Is there a way to have a map with weakly-held values in JavaScript?

Question

Is there a way to create WeakMap of any other weak references in Javascript for storing key value pairs where key is String/Number and value is Object.

The referencing would have to work something like this:

const wMap = new WeakRefMap();
const referencer = {child: new WeakRefMap()}
wMap.set('child', temp.child);
wMap.has('child'); // true
delete referencer.child
wMap.has('child'); //false

I creating kind of a tree structure that holds track of references that are still used in the current scope.

I will do a lot of merging, and recursively cleaning up a deeply nested structure can be very inefficient for this use case.

wan2land · Accepted Answer · 2022-09-07 01:18:36Z

5

You can solve it by using WeakRef and FinalizationRegistry.

Here is an example written in TypeScript:

class InvertedWeakMap<K extends string | symbol, V extends object> {
  _map = new Map<K, WeakRef<V>>()
  _registry: FinalizationRegistry<K>

  constructor() {
    this._registry = new FinalizationRegistry<K>((key) => {
      this._map.delete(key)
    })
  }

  set(key: K, value: V) {
    this._map.set(key, new WeakRef(value))
    this._registry.register(value, key)
  }

  get(key: K): V | undefined {
    const ref = this._map.get(key)
    if (ref) {
      return ref.deref()
    }
  }

  has(key: K): boolean {
    return this._map.has(key) && this.get(key) !== undefined
  }
}

async function main() {
  const map = new InvertedWeakMap()
  let data = { hello: "world!" } as any
  map.set("string!", data)

  console.log('---before---')
  console.log(map.get("string!"))
  console.log(map.has("string!"))

  data = null
  await new Promise((resolve) => setTimeout(resolve, 0))
  global.gc() // call gc manually

  console.log('---after---')
  console.log(map.get("string!"))
  console.log(map.has("string!"))
}

main()

It must be run with the --expose-gc option in the node.js environment.

answered Sep 7, 2022 at 1:18

wan2land

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2 Comments

Nabuska Over a year ago

That's a nice example! Lot of new features in JavaScript since 5 years ago when I was trying to find a solution for this=). Here is a codepen of the code example above codepen.io/jEnbuska/pen/jOxzZbm

dumbass Over a year ago

There is a bug:

let a = {}, b = {}; const wm = new InvertedWeakMap(); wm.set('key', a); wm.set('key', b); a = null; system.gc(); console.info(wm.has('key'));

Jonas Wilms · Accepted Answer · 2017-07-19 19:18:47Z

1

You cannot catch a delete operation. What you could do would be encapsuling the data in another obj e.g.

function referenceTo(value){
 this.value=value;
}

So if this one reference is deleted, it cant be accessed anymore

var somedata=new referenceTo(5)
var anotherref=somedata;
//do whatever
delete somedata.value;
//cannot be accessed anymore
anotherref.value;//undefined

answered Jul 19, 2017 at 19:18

Jonas Wilms

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2 Comments

Nabuska Over a year ago

To my understanding that does not solve the issue of deleting nested structures when top one is deleted? ps. I fixed the example, the previous was invalid. I don't get it why es6 WeakMap does not take strings as keys and objects as values. Doesn't seem to add any value on everyday programming.

Jonas Wilms Over a year ago

@nabuska yeah you may need to filter somewhen.

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