manage to handle toJS circular reference.

type-definitions/immutable.d.ts

@@ -100,27 +100,25 @@ declare namespace Immutable {
   export type DeepCopy<T> = T extends Record<infer R>
     ? // convert Record to DeepCopy plain JS object
       {
-        [key in keyof R]: R[key] extends object ? unknown : R[key];
+        [key in keyof R]: DeepCopy<R[key]>;
       }
     : T extends Collection.Keyed<infer KeyedKey, infer V>
     ? // convert KeyedCollection to DeepCopy plain JS object
       {
         [key in KeyedKey extends string | number | symbol
           ? KeyedKey
-          : string]: V extends object ? unknown : V;
+          : string]: DeepCopy<V>;
       }
     : // convert IndexedCollection or Immutable.Set to DeepCopy plain JS array
     T extends Collection<infer _, infer V>
-    ? Array<V extends object ? unknown : V>
+    ? Array<DeepCopy<V>>
     : T extends string | number // Iterable scalar types : should be kept as is
     ? T
     : T extends Iterable<infer V> // Iterable are converted to plain JS array
-    ? Array<V extends object ? unknown : V>
+    ? Array<DeepCopy<V>>
     : T extends object // plain JS object are converted deeply
     ? {
-        [ObjectKey in keyof T]: T[ObjectKey] extends object
-          ? unknown
-          : T[ObjectKey];
+        [ObjectKey in keyof T]: DeepCopy<T[ObjectKey]>;
       }
     : // other case : should be kept as is
       T;

type-definitions/ts-tests/deepCopy.ts

@@ -23,12 +23,10 @@ import { List, Map, Record, Set, Seq, DeepCopy, Collection } from 'immutable';
   // $ExpectType { [x: string]: string; }
   type StringKey = DeepCopy<Map<string, string>>;
 
-  // should be `{ [x: string]: object; }` but there is an issue with circular references
-  // $ExpectType { [x: string]: unknown; }
+  // $ExpectType { [x: string]: object; }
   type ObjectKey = DeepCopy<Map<object, object>>;
 
-  // should be `{ [x: string]: object; [x: number]: object; }` but there is an issue with circular references
-  // $ExpectType { [x: string]: unknown; [x: number]: unknown; }
+  // $ExpectType { [x: string]: object; [x: number]: object; }
   type MixedKey = DeepCopy<Map<object | number, object>>;
 
   // $ExpectType string[]
@@ -57,12 +55,10 @@ import { List, Map, Record, Set, Seq, DeepCopy, Collection } from 'immutable';
 {
   // Nested
 
-  // should be `{ map: { [x: string]: string; }; list: string[]; set: string[]; }` but there is an issue with circular references
-  // $ExpectType { map: unknown; list: unknown; set: unknown; }
+  // $ExpectType { map: { [x: string]: string; }; list: string[]; set: string[]; }
   type NestedObject = DeepCopy<{ map: Map<string, string>; list: List<string>; set: Set<string>; }>;
 
-  // should be `{ map: { [x: string]: string; }; }`, but there is an issue with circular references
-  // $ExpectType { map: unknown; }
+  // $ExpectType { map: { [x: string]: string; }; }
   type NestedMap = DeepCopy<Map<'map', Map<string, string>>>;
 }
 
@@ -72,8 +68,6 @@ import { List, Map, Record, Set, Seq, DeepCopy, Collection } from 'immutable';
   type Article = Record<{ title: string; tag: Tag; }>;
   type Tag = Record<{ name: string; article: Article; }>;
 
-  // should handle circular references here somehow
-  // $ExpectType { title: string; tag: unknown; }
+  // $ExpectType { title: string; tag: { name: string; article: any; }; }
   type Circular = DeepCopy<Article>;
-  //   ^?
 }

type-definitions/ts-tests/record.ts

@@ -1,4 +1,4 @@
-import { List, Map, Record, Set } from 'immutable';
+import { List, Map, Record, RecordOf, Set } from 'immutable';
 
 {
   // Factory
@@ -88,7 +88,31 @@ import { List, Map, Record, Set } from 'immutable';
   // $ExpectType { map: Map<string, string>; list: List<string>; set: Set<string>; }
   withMap.toJSON();
 
-  // should be `{ map: { [x: string]: string; }; list: string[]; set: string[]; }` but there is an issue with circular references
-  // $ExpectType { map: unknown; list: unknown; set: unknown; }
+  // $ExpectType { map: { [x: string]: string; }; list: string[]; set: string[]; }
   withMap.toJS();
 }
+
+{
+  // optional properties
+
+  interface Size { distance: string; }
+
+  const Line = Record<{ size?: Size, color?: string }>({ size: undefined, color: 'red' });
+
+  const line = Line({});
+
+  // $ExpectType { size?: { distance: string; } | undefined; color?: string | undefined; }
+  line.toJS();
+}
+
+{
+  // similar properties, but one is optional. See https://github.com/immutable-js/immutable-js/issues/1930
+
+  interface Id { value: string; }
+
+  type A = RecordOf<{ id: Id }>;
+  type B = RecordOf<{ id?: Id }>;
+
+  const a: A = null as any;
+  const b: B = a;
+}