

And its WeakMap doesn't create weak references either, it's an ephemeron actually. It does not allow you to observe whether an object has been collected.