javascript - Would a "circular" reference be treated as "reachability" for a WeakMap? - Stac

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function f() {
  const w = new WeakMap();
  const o = {};

  w.set(o, { v: o });

  return w;
}

const weakMap = f();

For the given code, would the only weakMap item considered as reachable or not? Hence, will it be garbage collected or not?

PS: This question is asked from the perspective of the specification, not particular implementations.

function f() {
  const w = new WeakMap();
  const o = {};

  w.set(o, { v: o });

  return w;
}

const weakMap = f();

For the given code, would the only weakMap item considered as reachable or not? Hence, will it be garbage collected or not?

PS: This question is asked from the perspective of the specification, not particular implementations.

• javascript
• weak-references
• weakmap

Share Improve this question Follow edited Oct 13, 2016 at 1:36 zerkms asked Sep 21, 2015 at 3:53 zerkmszerkms 255k7373 gold badges447447 silver badges547547 bronze badges 24

• 1 It's not supposed to. Things that only have circular references are eligible for garbage collection so should not stay in the weakMap. – jfriend00 Commented Sep 21, 2015 at 3:55
• I was saying that it should get released because there are no external references to it and thus it is not reachable elsewhere. – jfriend00 Commented Sep 21, 2015 at 3:57
• 1 I see where you are ing from and the question itself is reasonable, but I don't think I'd personally categorize it as circular since w doesn't reference itself, and neither does o. – loganfsmyth Commented Sep 21, 2015 at 4:03
• 1 @zerkms "So the hash table not only stores the hashes but the original keys as well" Yep, I agree. That's why I said "w references o and { v: o }". It sounds like our disagreement is here: "which means that the linked list element contains both the data and the key and both of them refer to each other". What I'm saying is that the key and value do not refer to each other, the HashTable just refers to both of them. Neither of those objects contain references to each other. All references to those objects are within the HashTable itself. – Ajedi32 Commented Oct 22, 2015 at 23:45
• 1 Yep, now I see and now I agree that it's a DAG, not a cyclic graph. Not sure how to improve the subject though, since it's already put in quotes. – zerkms Commented Oct 22, 2015 at 23:45

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1 Answer 1

Sorted by: Reset to default 17

Quoting WeakMap Objects section,

If an object that is being used as the key of a WeakMap key/value pair is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible and is automatically removed from the WeakMap.

In your case, the only way to reach o would be to start from one of the keys in the weakMap, as there is no external references to it. So, it would be considered as inaccessible.

WeakMap implementations must detect and remove such key/value pairs and any associated resources.

So, it would be eventually garbage collected.

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