Stage 2 Draft / June 7, 2022

Symbol as WeakMap Keys Proposal

1 Executable Code and Execution Contexts

1.1 Processing Model of WeakRef and FinalizationRegistry Objects

Note
Full set of proposed changes here: https://arai-a.github.io/ecma262-compare/?pr=2777

1.2 AddToKeptObjects ( object: an Object, value: an Object or a Symbol, ): unused

  1. Let agentRecord be the surrounding agent's Agent Record.
  2. Append object to agentRecord.[[KeptAlive]].
  3. Append value to agentRecord.[[KeptAlive]].
  4. Return unused.
 Note
When the abstract operation AddToKeptObjects is called with a target object , or symbol, reference, it adds the target to a list that will point strongly at the target until ClearKeptObjects is called.

2 CanBeHeldWeakly ( v: unknown ): a Boolean

  1. If Type(v) is Object, return true.
  2. If Type(v) is Symbol, then
    1. For each element e of the GlobalSymbolRegistry List (see 19.4.2.2), do
      1. If SameValue(e.[[Symbol]], v) is true, return false.
    2. Return true.
  3. Return false.
 Editor's Note

This abstract operation determines if v is considered to have an identity suitable for a weak reference. i.e. Is v a valid candidate key of a WeakMap, or entry in a WeakSet, or target of a WeakRef or FinalizationRegistry.

'Registered symbols' are not considered to have a suitable identity because their identity is similar to strings. Implementations may attempt to collect them when they are unreachable.

'Well-Known symbols' 6.1.5.1 return true - this should not be interpreted as evidence that using them as a key in a WeakMap will not result in a memory leak.

3 Modifications to the properties of WeakMap.prototype

3.1 WeakMap.prototype.delete ( key )

The following steps are taken:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be the List that is M.[[WeakMapData]].
  4. If Type(key) is not Object, return false.
  5. If CanBeHeldWeakly(key) is false, return false.
  6. For each Record { [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
      1. Set p.[[Key]] to empty.
      2. Set p.[[Value]] to empty.
      3. Return true.
  7. Return false.

3.2 WeakMap.prototype.get ( key )

The following steps are taken:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be the List that is M.[[WeakMapData]].
  4. If Type(key) is not Object, return undefined.
  5. If CanBeHeldWeakly(key) is false, return undefined.
  6. For each Record { [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return p.[[Value]].
  7. Return undefined.

3.3 WeakMap.prototype.has ( key )

The following steps are taken:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be the List that is M.[[WeakMapData]].
  4. If Type(key) is not Object, return false.
  5. If CanBeHeldWeakly(key) is false, return false.
  6. For each Record { [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, return true.
  7. Return false.

3.4 WeakMap.prototype.set ( key, value )

The following steps are taken:

  1. Let M be the this value.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be the List that is M.[[WeakMapData]].
  4. If Type(key) is not Object, throw a TypeError exception.
  5. If CanBeHeldWeakly(key) is false, throw a TypeError exception.
  6. For each Record { [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
      1. Set p.[[Value]] to value.
      2. Return M.
  7. Let p be the Record { [[Key]]: key, [[Value]]: value }.
  8. Append p as the last element of entries.
  9. Return M.

4 Modifications to the properties of WeakSet.prototype

4.1 WeakSet.prototype.add ( value )

The following steps are taken:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. If Type(value) is not Object, throw a TypeError exception.
  4. If CanBeHeldWeakly(value) is false, throw a TypeError exception.
  5. Let entries be the List that is S.[[WeakSetData]].
  6. For each element e of entries, do
    1. If e is not empty and SameValue(e, value) is true, then
      1. Return S.
  7. Append value as the last element of entries.
  8. Return S.

4.2 WeakSet.prototype.delete ( value )

The following steps are taken:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. If Type(value) is not Object, return false.
  4. If CanBeHeldWeakly(value) is false, return false.
  5. Let entries be the List that is S.[[WeakSetData]].
  6. For each element e of entries, do
    1. If e is not empty and SameValue(e, value) is true, then
      1. Replace the element of entries whose value is e with an element whose value is empty.
      2. Return true.
  7. Return false.

4.3 WeakSet.prototype.has ( value )

The following steps are taken:

  1. Let S be the this value.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. Let entries be the List that is S.[[WeakSetData]].
  4. If Type(value) is not Object, return false.
  5. If CanBeHeldWeakly(value) is false, return false.
  6. For each element e of entries, do
    1. If e is not empty and SameValue(e, value) is true, return true.
  7. Return false.

5 Modifications to WeakRef and FinalizationRegistry

5.1 WeakRef ( target )

When the WeakRef function is called with argument target, the following steps are taken:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. If Type(target) is not Object, throw a TypeError exception.
  3. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
  4. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakRef.prototype%", « [[WeakRefTarget]] »).
  5. Perform ! AddToKeptObjects(target).
  6. Set weakRef.[[WeakRefTarget]] to target.
  7. Return weakRef.

5.2 FinalizationRegistry.prototype.register ( target, heldValue [ , unregisterToken ] )

The following steps are taken:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If Type(target) is not Object, throw a TypeError exception.
  4. If CanBeHeldWeakly(target) is false, throw a TypeError exception.
  5. If SameValue(target, heldValue) is true, throw a TypeError exception.
  6. If Type(unregisterToken) is not Object, then
  7. If CanBeHeldWeakly(unregisterToken) is false, then
    1. If unregisterToken is not undefined, throw a TypeError exception.
    2. Set unregisterToken to empty.
  8. Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
  9. Append cell to finalizationRegistry.[[Cells]].
  10. Return undefined.

5.3 FinalizationRegistry.prototype.unregister ( unregisterToken )

The following steps are taken:

  1. Let finalizationRegistry be the this value.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. If Type(unregisterToken) is not Object, throw a TypeError exception.
  4. If CanBeHeldWeakly(unregisterToken) is false, throw a TypeError exception.
  5. Let removed be false.
  6. For each Record { [[WeakRefTarget]], [[HeldValue]], [[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do
    1. If cell.[[UnregisterToken]] is not empty and SameValue(cell.[[UnregisterToken]], unregisterToken) is true, then
      1. Remove cell from finalizationRegistry.[[Cells]].
      2. Set removed to true.
  7. Return removed.

A Copyright & Software License

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© 2022 Robin Ricard,Rick Button,Daniel Ehrenberg,Leo Balter,Caridy Patiño,Rick Waldron,Ashley Claymore

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