# What This DoesSome state trees may need to reference an object more than once (such as the tree for my [fomod](https://www.npmjs.com/package/fomod) library). In essence, we store existing drafts when an off-by-default Immer class configuration option is enabled. This should be a painless solution. Specifics are described below.## Implementation Details* Two `WeakMap` are used to keep track of draft states and related data at different parts of the immerification process:  * `existingStateMap_` maps a given base object to the first draft state created for it. This state includes a reference to the revokable draft.    * If a state is referenced multiple times, it will be given a new `revoke_()` function that, once called the first time, calls the old `revoke_()` function. The result is that the final `revoke_()` must be called once for every requested draft before the Proxy is finally revoked. Since a proxy which has has its `revoke_()` method called should be considered revoked by all code paths, duplicate calls should not be an issue.  * During finalization, `encounteredObjects` keeps track of objects we've finalized and doesn't traverse an object if it's already seen it. It prevents infinite recursion when circular references are present.* Introduced the `extraParents_` property to the `ImmerBaseState` interface. This keeps track of additional values that would normally be attached to `parent_` so that functionality such as marking the parent state as modified is retained for objects with multiple parent objects* For Maps and Sets, a proxy is established between the state and DraftMap/DraftSet classes to handle multiple references to these native classes while preserving the idea of having one DraftSet per reference.* For Sets, each child draft has a single symbol value set so that a copy is prepared. (discussion needed; see TODOs below)* During finalization, objects may have drafted children and, thus, even unmodified children are finalized in multi-ref mode* To enable the feature, it is the same as other Immer class configuration options (such as `useStrictShallowCopy`). That is, either specify it in the config object passed to the class's constructor OR call the relevant method, `setAllowMultiRefs()`> [!NOTE]> Because of the extra computation involved with checking every proxied object against a map and traversing every object in a tree, enabling multi-ref will have a significant performance impact—even on trees which contain no repeated references.# TestsThe file `__tests__/multiref.ts` contains a number of tests related to this multi-reference support implementation. Such tests seek to verify that:* Direct circular references (which Immer tests for normally) do not throw an error when multi-ref is enabled* When the properties of multiple references are modified, all references are modified* Unmodified references to the same object are kept* The same copy is provided for every reference (new references are strictly equivalent [`===`] just as the references before `produce()` would have been)Tests are performed on all relevant object archetypes where applicable.# Outstanding Discussion TODOs* [ ] What to do regarding documentation* [ ] Possible alternate solution for preparing copies for multi-reference DraftSet children* [ ] Add an error for when WeakMap isn't supported in the current environment? (supported in every noteworthy browser and server environment since late 2015)

Cuts down on value passing and works better with nested producers.

See new tests and comments in multiref.ts for elaboration regarding the extended multiref support included in this commit.

👀 left those in by mistake