Improve anonymizer return type

The anonymizer function can now return a _recursive partial_ of the
input type. So any properties can be omitted in the state property
directly or in any nested object.

Note that for primitive values and arrays, the return type must still
match the state. Similarly, any objects still need to be returned as
objects, though they're allowed to have a few less properties.

src/BaseControllerV2.ts

@@ -11,7 +11,18 @@ enablePatches();
  */
 export type Listener<T> = (state: T, patches: Patch[]) => void;
 
-export type Anonymizer<T> = (value: T) => T;
+type Primitive = boolean | string | number | null;
+
+// Based upon this StackOverflow answer: https://stackoverflow.com/a/64060332
+type RecursivePartial<T> = {
+  [P in keyof T]?: T[P] extends (infer U)[]
+    ? RecursivePartial<U>[]
+    : T[P] extends Primitive
+    ? T[P]
+    : RecursivePartial<T>;
+};
+
+export type Anonymizer<T> = (value: T) => T extends Primitive ? T : RecursivePartial<T>;
 
 export type StateMetadata<T> = {
   [P in keyof T]: {
@@ -109,7 +120,10 @@ function isAnonymizingFunction<T>(x: boolean | Anonymizer<T>): x is Anonymizer<T
   return typeof x === 'function';
 }
 
-export function getAnonymizedState<S extends Record<string, any>>(state: S, metadata: StateMetadata<S>) {
+export function getAnonymizedState<S extends Record<string, any>>(
+  state: S,
+  metadata: StateMetadata<S>,
+): RecursivePartial<S> {
   return Object.keys(state).reduce((anonymizedState, _key) => {
     const key: keyof S = _key; // https://stackoverflow.com/questions/63893394/string-cannot-be-used-to-index-type-t
     const metadataValue = metadata[key].anonymous;
@@ -119,15 +133,18 @@ export function getAnonymizedState<S extends Record<string, any>>(state: S, meta
       anonymizedState[key] = state[key];
     }
     return anonymizedState;
-  }, {} as Partial<S>);
+  }, {} as RecursivePartial<S>);
 }
 
-export function getPersistentState<S extends Record<string, any>>(state: S, metadata: StateMetadata<S>) {
+export function getPersistentState<S extends Record<string, any>>(
+  state: S,
+  metadata: StateMetadata<S>,
+): RecursivePartial<S> {
   return Object.keys(state).reduce((persistedState, _key) => {
     const key: keyof S = _key; // https://stackoverflow.com/questions/63893394/string-cannot-be-used-to-index-type-t
     if (metadata[key].persist) {
       persistedState[key] = state[key];
     }
     return persistedState;
-  }, {} as Partial<S>);
+  }, {} as RecursivePartial<S>);
 }